From 0ad1239522c70418990dc7b9da4e128da7cdd1d5 Mon Sep 17 00:00:00 2001
From: Andrew Kelley <superjoe30@gmail.com>
Date: Sun, 3 Dec 2017 20:43:56 -0500
Subject: [PATCH] rework enums and unions and their relationship to each other

 * @enumTagName renamed to @tagName and it works on enums and
   union-enums
 * Remove the EnumTag type. Now there is only enum and union,
   and the tag type of a union is always an enum.
 * unions support specifying the tag enum type, and they support
   inferring an enum tag type.
 * Enums no longer support field types but they do support
   setting the tag values. Likewise union-enums when inferring
   an enum tag type support setting the tag values.
 * It is now an error for enums and unions to have 0 fields.
 * switch statements support union-enums

closes #618
---
 doc/langref.html.in                       |  21 +-
 src/all_types.hpp                         |  59 +--
 src/analyze.cpp                           | 607 +++++++++++-----------
 src/analyze.hpp                           |   6 +-
 src/ast_render.cpp                        |   7 +
 src/codegen.cpp                           | 245 +++------
 src/ir.cpp                                | 488 ++++++++---------
 src/ir_print.cpp                          |  31 +-
 src/parser.cpp                            |  29 +-
 std/build.zig                             |  42 +-
 std/debug.zig                             |  28 +-
 std/os/child_process.zig                  |  10 +-
 std/os/path.zig                           |   2 +-
 test/cases/bugs/394.zig                   |   4 +-
 test/cases/enum.zig                       |  24 +-
 test/cases/enum_with_members.zig          |   6 +-
 test/cases/misc.zig                       |  16 +-
 test/cases/reflection.zig                 |   4 +-
 test/cases/switch.zig                     |  16 +-
 test/cases/switch_prong_err_enum.zig      |   4 +-
 test/cases/switch_prong_implicit_cast.zig |   8 +-
 test/cases/union.zig                      |  35 +-
 test/compile_errors.zig                   |  54 +-
 test/tests.zig                            |  10 +-
 24 files changed, 803 insertions(+), 953 deletions(-)

diff --git a/doc/langref.html.in b/doc/langref.html.in
index 20b8ae1eee..76bf0ce237 100644
--- a/doc/langref.html.in
+++ b/doc/langref.html.in
@@ -136,7 +136,7 @@
             <li><a href="#builtin-divFloor">@divFloor</a></li>
             <li><a href="#builtin-divTrunc">@divTrunc</a></li>
             <li><a href="#builtin-embedFile">@embedFile</a></li>
-            <li><a href="#builtin-enumTagName">@enumTagName</a></li>
+            <li><a href="#builtin-tagName">@tagName</a></li>
             <li><a href="#builtin-EnumTagType">@EnumTagType</a></li>
             <li><a href="#builtin-errorName">@errorName</a></li>
             <li><a href="#builtin-fence">@fence</a></li>
@@ -2165,7 +2165,7 @@ test "enum variant switch" {
     };
 }
 
-// The @enumTagName and @memberCount builtin functions can be used to
+// The @memberName and @memberCount builtin functions can be used to
 // the string representation and number of members respectively.
 const BuiltinType = enum {
     A: f32,
@@ -2174,8 +2174,8 @@ const BuiltinType = enum {
 };
 
 test "enum builtins" {
-    assert(mem.eql(u8, @enumTagName(BuiltinType.A { 0 }), "A"));
-    assert(mem.eql(u8, @enumTagName(BuiltinType.C), "C"));
+    assert(mem.eql(u8, @memberName(BuiltinType.A { 0 }), "A"));
+    assert(mem.eql(u8, @memberName(BuiltinType.C), "C"));
     assert(@memberCount(BuiltinType) == 3);
 }</code></pre>
       <pre><code class="sh">$ zig test enum.zig
@@ -2189,8 +2189,9 @@ Test 4/4 enum builtins...OK</code></pre>
       </p>
       <p>See also:</p>
       <ul>
-        <li><a href="#builtin-enumTagName">@enumTagName</a></li>
+        <li><a href="#builtin-memberName">@memberName</a></li>
         <li><a href="#builtin-memberCount">@memberCount</a></li>
+        <li><a href="#builtin-tagName">@tagName</a></li>
       </ul>
       <h2 id="union">union</h2>
       <p>TODO union documentation</p>
@@ -4252,10 +4253,10 @@ test.zig:6:2: error: found compile log statement
       <ul>
         <li><a href="#builtin-import">@import</a></li>
       </ul>
-      <h3 id="builtin-enumTagName">@enumTagName</h3>
-      <pre><code class="zig">@enumTagName(value: var) -&gt; []const u8</code></pre>
+      <h3 id="builtin-tagName">@tagName</h3>
+      <pre><code class="zig">@tagName(value: var) -&gt; []const u8</code></pre>
       <p>
-      Converts an enum tag name to a slice of bytes.
+      Converts an enum value or union value to a slice of bytes representing the name.
       </p>
       <h3 id="builtin-EnumTagType">@EnumTagType</h3>
       <pre><code class="zig">@EnumTagType(T: type) -&gt; type</code></pre>
@@ -5843,7 +5844,9 @@ GroupedExpression = "(" Expression ")"
 
 KeywordLiteral = "true" | "false" | "null" | "continue" | "undefined" | "error" | "this" | "unreachable"
 
-ContainerDecl = option("extern" | "packed") ("struct" | "union" | ("enum" option(GroupedExpression))) "{" many(ContainerMember) "}"</code></pre>
+ContainerDecl = option("extern" | "packed")
+  ("struct" option(GroupedExpression) | "union" option("enum" option(GroupedExpression) | GroupedExpression) | ("enum" option(GroupedExpression)))
+  "{" many(ContainerMember) "}"</code></pre>
       <h2 id="zen">Zen</h2>
       <ul>
         <li>Communicate intent precisely.</li>
diff --git a/src/all_types.hpp b/src/all_types.hpp
index 8d4ffc5b84..6061f63da6 100644
--- a/src/all_types.hpp
+++ b/src/all_types.hpp
@@ -97,11 +97,6 @@ struct ConstParent {
     } data;
 };
 
-struct ConstEnumValue {
-    BigInt tag;
-    ConstExprValue *payload;
-};
-
 struct ConstStructValue {
     ConstExprValue *fields;
     ConstParent parent;
@@ -249,7 +244,7 @@ struct ConstExprValue {
         ConstExprValue *x_maybe;
         ConstErrValue x_err_union;
         ErrorTableEntry *x_pure_err;
-        ConstEnumValue x_enum;
+        BigInt x_enum_tag;
         ConstStructValue x_struct;
         ConstUnionValue x_union;
         ConstArrayValue x_array;
@@ -345,15 +340,14 @@ struct TldCompTime {
 
 struct TypeEnumField {
     Buf *name;
-    TypeTableEntry *type_entry;
     BigInt value;
-    uint32_t gen_index;
+    uint32_t decl_index;
 };
 
 struct TypeUnionField {
     Buf *name;
+    TypeEnumField *enum_field;
     TypeTableEntry *type_entry;
-    BigInt value;
     uint32_t gen_index;
 };
 
@@ -773,7 +767,8 @@ struct AstNodeContainerDecl {
     ZigList<AstNode *> fields;
     ZigList<AstNode *> decls;
     ContainerLayout layout;
-    AstNode *init_arg_expr; // enum(T) or struct(endianness)
+    AstNode *init_arg_expr; // enum(T), struct(endianness), or union(T), or union(enum(T))
+    bool auto_enum; // union(enum)
 };
 
 struct AstNodeStructField {
@@ -1010,13 +1005,9 @@ struct TypeTableEntryEnum {
     AstNode *decl_node;
     ContainerLayout layout;
     uint32_t src_field_count;
-    // number of fields in the union. 0 if enum with no payload
-    uint32_t gen_field_count;
     TypeEnumField *fields;
     bool is_invalid; // true if any fields are invalid
-    TypeTableEntry *tag_type;
     TypeTableEntry *tag_int_type;
-    LLVMTypeRef union_type_ref;
 
     ScopeDecls *decls_scope;
 
@@ -1028,18 +1019,7 @@ struct TypeTableEntryEnum {
 
     bool zero_bits_loop_flag;
     bool zero_bits_known;
-    uint32_t abi_alignment; // also figured out with zero_bits pass
 
-    size_t gen_union_index;
-    size_t gen_tag_index;
-
-    uint32_t union_size_bytes;
-    TypeTableEntry *most_aligned_union_member;
-};
-
-struct TypeTableEntryEnumTag {
-    TypeTableEntry *enum_type;
-    TypeTableEntry *int_type;
     bool generate_name_table;
     LLVMValueRef name_table;
 };
@@ -1054,7 +1034,7 @@ struct TypeTableEntryUnion {
     uint32_t gen_field_count;
     TypeUnionField *fields;
     bool is_invalid; // true if any fields are invalid
-    TypeTableEntry *tag_type;
+    TypeTableEntry *tag_type; // always an enum or null
     LLVMTypeRef union_type_ref;
 
     ScopeDecls *decls_scope;
@@ -1119,7 +1099,6 @@ enum TypeTableEntryId {
     TypeTableEntryIdErrorUnion,
     TypeTableEntryIdPureError,
     TypeTableEntryIdEnum,
-    TypeTableEntryIdEnumTag,
     TypeTableEntryIdUnion,
     TypeTableEntryIdFn,
     TypeTableEntryIdNamespace,
@@ -1148,7 +1127,6 @@ struct TypeTableEntry {
         TypeTableEntryMaybe maybe;
         TypeTableEntryError error;
         TypeTableEntryEnum enumeration;
-        TypeTableEntryEnumTag enum_tag;
         TypeTableEntryUnion unionation;
         TypeTableEntryFn fn;
         TypeTableEntryBoundFn bound_fn;
@@ -1287,7 +1265,7 @@ enum BuiltinFnId {
     BuiltinFnIdBitCast,
     BuiltinFnIdIntToPtr,
     BuiltinFnIdPtrToInt,
-    BuiltinFnIdEnumTagName,
+    BuiltinFnIdTagName,
     BuiltinFnIdEnumTagType,
     BuiltinFnIdFieldParentPtr,
     BuiltinFnIdOffsetOf,
@@ -1832,7 +1810,6 @@ enum IrInstructionId {
     IrInstructionIdStorePtr,
     IrInstructionIdFieldPtr,
     IrInstructionIdStructFieldPtr,
-    IrInstructionIdEnumFieldPtr,
     IrInstructionIdUnionFieldPtr,
     IrInstructionIdElemPtr,
     IrInstructionIdVarPtr,
@@ -1857,7 +1834,7 @@ enum IrInstructionId {
     IrInstructionIdTestNonNull,
     IrInstructionIdUnwrapMaybe,
     IrInstructionIdMaybeWrap,
-    IrInstructionIdEnumTag,
+    IrInstructionIdUnionTag,
     IrInstructionIdClz,
     IrInstructionIdCtz,
     IrInstructionIdImport,
@@ -1896,7 +1873,6 @@ enum IrInstructionId {
     IrInstructionIdErrWrapPayload,
     IrInstructionIdFnProto,
     IrInstructionIdTestComptime,
-    IrInstructionIdInitEnum,
     IrInstructionIdPtrCast,
     IrInstructionIdBitCast,
     IrInstructionIdWidenOrShorten,
@@ -2092,14 +2068,6 @@ struct IrInstructionStructFieldPtr {
     bool is_const;
 };
 
-struct IrInstructionEnumFieldPtr {
-    IrInstruction base;
-
-    IrInstruction *enum_ptr;
-    TypeEnumField *field;
-    bool is_const;
-};
-
 struct IrInstructionUnionFieldPtr {
     IrInstruction base;
 
@@ -2303,7 +2271,7 @@ struct IrInstructionClz {
     IrInstruction *value;
 };
 
-struct IrInstructionEnumTag {
+struct IrInstructionUnionTag {
     IrInstruction base;
 
     IrInstruction *value;
@@ -2573,15 +2541,6 @@ struct IrInstructionTestComptime {
     IrInstruction *value;
 };
 
-struct IrInstructionInitEnum {
-    IrInstruction base;
-
-    TypeTableEntry *enum_type;
-    TypeEnumField *field;
-    IrInstruction *init_value;
-    LLVMValueRef tmp_ptr;
-};
-
 struct IrInstructionPtrCast {
     IrInstruction base;
 
diff --git a/src/analyze.cpp b/src/analyze.cpp
index b7d12443a0..9d2d4f1a8f 100644
--- a/src/analyze.cpp
+++ b/src/analyze.cpp
@@ -223,7 +223,6 @@ bool type_is_complete(TypeTableEntry *type_entry) {
         case TypeTableEntryIdNamespace:
         case TypeTableEntryIdBlock:
         case TypeTableEntryIdBoundFn:
-        case TypeTableEntryIdEnumTag:
         case TypeTableEntryIdArgTuple:
             return true;
     }
@@ -260,7 +259,6 @@ bool type_has_zero_bits_known(TypeTableEntry *type_entry) {
         case TypeTableEntryIdNamespace:
         case TypeTableEntryIdBlock:
         case TypeTableEntryIdBoundFn:
-        case TypeTableEntryIdEnumTag:
         case TypeTableEntryIdArgTuple:
         case TypeTableEntryIdOpaque:
             return true;
@@ -1175,7 +1173,6 @@ static TypeTableEntry *analyze_fn_type(CodeGen *g, AstNode *proto_node, Scope *c
             case TypeTableEntryIdEnum:
             case TypeTableEntryIdUnion:
             case TypeTableEntryIdFn:
-            case TypeTableEntryIdEnumTag:
                 ensure_complete_type(g, type_entry);
                 if (fn_type_id.cc == CallingConventionUnspecified && !type_is_copyable(g, type_entry)) {
                     add_node_error(g, param_node->data.param_decl.type,
@@ -1239,7 +1236,6 @@ static TypeTableEntry *analyze_fn_type(CodeGen *g, AstNode *proto_node, Scope *c
         case TypeTableEntryIdEnum:
         case TypeTableEntryIdUnion:
         case TypeTableEntryIdFn:
-        case TypeTableEntryIdEnumTag:
             break;
     }
 
@@ -1263,22 +1259,6 @@ bool type_is_invalid(TypeTableEntry *type_entry) {
 }
 
 
-TypeTableEntry *create_enum_tag_type(CodeGen *g, TypeTableEntry *enum_type, TypeTableEntry *int_type) {
-    TypeTableEntry *entry = new_type_table_entry(TypeTableEntryIdEnumTag);
-
-    buf_resize(&entry->name, 0);
-    buf_appendf(&entry->name, "@EnumTagType(%s)", buf_ptr(&enum_type->name));
-
-    entry->is_copyable = true;
-    entry->data.enum_tag.enum_type = enum_type;
-    entry->data.enum_tag.int_type = int_type;
-    entry->type_ref = int_type->type_ref;
-    entry->di_type = int_type->di_type;
-    entry->zero_bits = int_type->zero_bits;
-
-    return entry;
-}
-
 static void resolve_enum_type(CodeGen *g, TypeTableEntry *enum_type) {
     assert(enum_type->id == TypeTableEntryIdEnum);
 
@@ -1308,14 +1288,6 @@ static void resolve_enum_type(CodeGen *g, TypeTableEntry *enum_type) {
     assert(enum_type->data.enumeration.fields);
     ZigLLVMDIEnumerator **di_enumerators = allocate<ZigLLVMDIEnumerator*>(field_count);
 
-    uint32_t gen_field_count = enum_type->data.enumeration.gen_field_count;
-    ZigLLVMDIType **union_inner_di_types = allocate<ZigLLVMDIType*>(gen_field_count);
-
-    TypeTableEntry *most_aligned_union_member = nullptr;
-    uint64_t size_of_most_aligned_member_in_bits = 0;
-    uint64_t biggest_align_in_bits = 0;
-    uint64_t biggest_size_in_bits = 0;
-
     Scope *scope = &enum_type->data.enumeration.decls_scope->base;
     ImportTableEntry *import = get_scope_import(scope);
 
@@ -1323,49 +1295,17 @@ static void resolve_enum_type(CodeGen *g, TypeTableEntry *enum_type) {
     enum_type->data.enumeration.embedded_in_current = true;
 
     for (uint32_t i = 0; i < field_count; i += 1) {
-        AstNode *field_node = decl_node->data.container_decl.fields.at(i);
-        TypeEnumField *type_enum_field = &enum_type->data.enumeration.fields[i];
-        TypeTableEntry *field_type = type_enum_field->type_entry;
+        TypeEnumField *enum_field = &enum_type->data.enumeration.fields[i];
 
-        di_enumerators[i] = ZigLLVMCreateDebugEnumerator(g->dbuilder, buf_ptr(type_enum_field->name), i);
-
-        ensure_complete_type(g, field_type);
-        if (type_is_invalid(field_type)) {
-            enum_type->data.enumeration.is_invalid = true;
-            continue;
-        }
-
-        if (!type_has_bits(field_type))
-            continue;
-
-        uint64_t store_size_in_bits = 8*LLVMStoreSizeOfType(g->target_data_ref, field_type->type_ref);
-        uint64_t abi_align_in_bits = 8*LLVMABIAlignmentOfType(g->target_data_ref, field_type->type_ref);
-
-        assert(store_size_in_bits > 0);
-        assert(abi_align_in_bits > 0);
-
-        union_inner_di_types[type_enum_field->gen_index] = ZigLLVMCreateDebugMemberType(g->dbuilder,
-                ZigLLVMTypeToScope(enum_type->di_type), buf_ptr(type_enum_field->name),
-                import->di_file, (unsigned)(field_node->line + 1),
-                store_size_in_bits,
-                abi_align_in_bits,
-                0,
-                0, field_type->di_type);
-
-        biggest_size_in_bits = max(biggest_size_in_bits, store_size_in_bits);
-
-        if (!most_aligned_union_member || abi_align_in_bits > biggest_align_in_bits) {
-            most_aligned_union_member = field_type;
-            biggest_align_in_bits = abi_align_in_bits;
-            size_of_most_aligned_member_in_bits = store_size_in_bits;
-        }
+        // TODO send patch to LLVM to support APInt in createEnumerator instead of int64_t
+        // http://lists.llvm.org/pipermail/llvm-dev/2017-December/119456.html
+        di_enumerators[i] = ZigLLVMCreateDebugEnumerator(g->dbuilder, buf_ptr(enum_field->name),
+                bigint_as_signed(&enum_field->value));
     }
 
     // unset temporary flag
     enum_type->data.enumeration.embedded_in_current = false;
     enum_type->data.enumeration.complete = true;
-    enum_type->data.enumeration.union_size_bytes = biggest_size_in_bits / 8;
-    enum_type->data.enumeration.most_aligned_union_member = most_aligned_union_member;
 
     if (enum_type->data.enumeration.is_invalid)
         return;
@@ -1391,117 +1331,20 @@ static void resolve_enum_type(CodeGen *g, TypeTableEntry *enum_type) {
     }
 
     TypeTableEntry *tag_int_type = enum_type->data.enumeration.tag_int_type;
-    TypeTableEntry *tag_type_entry = create_enum_tag_type(g, enum_type, tag_int_type);
-    enum_type->data.enumeration.tag_type = tag_type_entry;
 
-    uint64_t align_of_tag_in_bits = 8*LLVMABIAlignmentOfType(g->target_data_ref, tag_int_type->type_ref);
+    // create debug type for tag
+    uint64_t tag_debug_size_in_bits = 8*LLVMStoreSizeOfType(g->target_data_ref, tag_int_type->type_ref);
+    uint64_t tag_debug_align_in_bits = 8*LLVMABIAlignmentOfType(g->target_data_ref, tag_int_type->type_ref);
+    ZigLLVMDIType *tag_di_type = ZigLLVMCreateDebugEnumerationType(g->dbuilder,
+            ZigLLVMFileToScope(import->di_file), buf_ptr(&enum_type->name),
+            import->di_file, (unsigned)(decl_node->line + 1),
+            tag_debug_size_in_bits,
+            tag_debug_align_in_bits,
+            di_enumerators, field_count,
+            tag_int_type->di_type, "");
 
-    if (most_aligned_union_member) {
-        // create llvm type for union
-        uint64_t padding_in_bits = biggest_size_in_bits - size_of_most_aligned_member_in_bits;
-        LLVMTypeRef union_type_ref;
-        if (padding_in_bits > 0) {
-            TypeTableEntry *u8_type = get_int_type(g, false, 8);
-            TypeTableEntry *padding_array = get_array_type(g, u8_type, padding_in_bits / 8);
-            LLVMTypeRef union_element_types[] = {
-                most_aligned_union_member->type_ref,
-                padding_array->type_ref,
-            };
-            union_type_ref = LLVMStructType(union_element_types, 2, false);
-        } else {
-            union_type_ref = most_aligned_union_member->type_ref;
-        }
-        enum_type->data.enumeration.union_type_ref = union_type_ref;
-
-        assert(8*LLVMABIAlignmentOfType(g->target_data_ref, union_type_ref) >= biggest_align_in_bits);
-        assert(8*LLVMStoreSizeOfType(g->target_data_ref, union_type_ref) >= biggest_size_in_bits);
-
-        if (align_of_tag_in_bits >= biggest_align_in_bits) {
-            enum_type->data.enumeration.gen_tag_index = 0;
-            enum_type->data.enumeration.gen_union_index = 1;
-        } else {
-            enum_type->data.enumeration.gen_union_index = 0;
-            enum_type->data.enumeration.gen_tag_index = 1;
-        }
-
-        // create llvm type for root struct
-        LLVMTypeRef root_struct_element_types[2];
-        root_struct_element_types[enum_type->data.enumeration.gen_tag_index] = tag_type_entry->type_ref;
-        root_struct_element_types[enum_type->data.enumeration.gen_union_index] = union_type_ref;
-        LLVMStructSetBody(enum_type->type_ref, root_struct_element_types, 2, false);
-
-        // create debug type for tag
-        uint64_t tag_debug_size_in_bits = 8*LLVMStoreSizeOfType(g->target_data_ref, tag_type_entry->type_ref);
-        uint64_t tag_debug_align_in_bits = 8*LLVMABIAlignmentOfType(g->target_data_ref, tag_type_entry->type_ref);
-        ZigLLVMDIType *tag_di_type = ZigLLVMCreateDebugEnumerationType(g->dbuilder,
-                ZigLLVMTypeToScope(enum_type->di_type), "AnonEnum",
-                import->di_file, (unsigned)(decl_node->line + 1),
-                tag_debug_size_in_bits, tag_debug_align_in_bits, di_enumerators, field_count,
-                tag_type_entry->di_type, "");
-
-        // create debug type for union
-        ZigLLVMDIType *union_di_type = ZigLLVMCreateDebugUnionType(g->dbuilder,
-                ZigLLVMTypeToScope(enum_type->di_type), "AnonUnion",
-                import->di_file, (unsigned)(decl_node->line + 1),
-                biggest_size_in_bits, biggest_align_in_bits, 0, union_inner_di_types,
-                gen_field_count, 0, "");
-
-        // create debug types for members of root struct
-        uint64_t tag_offset_in_bits = 8*LLVMOffsetOfElement(g->target_data_ref, enum_type->type_ref,
-                enum_type->data.enumeration.gen_tag_index);
-        ZigLLVMDIType *tag_member_di_type = ZigLLVMCreateDebugMemberType(g->dbuilder,
-                ZigLLVMTypeToScope(enum_type->di_type), "tag_field",
-                import->di_file, (unsigned)(decl_node->line + 1),
-                tag_debug_size_in_bits,
-                tag_debug_align_in_bits,
-                tag_offset_in_bits,
-                0, tag_di_type);
-
-        uint64_t union_offset_in_bits = 8*LLVMOffsetOfElement(g->target_data_ref, enum_type->type_ref,
-                enum_type->data.enumeration.gen_union_index);
-        ZigLLVMDIType *union_member_di_type = ZigLLVMCreateDebugMemberType(g->dbuilder,
-                ZigLLVMTypeToScope(enum_type->di_type), "union_field",
-                import->di_file, (unsigned)(decl_node->line + 1),
-                biggest_size_in_bits,
-                biggest_align_in_bits,
-                union_offset_in_bits,
-                0, union_di_type);
-
-        // create debug type for root struct
-        ZigLLVMDIType *di_root_members[2];
-        di_root_members[enum_type->data.enumeration.gen_tag_index] = tag_member_di_type;
-        di_root_members[enum_type->data.enumeration.gen_union_index] = union_member_di_type;
-
-        uint64_t debug_size_in_bits = 8*LLVMStoreSizeOfType(g->target_data_ref, enum_type->type_ref);
-        uint64_t debug_align_in_bits = 8*LLVMABISizeOfType(g->target_data_ref, enum_type->type_ref);
-        ZigLLVMDIType *replacement_di_type = ZigLLVMCreateDebugStructType(g->dbuilder,
-                ZigLLVMFileToScope(import->di_file),
-                buf_ptr(&enum_type->name),
-                import->di_file, (unsigned)(decl_node->line + 1),
-                debug_size_in_bits,
-                debug_align_in_bits,
-                0, nullptr, di_root_members, 2, 0, nullptr, "");
-
-        ZigLLVMReplaceTemporary(g->dbuilder, enum_type->di_type, replacement_di_type);
-        enum_type->di_type = replacement_di_type;
-    } else {
-        // create llvm type for root struct
-        enum_type->type_ref = tag_type_entry->type_ref;
-
-        // create debug type for tag
-        uint64_t tag_debug_size_in_bits = 8*LLVMStoreSizeOfType(g->target_data_ref, tag_type_entry->type_ref);
-        uint64_t tag_debug_align_in_bits = 8*LLVMABIAlignmentOfType(g->target_data_ref, tag_type_entry->type_ref);
-        ZigLLVMDIType *tag_di_type = ZigLLVMCreateDebugEnumerationType(g->dbuilder,
-                ZigLLVMFileToScope(import->di_file), buf_ptr(&enum_type->name),
-                import->di_file, (unsigned)(decl_node->line + 1),
-                tag_debug_size_in_bits,
-                tag_debug_align_in_bits,
-                di_enumerators, field_count,
-                tag_type_entry->di_type, "");
-
-        ZigLLVMReplaceTemporary(g->dbuilder, enum_type->di_type, tag_di_type);
-        enum_type->di_type = tag_di_type;
-    }
+    ZigLLVMReplaceTemporary(g->dbuilder, enum_type->di_type, tag_di_type);
+    enum_type->di_type = tag_di_type;
 }
 
 static bool type_allowed_in_packed_struct(TypeTableEntry *type_entry) {
@@ -1517,7 +1360,6 @@ static bool type_allowed_in_packed_struct(TypeTableEntry *type_entry) {
         case TypeTableEntryIdNullLit:
         case TypeTableEntryIdErrorUnion:
         case TypeTableEntryIdPureError:
-        case TypeTableEntryIdEnumTag:
         case TypeTableEntryIdNamespace:
         case TypeTableEntryIdBlock:
         case TypeTableEntryIdBoundFn:
@@ -1541,8 +1383,7 @@ static bool type_allowed_in_packed_struct(TypeTableEntry *type_entry) {
                 return child_type->id == TypeTableEntryIdPointer || child_type->id == TypeTableEntryIdFn;
             }
         case TypeTableEntryIdEnum:
-            return type_entry->data.enumeration.gen_field_count == 0 &&
-                type_entry->data.enumeration.decl_node->data.container_decl.init_arg_expr != nullptr;
+            return type_entry->data.enumeration.decl_node->data.container_decl.init_arg_expr != nullptr;
     }
     zig_unreachable();
 }
@@ -1850,6 +1691,7 @@ static void resolve_union_type(CodeGen *g, TypeTableEntry *union_type) {
     if (union_type->data.unionation.embedded_in_current) {
         if (!union_type->data.unionation.reported_infinite_err) {
             union_type->data.unionation.reported_infinite_err = true;
+            union_type->data.unionation.is_invalid = true;
             add_node_error(g, decl_node, buf_sprintf("union '%s' contains itself", buf_ptr(&union_type->name)));
         }
         return;
@@ -1871,8 +1713,7 @@ static void resolve_union_type(CodeGen *g, TypeTableEntry *union_type) {
     uint64_t biggest_align_in_bits = 0;
     uint64_t biggest_size_in_bits = 0;
 
-    bool auto_layout = (union_type->data.unionation.layout == ContainerLayoutAuto);
-    ZigLLVMDIEnumerator **di_enumerators = allocate<ZigLLVMDIEnumerator*>(field_count);
+    ZigLLVMDIEnumerator **di_enumerators;
 
     Scope *scope = &union_type->data.unionation.decls_scope->base;
     ImportTableEntry *import = get_scope_import(scope);
@@ -1880,10 +1721,77 @@ static void resolve_union_type(CodeGen *g, TypeTableEntry *union_type) {
     // set temporary flag
     union_type->data.unionation.embedded_in_current = true;
 
+    HashMap<BigInt, AstNode *, bigint_hash, bigint_eql> occupied_tag_values = {};
+
+    AstNode *enum_type_node = decl_node->data.container_decl.init_arg_expr;
+    bool auto_layout = (union_type->data.unionation.layout == ContainerLayoutAuto);
+    bool want_safety = (field_count >= 2) && (auto_layout || enum_type_node != nullptr);
+    TypeTableEntry *tag_type;
+    bool create_enum_type = decl_node->data.container_decl.auto_enum || (enum_type_node == nullptr && want_safety);
+    bool *covered_enum_fields;
+    if (create_enum_type) {
+        occupied_tag_values.init(field_count);
+
+        di_enumerators = allocate<ZigLLVMDIEnumerator*>(field_count);
+
+        TypeTableEntry *tag_int_type;
+        if (enum_type_node != nullptr) {
+            tag_int_type = analyze_type_expr(g, scope, enum_type_node);
+            if (type_is_invalid(tag_int_type)) {
+                union_type->data.unionation.is_invalid = true;
+                return;
+            }
+            if (tag_int_type->id != TypeTableEntryIdInt) {
+                add_node_error(g, enum_type_node,
+                    buf_sprintf("expected integer tag type, found '%s'", buf_ptr(&tag_int_type->name)));
+                union_type->data.unionation.is_invalid = true;
+                return;
+            }
+        } else {
+            tag_int_type = get_smallest_unsigned_int_type(g, field_count - 1);
+        }
+
+        tag_type = new_type_table_entry(TypeTableEntryIdEnum);
+        buf_resize(&tag_type->name, 0);
+        buf_appendf(&tag_type->name, "@EnumTagType(%s)", buf_ptr(&union_type->name));
+        tag_type->is_copyable = true;
+        tag_type->type_ref = tag_int_type->type_ref;
+        tag_type->zero_bits = tag_int_type->zero_bits;
+
+        tag_type->data.enumeration.tag_int_type = tag_int_type;
+        tag_type->data.enumeration.zero_bits_known = true;
+        tag_type->data.enumeration.decl_node = decl_node;
+        tag_type->data.enumeration.layout = ContainerLayoutAuto;
+        tag_type->data.enumeration.src_field_count = field_count;
+        tag_type->data.enumeration.fields = allocate<TypeEnumField>(field_count);
+        tag_type->data.enumeration.decls_scope = union_type->data.unionation.decls_scope;
+        tag_type->data.enumeration.complete = true;
+    } else if (enum_type_node != nullptr) {
+        TypeTableEntry *enum_type = analyze_type_expr(g, scope, enum_type_node);
+        if (type_is_invalid(enum_type)) {
+            union_type->data.unionation.is_invalid = true;
+            union_type->data.unionation.embedded_in_current = false;
+            return;
+        }
+        if (enum_type->id != TypeTableEntryIdEnum) {
+            union_type->data.unionation.is_invalid = true;
+            union_type->data.unionation.embedded_in_current = false;
+            add_node_error(g, enum_type_node,
+                buf_sprintf("expected enum tag type, found '%s'", buf_ptr(&enum_type->name)));
+            return;
+        }
+        tag_type = enum_type;
+        covered_enum_fields = allocate<bool>(enum_type->data.enumeration.src_field_count);
+    } else {
+        tag_type = nullptr;
+    }
+    union_type->data.unionation.tag_type = tag_type;
+
     for (uint32_t i = 0; i < field_count; i += 1) {
         AstNode *field_node = decl_node->data.container_decl.fields.at(i);
-        TypeUnionField *type_union_field = &union_type->data.unionation.fields[i];
-        TypeTableEntry *field_type = type_union_field->type_entry;
+        TypeUnionField *union_field = &union_type->data.unionation.fields[i];
+        Buf *field_name = field_node->data.struct_field.name;
+        TypeTableEntry *field_type = union_field->type_entry;
 
         ensure_complete_type(g, field_type);
         if (type_is_invalid(field_type)) {
@@ -1891,19 +1799,68 @@ static void resolve_union_type(CodeGen *g, TypeTableEntry *union_type) {
             continue;
         }
 
+        if (create_enum_type) {
+            di_enumerators[i] = ZigLLVMCreateDebugEnumerator(g->dbuilder, buf_ptr(field_name), i);
+            union_field->enum_field = &tag_type->data.enumeration.fields[i];
+            union_field->enum_field->name = field_name;
+            union_field->enum_field->decl_index = i;
+
+            AstNode *tag_value = field_node->data.struct_field.value;
+            // In this first pass we resolve explicit tag values.
+            // In a second pass we will fill in the unspecified ones.
+            if (tag_value != nullptr) {
+                TypeTableEntry *tag_int_type = tag_type->data.enumeration.tag_int_type;
+                IrInstruction *result_inst = analyze_const_value(g, scope, tag_value, tag_int_type, nullptr);
+                if (result_inst->value.type->id == TypeTableEntryIdInvalid) {
+                    union_type->data.unionation.is_invalid = true;
+                    continue;
+                }
+                assert(result_inst->value.special != ConstValSpecialRuntime);
+                assert(result_inst->value.type->id == TypeTableEntryIdInt);
+                auto entry = occupied_tag_values.put_unique(result_inst->value.data.x_bigint, tag_value);
+                if (entry == nullptr) {
+                    bigint_init_bigint(&union_field->enum_field->value, &result_inst->value.data.x_bigint);
+                } else {
+                    Buf *val_buf = buf_alloc();
+                    bigint_append_buf(val_buf, &result_inst->value.data.x_bigint, 10);
+
+                    ErrorMsg *msg = add_node_error(g, tag_value,
+                            buf_sprintf("enum tag value %s already taken", buf_ptr(val_buf)));
+                    add_error_note(g, msg, entry->value,
+                            buf_sprintf("other occurrence here"));
+                    union_type->data.unionation.is_invalid = true;
+                    continue;
+                }
+            }
+        } else if (enum_type_node != nullptr) {
+            union_field->enum_field = find_enum_type_field(tag_type, field_name);
+            if (union_field->enum_field == nullptr) {
+                ErrorMsg *msg = add_node_error(g, field_node,
+                    buf_sprintf("enum field not found: '%s'", buf_ptr(field_name)));
+                add_error_note(g, msg, tag_type->data.enumeration.decl_node,
+                        buf_sprintf("enum declared here"));
+                union_type->data.unionation.is_invalid = true;
+                continue;
+            }
+            covered_enum_fields[union_field->enum_field->decl_index] = true;
+        } else {
+            union_field->enum_field = allocate<TypeEnumField>(1);
+            union_field->enum_field->name = field_name;
+            union_field->enum_field->decl_index = i;
+            bigint_init_unsigned(&union_field->enum_field->value, i);
+        }
+
         if (!type_has_bits(field_type))
             continue;
 
-        di_enumerators[i] = ZigLLVMCreateDebugEnumerator(g->dbuilder, buf_ptr(type_union_field->name), i);
-
         uint64_t store_size_in_bits = 8*LLVMStoreSizeOfType(g->target_data_ref, field_type->type_ref);
         uint64_t abi_align_in_bits = 8*LLVMABIAlignmentOfType(g->target_data_ref, field_type->type_ref);
 
         assert(store_size_in_bits > 0);
         assert(abi_align_in_bits > 0);
 
-        union_inner_di_types[type_union_field->gen_index] = ZigLLVMCreateDebugMemberType(g->dbuilder,
-                ZigLLVMTypeToScope(union_type->di_type), buf_ptr(type_union_field->name),
+        union_inner_di_types[union_field->gen_index] = ZigLLVMCreateDebugMemberType(g->dbuilder,
+                ZigLLVMTypeToScope(union_type->di_type), buf_ptr(union_field->enum_field->name),
                 import->di_file, (unsigned)(field_node->line + 1),
                 store_size_in_bits,
                 abi_align_in_bits,
@@ -1919,6 +1876,49 @@ static void resolve_union_type(CodeGen *g, TypeTableEntry *union_type) {
         }
     }
 
+    if (create_enum_type) {
+        // Now iterate again and populate the unspecified tag values
+        uint32_t next_maybe_unoccupied_index = 0;
+
+        for (uint32_t field_i = 0; field_i < field_count; field_i += 1) {
+            AstNode *field_node = decl_node->data.container_decl.fields.at(field_i);
+            TypeUnionField *union_field = &union_type->data.unionation.fields[field_i];
+            AstNode *tag_value = field_node->data.struct_field.value;
+
+            if (tag_value == nullptr) {
+                if (occupied_tag_values.size() == 0) {
+                    bigint_init_unsigned(&union_field->enum_field->value, next_maybe_unoccupied_index);
+                    next_maybe_unoccupied_index += 1;
+                } else {
+                    BigInt proposed_value;
+                    for (;;) {
+                        bigint_init_unsigned(&proposed_value, next_maybe_unoccupied_index);
+                        next_maybe_unoccupied_index += 1;
+                        auto entry = occupied_tag_values.put_unique(proposed_value, field_node);
+                        if (entry != nullptr) {
+                            continue;
+                        }
+                        break;
+                    }
+                    bigint_init_bigint(&union_field->enum_field->value, &proposed_value);
+                }
+            }
+        }
+    } else if (enum_type_node != nullptr) {
+        for (uint32_t i = 0; i < tag_type->data.enumeration.src_field_count; i += 1) {
+            TypeEnumField *enum_field = &tag_type->data.enumeration.fields[i];
+            if (!covered_enum_fields[i]) {
+                AstNode *enum_decl_node = tag_type->data.enumeration.decl_node;
+                AstNode *field_node = enum_decl_node->data.container_decl.fields.at(i);
+                ErrorMsg *msg = add_node_error(g, decl_node,
+                    buf_sprintf("enum field missing: '%s'", buf_ptr(enum_field->name)));
+                add_error_note(g, msg, field_node,
+                        buf_sprintf("declared here"));
+                union_type->data.unionation.is_invalid = true;
+            }
+        }
+    }
+
     // unset temporary flag
     union_type->data.unionation.embedded_in_current = false;
     union_type->data.unionation.complete = true;
@@ -1950,11 +1950,9 @@ static void resolve_union_type(CodeGen *g, TypeTableEntry *union_type) {
 
     assert(most_aligned_union_member != nullptr);
 
-    bool want_safety = auto_layout && (field_count >= 2);
     uint64_t padding_in_bits = biggest_size_in_bits - size_of_most_aligned_member_in_bits;
 
-
-    if (!want_safety) {
+    if (tag_type == nullptr) {
         if (padding_in_bits > 0) {
             TypeTableEntry *u8_type = get_int_type(g, false, 8);
             TypeTableEntry *padding_array = get_array_type(g, u8_type, padding_in_bits / 8);
@@ -1994,6 +1992,8 @@ static void resolve_union_type(CodeGen *g, TypeTableEntry *union_type) {
             padding_array->type_ref,
         };
         union_type_ref = LLVMStructType(union_element_types, 2, false);
+    } else if (most_aligned_union_member == nullptr) {
+        zig_panic("TODO zero bit payload");
     } else {
         union_type_ref = most_aligned_union_member->type_ref;
     }
@@ -2003,9 +2003,7 @@ static void resolve_union_type(CodeGen *g, TypeTableEntry *union_type) {
     assert(8*LLVMStoreSizeOfType(g->target_data_ref, union_type_ref) >= biggest_size_in_bits);
 
     // create llvm type for root struct
-    TypeTableEntry *tag_int_type = get_smallest_unsigned_int_type(g, field_count - 1);
-    TypeTableEntry *tag_type_entry = tag_int_type;
-    union_type->data.unionation.tag_type = tag_type_entry;
+    TypeTableEntry *tag_int_type = tag_type->data.enumeration.tag_int_type;
     uint64_t align_of_tag_in_bits = 8*LLVMABIAlignmentOfType(g->target_data_ref, tag_int_type->type_ref);
 
     if (align_of_tag_in_bits >= biggest_align_in_bits) {
@@ -2017,21 +2015,24 @@ static void resolve_union_type(CodeGen *g, TypeTableEntry *union_type) {
     }
 
     LLVMTypeRef root_struct_element_types[2];
-    root_struct_element_types[union_type->data.unionation.gen_tag_index] = tag_type_entry->type_ref;
+    root_struct_element_types[union_type->data.unionation.gen_tag_index] = tag_type->type_ref;
     root_struct_element_types[union_type->data.unionation.gen_union_index] = union_type_ref;
     LLVMStructSetBody(union_type->type_ref, root_struct_element_types, 2, false);
 
 
     // create debug type for root struct
 
-    // create debug type for tag
-    uint64_t tag_debug_size_in_bits = 8*LLVMStoreSizeOfType(g->target_data_ref, tag_type_entry->type_ref);
-    uint64_t tag_debug_align_in_bits = 8*LLVMABIAlignmentOfType(g->target_data_ref, tag_type_entry->type_ref);
-    ZigLLVMDIType *tag_di_type = ZigLLVMCreateDebugEnumerationType(g->dbuilder,
-            ZigLLVMTypeToScope(union_type->di_type), "AnonEnum",
-            import->di_file, (unsigned)(decl_node->line + 1),
-            tag_debug_size_in_bits, tag_debug_align_in_bits, di_enumerators, field_count,
-            tag_type_entry->di_type, "");
+    uint64_t tag_debug_size_in_bits = 8*LLVMStoreSizeOfType(g->target_data_ref, tag_type->type_ref);
+    uint64_t tag_debug_align_in_bits = 8*LLVMABIAlignmentOfType(g->target_data_ref, tag_type->type_ref);
+    if (create_enum_type) {
+        // create debug type for tag
+        ZigLLVMDIType *tag_di_type = ZigLLVMCreateDebugEnumerationType(g->dbuilder,
+                ZigLLVMTypeToScope(union_type->di_type), "AnonEnum",
+                import->di_file, (unsigned)(decl_node->line + 1),
+                tag_debug_size_in_bits, tag_debug_align_in_bits, di_enumerators, field_count,
+                tag_type->di_type, "");
+        tag_type->di_type = tag_di_type;
+    }
 
     // create debug type for union
     ZigLLVMDIType *union_di_type = ZigLLVMCreateDebugUnionType(g->dbuilder,
@@ -2046,19 +2047,19 @@ static void resolve_union_type(CodeGen *g, TypeTableEntry *union_type) {
             union_type->data.unionation.gen_tag_index);
 
     ZigLLVMDIType *union_member_di_type = ZigLLVMCreateDebugMemberType(g->dbuilder,
-            ZigLLVMTypeToScope(union_type->di_type), "union_field",
+            ZigLLVMTypeToScope(union_type->di_type), "payload",
             import->di_file, (unsigned)(decl_node->line + 1),
             biggest_size_in_bits,
             biggest_align_in_bits,
             union_offset_in_bits,
             0, union_di_type);
     ZigLLVMDIType *tag_member_di_type = ZigLLVMCreateDebugMemberType(g->dbuilder,
-            ZigLLVMTypeToScope(union_type->di_type), "tag_field",
+            ZigLLVMTypeToScope(union_type->di_type), "tag",
             import->di_file, (unsigned)(decl_node->line + 1),
             tag_debug_size_in_bits,
             tag_debug_align_in_bits,
             tag_offset_in_bits,
-            0, tag_di_type);
+            0, tag_type->di_type);
 
     ZigLLVMDIType *di_root_members[2];
     di_root_members[union_type->data.unionation.gen_tag_index] = tag_member_di_type;
@@ -2104,7 +2105,6 @@ static void resolve_enum_zero_bits(CodeGen *g, TypeTableEntry *enum_type) {
         enum_type->data.enumeration.fields = nullptr;
         enum_type->data.enumeration.is_invalid = true;
         enum_type->data.enumeration.zero_bits_loop_flag = false;
-        enum_type->data.enumeration.gen_field_count = 0;
         enum_type->data.enumeration.zero_bits_known = true;
         return;
     }
@@ -2112,8 +2112,6 @@ static void resolve_enum_zero_bits(CodeGen *g, TypeTableEntry *enum_type) {
     enum_type->data.enumeration.src_field_count = field_count;
     enum_type->data.enumeration.fields = allocate<TypeEnumField>(field_count);
 
-    uint32_t biggest_align_bytes = 0;
-
     Scope *scope = &enum_type->data.enumeration.decls_scope->base;
 
     HashMap<BigInt, AstNode *, bigint_hash, bigint_eql> occupied_tag_values = {};
@@ -2143,14 +2141,20 @@ static void resolve_enum_zero_bits(CodeGen *g, TypeTableEntry *enum_type) {
         }
     }
     enum_type->data.enumeration.tag_int_type = tag_int_type;
+    enum_type->type_ref = tag_int_type->type_ref;
 
-    uint32_t gen_field_index = 0;
     for (uint32_t field_i = 0; field_i < field_count; field_i += 1) {
         AstNode *field_node = decl_node->data.container_decl.fields.at(field_i);
         TypeEnumField *type_enum_field = &enum_type->data.enumeration.fields[field_i];
         type_enum_field->name = field_node->data.struct_field.name;
-        TypeTableEntry *field_type = analyze_type_expr(g, scope, field_node->data.struct_field.type);
-        type_enum_field->type_entry = field_type;
+        type_enum_field->decl_index = field_i;
+
+        if (field_node->data.struct_field.type != nullptr) {
+            ErrorMsg *msg = add_node_error(g, field_node->data.struct_field.type,
+                buf_sprintf("structs and unions, not enums, support field types"));
+            add_error_note(g, msg, decl_node,
+                    buf_sprintf("consider 'union(enum)' here"));
+        }
 
         AstNode *tag_value = field_node->data.struct_field.value;
 
@@ -2179,23 +2183,6 @@ static void resolve_enum_zero_bits(CodeGen *g, TypeTableEntry *enum_type) {
                 continue;
             }
         }
-
-        type_ensure_zero_bits_known(g, field_type);
-        if (type_is_invalid(field_type)) {
-            enum_type->data.enumeration.is_invalid = true;
-            continue;
-        }
-
-        if (!type_has_bits(field_type))
-            continue;
-
-        type_enum_field->gen_index = gen_field_index;
-        gen_field_index += 1;
-
-        uint32_t field_align_bytes = get_abi_alignment(g, field_type);
-        if (field_align_bytes > biggest_align_bytes) {
-            biggest_align_bytes = field_align_bytes;
-        }
     }
 
     // Now iterate again and populate the unspecified tag values
@@ -2227,15 +2214,8 @@ static void resolve_enum_zero_bits(CodeGen *g, TypeTableEntry *enum_type) {
     }
 
     enum_type->data.enumeration.zero_bits_loop_flag = false;
-    enum_type->data.enumeration.gen_field_count = gen_field_index;
-    enum_type->zero_bits = (gen_field_index == 0 && field_count < 2);
+    enum_type->zero_bits = (field_count < 2);
     enum_type->data.enumeration.zero_bits_known = true;
-
-    // also compute abi_alignment
-    if (!enum_type->zero_bits) {
-        uint32_t align_of_tag_in_bytes = LLVMABIAlignmentOfType(g->target_data_ref, tag_int_type->type_ref);
-        enum_type->data.enumeration.abi_alignment = max(align_of_tag_in_bytes, biggest_align_bytes);
-    }
 }
 
 static void resolve_struct_zero_bits(CodeGen *g, TypeTableEntry *struct_type) {
@@ -2279,6 +2259,13 @@ static void resolve_struct_zero_bits(CodeGen *g, TypeTableEntry *struct_type) {
         AstNode *field_node = decl_node->data.container_decl.fields.at(i);
         TypeStructField *type_struct_field = &struct_type->data.structure.fields[i];
         type_struct_field->name = field_node->data.struct_field.name;
+
+        if (field_node->data.struct_field.type == nullptr) {
+            add_node_error(g, field_node, buf_sprintf("struct field missing type"));
+            struct_type->data.structure.is_invalid = true;
+            continue;
+        }
+
         TypeTableEntry *field_type = analyze_type_expr(g, scope, field_node->data.struct_field.type);
         type_struct_field->type_entry = field_type;
         type_struct_field->src_index = i;
@@ -2338,6 +2325,16 @@ static void resolve_union_zero_bits(CodeGen *g, TypeTableEntry *union_type) {
 
     assert(!union_type->data.unionation.fields);
     uint32_t field_count = (uint32_t)decl_node->data.container_decl.fields.length;
+    if (field_count == 0) {
+        add_node_error(g, decl_node, buf_sprintf("unions must have 1 or more fields"));
+
+        union_type->data.unionation.src_field_count = field_count;
+        union_type->data.unionation.fields = nullptr;
+        union_type->data.unionation.is_invalid = true;
+        union_type->data.unionation.zero_bits_loop_flag = false;
+        union_type->data.unionation.zero_bits_known = true;
+        return;
+    }
     union_type->data.unionation.src_field_count = field_count;
     union_type->data.unionation.fields = allocate<TypeUnionField>(field_count);
 
@@ -2348,17 +2345,23 @@ static void resolve_union_zero_bits(CodeGen *g, TypeTableEntry *union_type) {
     uint32_t gen_field_index = 0;
     for (uint32_t i = 0; i < field_count; i += 1) {
         AstNode *field_node = decl_node->data.container_decl.fields.at(i);
-        TypeUnionField *type_union_field = &union_type->data.unionation.fields[i];
-        type_union_field->name = field_node->data.struct_field.name;
+        TypeUnionField *union_field = &union_type->data.unionation.fields[i];
+        union_field->name = field_node->data.struct_field.name;
+
+        if (field_node->data.struct_field.type == nullptr) {
+            add_node_error(g, field_node, buf_sprintf("union field missing type"));
+            union_type->data.unionation.is_invalid = true;
+            continue;
+        }
+
         TypeTableEntry *field_type = analyze_type_expr(g, scope, field_node->data.struct_field.type);
-        type_union_field->type_entry = field_type;
+        union_field->type_entry = field_type;
 
-        // TODO look for enum arg to union
-        bigint_init_unsigned(&type_union_field->value, i);
-
-        if (field_node->data.struct_field.value != nullptr) {
-            add_node_error(g, field_node->data.struct_field.value,
-                    buf_sprintf("enums, not unions, support field assignment"));
+        if (field_node->data.struct_field.value != nullptr && !decl_node->data.container_decl.auto_enum) {
+            ErrorMsg *msg = add_node_error(g, field_node->data.struct_field.value,
+                    buf_sprintf("non-enum union field assignment"));
+            add_error_note(g, msg, decl_node,
+                    buf_sprintf("consider 'union(enum)' here"));
         }
 
         type_ensure_zero_bits_known(g, field_type);
@@ -2370,7 +2373,7 @@ static void resolve_union_zero_bits(CodeGen *g, TypeTableEntry *union_type) {
         if (!type_has_bits(field_type))
             continue;
 
-        type_union_field->gen_index = gen_field_index;
+        union_field->gen_index = gen_field_index;
         gen_field_index += 1;
 
         uint32_t field_align_bytes = get_abi_alignment(g, field_type);
@@ -2379,11 +2382,32 @@ static void resolve_union_zero_bits(CodeGen *g, TypeTableEntry *union_type) {
         }
     }
 
-    bool auto_layout = (union_type->data.unionation.layout == ContainerLayoutAuto);
+    bool src_have_tag = decl_node->data.container_decl.auto_enum ||
+        decl_node->data.container_decl.init_arg_expr != nullptr;
+
+    if (src_have_tag && union_type->data.unionation.layout != ContainerLayoutAuto) {
+        const char *qual_str;
+        switch (union_type->data.unionation.layout) {
+            case ContainerLayoutAuto:
+                zig_unreachable();
+            case ContainerLayoutPacked:
+                qual_str = "packed";
+                break;
+            case ContainerLayoutExtern:
+                qual_str = "extern";
+                break;
+        }
+        AstNode *source_node = (decl_node->data.container_decl.init_arg_expr != nullptr) ? 
+            decl_node->data.container_decl.init_arg_expr : decl_node;
+        add_node_error(g, source_node,
+            buf_sprintf("%s union does not support enum tag type", qual_str));
+        union_type->data.unionation.is_invalid = true;
+        return;
+    }
 
     union_type->data.unionation.zero_bits_loop_flag = false;
     union_type->data.unionation.gen_field_count = gen_field_index;
-    union_type->zero_bits = (gen_field_index == 0 && (field_count < 2 || !auto_layout));
+    union_type->zero_bits = (gen_field_index == 0 && (field_count < 2 || !src_have_tag));
     union_type->data.unionation.zero_bits_known = true;
 
     // also compute abi_alignment
@@ -2848,7 +2872,6 @@ TypeTableEntry *validate_var_type(CodeGen *g, AstNode *source_node, TypeTableEnt
         case TypeTableEntryIdUnion:
         case TypeTableEntryIdFn:
         case TypeTableEntryIdBoundFn:
-        case TypeTableEntryIdEnumTag:
             return type_entry;
     }
     zig_unreachable();
@@ -3265,19 +3288,20 @@ TypeUnionField *find_union_type_field(TypeTableEntry *type_entry, Buf *name) {
     assert(type_entry->data.unionation.complete);
     for (uint32_t i = 0; i < type_entry->data.unionation.src_field_count; i += 1) {
         TypeUnionField *field = &type_entry->data.unionation.fields[i];
-        if (buf_eql_buf(field->name, name)) {
+        if (buf_eql_buf(field->enum_field->name, name)) {
             return field;
         }
     }
     return nullptr;
 }
 
-static TypeUnionField *find_union_field_by_tag(TypeTableEntry *type_entry, const BigInt *tag) {
+TypeUnionField *find_union_field_by_tag(TypeTableEntry *type_entry, const BigInt *tag) {
     assert(type_entry->id == TypeTableEntryIdUnion);
     assert(type_entry->data.unionation.complete);
+    assert(type_entry->data.unionation.gen_tag_index != SIZE_MAX);
     for (uint32_t i = 0; i < type_entry->data.unionation.src_field_count; i += 1) {
         TypeUnionField *field = &type_entry->data.unionation.fields[i];
-        if (bigint_cmp(&field->value, tag) == CmpEQ) {
+        if (bigint_cmp(&field->enum_field->value, tag) == CmpEQ) {
             return field;
         }
     }
@@ -3323,7 +3347,6 @@ static bool is_container(TypeTableEntry *type_entry) {
         case TypeTableEntryIdNamespace:
         case TypeTableEntryIdBlock:
         case TypeTableEntryIdBoundFn:
-        case TypeTableEntryIdEnumTag:
         case TypeTableEntryIdArgTuple:
         case TypeTableEntryIdOpaque:
             return false;
@@ -3374,7 +3397,6 @@ void resolve_container_type(CodeGen *g, TypeTableEntry *type_entry) {
         case TypeTableEntryIdBoundFn:
         case TypeTableEntryIdInvalid:
         case TypeTableEntryIdVar:
-        case TypeTableEntryIdEnumTag:
         case TypeTableEntryIdArgTuple:
         case TypeTableEntryIdOpaque:
             zig_unreachable();
@@ -3828,7 +3850,7 @@ bool handle_is_ptr(TypeTableEntry *type_entry) {
         case TypeTableEntryIdPointer:
         case TypeTableEntryIdPureError:
         case TypeTableEntryIdFn:
-        case TypeTableEntryIdEnumTag:
+        case TypeTableEntryIdEnum:
              return false;
         case TypeTableEntryIdArray:
         case TypeTableEntryIdStruct:
@@ -3836,9 +3858,6 @@ bool handle_is_ptr(TypeTableEntry *type_entry) {
              return type_has_bits(type_entry);
         case TypeTableEntryIdErrorUnion:
              return type_has_bits(type_entry->data.error.child_type);
-        case TypeTableEntryIdEnum:
-             assert(type_entry->data.enumeration.complete);
-             return type_entry->data.enumeration.gen_field_count != 0;
         case TypeTableEntryIdMaybe:
              return type_has_bits(type_entry->data.maybe.child_type) &&
                     type_entry->data.maybe.child_type->id != TypeTableEntryIdPointer &&
@@ -3980,7 +3999,6 @@ static uint32_t hash_const_val(ConstExprValue *const_val) {
             return (uint32_t)4149439618;
         case TypeTableEntryIdInt:
         case TypeTableEntryIdNumLitInt:
-        case TypeTableEntryIdEnumTag:
             {
                 uint32_t result = 1331471175;
                 for (size_t i = 0; i < const_val->data.x_bigint.digit_count; i += 1) {
@@ -3989,6 +4007,15 @@ static uint32_t hash_const_val(ConstExprValue *const_val) {
                 }
                 return result;
             }
+        case TypeTableEntryIdEnum:
+            {
+                uint32_t result = 31643936;
+                for (size_t i = 0; i < const_val->data.x_enum_tag.digit_count; i += 1) {
+                    uint64_t digit = bigint_ptr(&const_val->data.x_enum_tag)[i];
+                    result ^= ((uint32_t)(digit >> 32)) ^ (uint32_t)(result);
+                }
+                return result;
+            }
         case TypeTableEntryIdFloat:
             switch (const_val->type->data.floating.bit_count) {
                 case 32:
@@ -4089,9 +4116,6 @@ static uint32_t hash_const_val(ConstExprValue *const_val) {
         case TypeTableEntryIdPureError:
             // TODO better hashing algorithm
             return 2630160122;
-        case TypeTableEntryIdEnum:
-            // TODO better hashing algorithm
-            return 31643936;
         case TypeTableEntryIdFn:
             return 4133894920 ^ hash_ptr(const_val->data.x_fn.fn_entry);
         case TypeTableEntryIdNamespace:
@@ -4224,7 +4248,6 @@ bool type_requires_comptime(TypeTableEntry *type_entry) {
         case TypeTableEntryIdInt:
         case TypeTableEntryIdFloat:
         case TypeTableEntryIdPointer:
-        case TypeTableEntryIdEnumTag:
         case TypeTableEntryIdVoid:
         case TypeTableEntryIdUnreachable:
             return false;
@@ -4295,6 +4318,7 @@ ConstExprValue *create_const_bigint(TypeTableEntry *type, const BigInt *bigint)
     return const_val;
 }
 
+
 void init_const_unsigned_negative(ConstExprValue *const_val, TypeTableEntry *type, uint64_t x, bool negative) {
     const_val->special = ConstValSpecialStatic;
     const_val->type = type;
@@ -4358,18 +4382,19 @@ ConstExprValue *create_const_float(TypeTableEntry *type, double value) {
     return const_val;
 }
 
-void init_const_enum_tag(ConstExprValue *const_val, TypeTableEntry *type, const BigInt *tag) {
+void init_const_enum(ConstExprValue *const_val, TypeTableEntry *type, const BigInt *tag) {
     const_val->special = ConstValSpecialStatic;
     const_val->type = type;
-    bigint_init_bigint(&const_val->data.x_enum.tag, tag);
+    bigint_init_bigint(&const_val->data.x_enum_tag, tag);
 }
 
-ConstExprValue *create_const_enum_tag(TypeTableEntry *type, const BigInt *tag) {
+ConstExprValue *create_const_enum(TypeTableEntry *type, const BigInt *tag) {
     ConstExprValue *const_val = create_const_vals(1);
-    init_const_enum_tag(const_val, type, tag);
+    init_const_enum(const_val, type, tag);
     return const_val;
 }
 
+
 void init_const_bool(CodeGen *g, ConstExprValue *const_val, bool value) {
     const_val->special = ConstValSpecialStatic;
     const_val->type = g->builtin_types.entry_bool;
@@ -4567,20 +4592,8 @@ bool const_values_equal(ConstExprValue *a, ConstExprValue *b) {
     switch (a->type->id) {
         case TypeTableEntryIdOpaque:
             zig_unreachable();
-        case TypeTableEntryIdEnum: {
-            ConstEnumValue *enum1 = &a->data.x_enum;
-            ConstEnumValue *enum2 = &b->data.x_enum;
-            if (bigint_cmp(&enum1->tag, &enum2->tag) == CmpEQ) {
-                TypeEnumField *field = find_enum_field_by_tag(a->type, &enum1->tag);
-                assert(field != nullptr);
-                if (type_has_bits(field->type_entry)) {
-                    zig_panic("TODO const expr analyze enum field value for equality");
-                } else {
-                    return true;
-                }
-            }
-            return false;
-        }
+        case TypeTableEntryIdEnum:
+            return bigint_cmp(&a->data.x_enum_tag, &b->data.x_enum_tag) == CmpEQ;
         case TypeTableEntryIdUnion: {
             ConstUnionValue *union1 = &a->data.x_union;
             ConstUnionValue *union2 = &b->data.x_union;
@@ -4622,7 +4635,6 @@ bool const_values_equal(ConstExprValue *a, ConstExprValue *b) {
             return bigfloat_cmp(&a->data.x_bigfloat, &b->data.x_bigfloat) == CmpEQ;
         case TypeTableEntryIdInt:
         case TypeTableEntryIdNumLitInt:
-        case TypeTableEntryIdEnumTag:
             return bigint_cmp(&a->data.x_bigint, &b->data.x_bigint) == CmpEQ;
         case TypeTableEntryIdPointer:
             if (a->data.x_ptr.special != b->data.x_ptr.special)
@@ -4949,7 +4961,8 @@ void render_const_value(CodeGen *g, Buf *buf, ConstExprValue *const_val) {
             }
         case TypeTableEntryIdEnum:
             {
-                buf_appendf(buf, "(enum %s constant)", buf_ptr(&type_entry->name));
+                TypeEnumField *field = find_enum_field_by_tag(type_entry, &const_val->data.x_enum_tag);
+                buf_appendf(buf, "%s.%s", buf_ptr(&type_entry->name), buf_ptr(field->name));
                 return;
             }
         case TypeTableEntryIdErrorUnion:
@@ -4967,14 +4980,6 @@ void render_const_value(CodeGen *g, Buf *buf, ConstExprValue *const_val) {
                 buf_appendf(buf, "(pure error constant)");
                 return;
             }
-        case TypeTableEntryIdEnumTag:
-            {
-                TypeTableEntry *enum_type = type_entry->data.enum_tag.enum_type;
-                size_t field_index = bigint_as_unsigned(&const_val->data.x_bigint);
-                TypeEnumField *field = &enum_type->data.enumeration.fields[field_index];
-                buf_appendf(buf, "%s.%s", buf_ptr(&enum_type->name), buf_ptr(field->name));
-                return;
-            }
         case TypeTableEntryIdArgTuple:
             {
                 buf_appendf(buf, "(args value)");
@@ -5036,7 +5041,6 @@ uint32_t type_id_hash(TypeId x) {
         case TypeTableEntryIdErrorUnion:
         case TypeTableEntryIdPureError:
         case TypeTableEntryIdEnum:
-        case TypeTableEntryIdEnumTag:
         case TypeTableEntryIdUnion:
         case TypeTableEntryIdFn:
         case TypeTableEntryIdNamespace:
@@ -5081,7 +5085,6 @@ bool type_id_eql(TypeId a, TypeId b) {
         case TypeTableEntryIdErrorUnion:
         case TypeTableEntryIdPureError:
         case TypeTableEntryIdEnum:
-        case TypeTableEntryIdEnumTag:
         case TypeTableEntryIdUnion:
         case TypeTableEntryIdFn:
         case TypeTableEntryIdNamespace:
@@ -5196,7 +5199,6 @@ static const TypeTableEntryId all_type_ids[] = {
     TypeTableEntryIdErrorUnion,
     TypeTableEntryIdPureError,
     TypeTableEntryIdEnum,
-    TypeTableEntryIdEnumTag,
     TypeTableEntryIdUnion,
     TypeTableEntryIdFn,
     TypeTableEntryIdNamespace,
@@ -5254,22 +5256,20 @@ size_t type_id_index(TypeTableEntryId id) {
             return 15;
         case TypeTableEntryIdEnum:
             return 16;
-        case TypeTableEntryIdEnumTag:
-            return 17;
         case TypeTableEntryIdUnion:
-            return 18;
+            return 17;
         case TypeTableEntryIdFn:
-            return 19;
+            return 18;
         case TypeTableEntryIdNamespace:
-            return 20;
+            return 19;
         case TypeTableEntryIdBlock:
-            return 21;
+            return 20;
         case TypeTableEntryIdBoundFn:
-            return 22;
+            return 21;
         case TypeTableEntryIdArgTuple:
-            return 23;
+            return 22;
         case TypeTableEntryIdOpaque:
-            return 24;
+            return 23;
     }
     zig_unreachable();
 }
@@ -5313,8 +5313,6 @@ const char *type_id_name(TypeTableEntryId id) {
             return "Error";
         case TypeTableEntryIdEnum:
             return "Enum";
-        case TypeTableEntryIdEnumTag:
-            return "EnumTag";
         case TypeTableEntryIdUnion:
             return "Union";
         case TypeTableEntryIdFn:
@@ -5381,9 +5379,6 @@ uint32_t get_abi_alignment(CodeGen *g, TypeTableEntry *type_entry) {
     if (type_entry->id == TypeTableEntryIdStruct) {
         assert(type_entry->data.structure.abi_alignment != 0);
         return type_entry->data.structure.abi_alignment;
-    } else if (type_entry->id == TypeTableEntryIdEnum) {
-        assert(type_entry->data.enumeration.abi_alignment != 0);
-        return type_entry->data.enumeration.abi_alignment;
     } else if (type_entry->id == TypeTableEntryIdUnion) {
         assert(type_entry->data.unionation.abi_alignment != 0);
         return type_entry->data.unionation.abi_alignment;
diff --git a/src/analyze.hpp b/src/analyze.hpp
index 50eb2a800f..e6100c692c 100644
--- a/src/analyze.hpp
+++ b/src/analyze.hpp
@@ -65,6 +65,7 @@ ScopeDecls *get_container_scope(TypeTableEntry *type_entry);
 TypeEnumField *find_enum_type_field(TypeTableEntry *enum_type, Buf *name);
 TypeUnionField *find_union_type_field(TypeTableEntry *type_entry, Buf *name);
 TypeEnumField *find_enum_field_by_tag(TypeTableEntry *enum_type, const BigInt *tag);
+TypeUnionField *find_union_field_by_tag(TypeTableEntry *type_entry, const BigInt *tag);
 
 bool is_container_ref(TypeTableEntry *type_entry);
 void scan_decls(CodeGen *g, ScopeDecls *decls_scope, AstNode *node);
@@ -126,8 +127,8 @@ ConstExprValue *create_const_usize(CodeGen *g, uint64_t x);
 void init_const_float(ConstExprValue *const_val, TypeTableEntry *type, double value);
 ConstExprValue *create_const_float(TypeTableEntry *type, double value);
 
-void init_const_enum_tag(ConstExprValue *const_val, TypeTableEntry *type, const BigInt *tag);
-ConstExprValue *create_const_enum_tag(TypeTableEntry *type, const BigInt *tag);
+void init_const_enum(ConstExprValue *const_val, TypeTableEntry *type, const BigInt *tag);
+ConstExprValue *create_const_enum(TypeTableEntry *type, const BigInt *tag);
 
 void init_const_bool(CodeGen *g, ConstExprValue *const_val, bool value);
 ConstExprValue *create_const_bool(CodeGen *g, bool value);
@@ -163,7 +164,6 @@ ConstExprValue *create_const_vals(size_t count);
 
 TypeTableEntry *make_int_type(CodeGen *g, bool is_signed, uint32_t size_in_bits);
 ConstParent *get_const_val_parent(CodeGen *g, ConstExprValue *value);
-TypeTableEntry *create_enum_tag_type(CodeGen *g, TypeTableEntry *enum_type, TypeTableEntry *int_type);
 void expand_undef_array(CodeGen *g, ConstExprValue *const_val);
 void update_compile_var(CodeGen *g, Buf *name, ConstExprValue *value);
 
diff --git a/src/ast_render.cpp b/src/ast_render.cpp
index cdc18c7252..4f4dc1decd 100644
--- a/src/ast_render.cpp
+++ b/src/ast_render.cpp
@@ -661,11 +661,18 @@ static void render_node_extra(AstRender *ar, AstNode *node, bool grouped) {
                 const char *layout_str = layout_string(node->data.container_decl.layout);
                 const char *container_str = container_string(node->data.container_decl.kind);
                 fprintf(ar->f, "%s%s", layout_str, container_str);
+                if (node->data.container_decl.auto_enum) {
+                    fprintf(ar->f, "(enum");
+                }
                 if (node->data.container_decl.init_arg_expr != nullptr) {
                     fprintf(ar->f, "(");
                     render_node_grouped(ar, node->data.container_decl.init_arg_expr);
                     fprintf(ar->f, ")");
                 }
+                if (node->data.container_decl.auto_enum) {
+                    fprintf(ar->f, ")");
+                }
+
                 fprintf(ar->f, " {\n");
                 ar->indent += ar->indent_size;
                 for (size_t field_i = 0; field_i < node->data.container_decl.fields.length; field_i += 1) {
diff --git a/src/codegen.cpp b/src/codegen.cpp
index 0ac8ffb7e1..dbf4f8522b 100644
--- a/src/codegen.cpp
+++ b/src/codegen.cpp
@@ -1631,12 +1631,8 @@ static LLVMValueRef ir_render_bin_op(CodeGen *g, IrExecutable *executable,
                 LLVMIntPredicate pred = cmp_op_to_int_predicate(op_id, type_entry->data.integral.is_signed);
                 return LLVMBuildICmp(g->builder, pred, op1_value, op2_value, "");
             } else if (type_entry->id == TypeTableEntryIdEnum) {
-                if (type_entry->data.enumeration.gen_field_count == 0) {
-                    LLVMIntPredicate pred = cmp_op_to_int_predicate(op_id, false);
-                    return LLVMBuildICmp(g->builder, pred, op1_value, op2_value, "");
-                } else {
-                    zig_unreachable();
-                }
+                LLVMIntPredicate pred = cmp_op_to_int_predicate(op_id, false);
+                return LLVMBuildICmp(g->builder, pred, op1_value, op2_value, "");
             } else if (type_entry->id == TypeTableEntryIdPureError ||
                     type_entry->id == TypeTableEntryIdPointer ||
                     type_entry->id == TypeTableEntryIdBool)
@@ -1920,9 +1916,7 @@ static LLVMValueRef ir_render_widen_or_shorten(CodeGen *g, IrExecutable *executa
     // enum_tag to the underlying int type
     TypeTableEntry *int_type;
     if (actual_type->id == TypeTableEntryIdEnum) {
-        TypeTableEntry *tag_type = actual_type->data.enumeration.tag_type;
-        assert(tag_type->id == TypeTableEntryIdEnumTag);
-        int_type = tag_type->data.enum_tag.int_type;
+        int_type = actual_type->data.enumeration.tag_int_type;
     } else {
         int_type = actual_type;
     }
@@ -1946,19 +1940,11 @@ static LLVMValueRef ir_render_ptr_to_int(CodeGen *g, IrExecutable *executable, I
 static LLVMValueRef ir_render_int_to_enum(CodeGen *g, IrExecutable *executable, IrInstructionIntToEnum *instruction) {
     TypeTableEntry *wanted_type = instruction->base.value.type;
     assert(wanted_type->id == TypeTableEntryIdEnum);
-    TypeTableEntry *tag_type = wanted_type->data.enumeration.tag_type;
-    TypeTableEntry *wanted_int_type;
-    if (tag_type->id == TypeTableEntryIdEnumTag) {
-        wanted_int_type = tag_type->data.enum_tag.int_type;
-    } else if (tag_type->id == TypeTableEntryIdInt) {
-        wanted_int_type = tag_type;
-    } else {
-        zig_unreachable();
-    }
+    TypeTableEntry *tag_int_type = wanted_type->data.enumeration.tag_int_type;
 
     LLVMValueRef target_val = ir_llvm_value(g, instruction->target);
     return gen_widen_or_shorten(g, ir_want_debug_safety(g, &instruction->base),
-            instruction->target->value.type, wanted_int_type, target_val);
+            instruction->target->value.type, tag_int_type, target_val);
 }
 
 static LLVMValueRef ir_render_int_to_err(CodeGen *g, IrExecutable *executable, IrInstructionIntToErr *instruction) {
@@ -2378,27 +2364,6 @@ static LLVMValueRef ir_render_struct_field_ptr(CodeGen *g, IrExecutable *executa
     return LLVMBuildStructGEP(g->builder, struct_ptr, (unsigned)field->gen_index, "");
 }
 
-static LLVMValueRef ir_render_enum_field_ptr(CodeGen *g, IrExecutable *executable,
-    IrInstructionEnumFieldPtr *instruction)
-{
-    TypeTableEntry *enum_ptr_type = instruction->enum_ptr->value.type;
-    assert(enum_ptr_type->id == TypeTableEntryIdPointer);
-    TypeTableEntry *enum_type = enum_ptr_type->data.pointer.child_type;
-    assert(enum_type->id == TypeTableEntryIdEnum);
-
-    TypeEnumField *field = instruction->field;
-
-    if (!type_has_bits(field->type_entry))
-        return nullptr;
-
-    LLVMValueRef enum_ptr = ir_llvm_value(g, instruction->enum_ptr);
-    LLVMTypeRef field_type_ref = LLVMPointerType(field->type_entry->type_ref, 0);
-    LLVMValueRef union_field_ptr = LLVMBuildStructGEP(g->builder, enum_ptr, enum_type->data.enumeration.gen_union_index, "");
-    LLVMValueRef bitcasted_union_field_ptr = LLVMBuildBitCast(g->builder, union_field_ptr, field_type_ref, "");
-
-    return bitcasted_union_field_ptr;
-}
-
 static LLVMValueRef ir_render_union_field_ptr(CodeGen *g, IrExecutable *executable,
     IrInstructionUnionFieldPtr *instruction)
 {
@@ -2427,7 +2392,7 @@ static LLVMValueRef ir_render_union_field_ptr(CodeGen *g, IrExecutable *executab
 
 
         LLVMValueRef expected_tag_value = bigint_to_llvm_const(union_type->data.unionation.tag_type->type_ref,
-                &field->value);
+                &field->enum_field->value);
         LLVMBasicBlockRef ok_block = LLVMAppendBasicBlock(g->cur_fn_val, "UnionCheckOk");
         LLVMBasicBlockRef bad_block = LLVMAppendBasicBlock(g->cur_fn_val, "UnionCheckFail");
         LLVMValueRef ok_val = LLVMBuildICmp(g->builder, LLVMIntEQ, tag_value, expected_tag_value, "");
@@ -2754,19 +2719,19 @@ static LLVMValueRef ir_render_err_name(CodeGen *g, IrExecutable *executable, IrI
 static LLVMValueRef ir_render_enum_tag_name(CodeGen *g, IrExecutable *executable,
         IrInstructionEnumTagName *instruction)
 {
-    TypeTableEntry *enum_tag_type = instruction->target->value.type;
-    assert(enum_tag_type->data.enum_tag.generate_name_table);
+    TypeTableEntry *enum_type = instruction->target->value.type;
+    assert(enum_type->id == TypeTableEntryIdEnum);
+    assert(enum_type->data.enumeration.generate_name_table);
 
+    TypeTableEntry *tag_int_type = enum_type->data.enumeration.tag_int_type;
     LLVMValueRef enum_tag_value = ir_llvm_value(g, instruction->target);
     if (ir_want_debug_safety(g, &instruction->base)) {
-        TypeTableEntry *enum_type = enum_tag_type->data.enum_tag.enum_type;
         size_t field_count = enum_type->data.enumeration.src_field_count;
 
-        // if the field_count can't fit in the bits of the enum_tag_type, then it can't possibly
+        // if the field_count can't fit in the bits of the enum_type, then it can't possibly
         // be the wrong value
         BigInt field_bi;
         bigint_init_unsigned(&field_bi, field_count);
-        TypeTableEntry *tag_int_type = enum_tag_type->data.enum_tag.int_type;
         if (bigint_fits_in_bits(&field_bi, tag_int_type->data.integral.bit_count, false)) {
             LLVMValueRef end_val = LLVMConstInt(LLVMTypeOf(enum_tag_value), field_count, false);
             add_bounds_check(g, enum_tag_value, LLVMIntEQ, nullptr, LLVMIntULT, end_val);
@@ -2775,10 +2740,10 @@ static LLVMValueRef ir_render_enum_tag_name(CodeGen *g, IrExecutable *executable
 
     LLVMValueRef indices[] = {
         LLVMConstNull(g->builtin_types.entry_usize->type_ref),
-        gen_widen_or_shorten(g, false, enum_tag_type->data.enum_tag.int_type,
+        gen_widen_or_shorten(g, false, tag_int_type,
                 g->builtin_types.entry_usize, enum_tag_value),
     };
-    return LLVMBuildInBoundsGEP(g->builder, enum_tag_type->data.enum_tag.name_table, indices, 2, "");
+    return LLVMBuildInBoundsGEP(g->builder, enum_type->data.enumeration.name_table, indices, 2, "");
 }
 
 static LLVMValueRef ir_render_field_parent_ptr(CodeGen *g, IrExecutable *executable,
@@ -3352,48 +3317,24 @@ static LLVMValueRef ir_render_err_wrap_payload(CodeGen *g, IrExecutable *executa
     return instruction->tmp_ptr;
 }
 
-static LLVMValueRef ir_render_enum_tag(CodeGen *g, IrExecutable *executable, IrInstructionEnumTag *instruction) {
-    TypeTableEntry *enum_type = instruction->value->value.type;
-    TypeTableEntry *tag_type = enum_type->data.enumeration.tag_type;
+static LLVMValueRef ir_render_union_tag(CodeGen *g, IrExecutable *executable, IrInstructionUnionTag *instruction) {
+    TypeTableEntry *union_type = instruction->value->value.type;
+    assert(union_type->data.unionation.gen_tag_index != SIZE_MAX);
+
+    TypeTableEntry *tag_type = union_type->data.unionation.tag_type;
     if (!type_has_bits(tag_type))
         return nullptr;
 
-    LLVMValueRef enum_val = ir_llvm_value(g, instruction->value);
-    if (enum_type->data.enumeration.gen_field_count == 0)
-        return enum_val;
+    LLVMValueRef union_val = ir_llvm_value(g, instruction->value);
+    if (union_type->data.unionation.gen_field_count == 0)
+        return union_val;
 
-    LLVMValueRef tag_field_ptr = LLVMBuildStructGEP(g->builder, enum_val, enum_type->data.enumeration.gen_tag_index, "");
+    LLVMValueRef tag_field_ptr = LLVMBuildStructGEP(g->builder, union_val,
+            union_type->data.unionation.gen_tag_index, "");
     TypeTableEntry *ptr_type = get_pointer_to_type(g, tag_type, false);
     return get_handle_value(g, tag_field_ptr, tag_type, ptr_type);
 }
 
-static LLVMValueRef ir_render_init_enum(CodeGen *g, IrExecutable *executable, IrInstructionInitEnum *instruction) {
-    TypeTableEntry *enum_type = instruction->enum_type;
-    LLVMTypeRef tag_type_ref = enum_type->data.enumeration.tag_type->type_ref;
-
-    LLVMValueRef tag_value = bigint_to_llvm_const(tag_type_ref, &instruction->field->value);
-
-    if (enum_type->data.enumeration.gen_field_count == 0)
-        return tag_value;
-
-    LLVMValueRef tmp_struct_ptr = instruction->tmp_ptr;
-
-    LLVMValueRef tag_field_ptr = LLVMBuildStructGEP(g->builder, tmp_struct_ptr, enum_type->data.enumeration.gen_tag_index, "");
-    gen_store_untyped(g, tag_value, tag_field_ptr, 0, false);
-
-    TypeTableEntry *union_val_type = instruction->field->type_entry;
-    if (type_has_bits(union_val_type)) {
-        LLVMValueRef new_union_val = ir_llvm_value(g, instruction->init_value);
-        LLVMValueRef union_field_ptr = LLVMBuildStructGEP(g->builder, tmp_struct_ptr, enum_type->data.enumeration.gen_union_index, "");
-        LLVMValueRef bitcasted_union_field_ptr = LLVMBuildBitCast(g->builder, union_field_ptr,
-                LLVMPointerType(union_val_type->type_ref, 0), "");
-
-        gen_assign_raw(g, bitcasted_union_field_ptr, get_pointer_to_type(g, union_val_type, false), new_union_val);
-    }
-
-    return tmp_struct_ptr;
-}
-
 static LLVMValueRef ir_render_struct_init(CodeGen *g, IrExecutable *executable, IrInstructionStructInit *instruction) {
     for (size_t i = 0; i < instruction->field_count; i += 1) {
         IrInstructionStructInitField *field = &instruction->fields[i];
@@ -3436,7 +3377,7 @@ static LLVMValueRef ir_render_union_init(CodeGen *g, IrExecutable *executable, I
                 union_type->data.unionation.gen_tag_index, "");
 
         LLVMValueRef tag_value = bigint_to_llvm_const(union_type->data.unionation.tag_type->type_ref,
-                &type_union_field->value);
+                &type_union_field->enum_field->value);
         gen_store_untyped(g, tag_value, tag_field_ptr, 0, false);
 
         uncasted_union_ptr = LLVMBuildStructGEP(g->builder, instruction->tmp_ptr,
@@ -3573,8 +3514,6 @@ static LLVMValueRef ir_render_instruction(CodeGen *g, IrExecutable *executable,
             return ir_render_call(g, executable, (IrInstructionCall *)instruction);
         case IrInstructionIdStructFieldPtr:
             return ir_render_struct_field_ptr(g, executable, (IrInstructionStructFieldPtr *)instruction);
-        case IrInstructionIdEnumFieldPtr:
-            return ir_render_enum_field_ptr(g, executable, (IrInstructionEnumFieldPtr *)instruction);
         case IrInstructionIdUnionFieldPtr:
             return ir_render_union_field_ptr(g, executable, (IrInstructionUnionFieldPtr *)instruction);
         case IrInstructionIdAsm:
@@ -3629,10 +3568,8 @@ static LLVMValueRef ir_render_instruction(CodeGen *g, IrExecutable *executable,
             return ir_render_err_wrap_code(g, executable, (IrInstructionErrWrapCode *)instruction);
         case IrInstructionIdErrWrapPayload:
             return ir_render_err_wrap_payload(g, executable, (IrInstructionErrWrapPayload *)instruction);
-        case IrInstructionIdEnumTag:
-            return ir_render_enum_tag(g, executable, (IrInstructionEnumTag *)instruction);
-        case IrInstructionIdInitEnum:
-            return ir_render_init_enum(g, executable, (IrInstructionInitEnum *)instruction);
+        case IrInstructionIdUnionTag:
+            return ir_render_union_tag(g, executable, (IrInstructionUnionTag *)instruction);
         case IrInstructionIdStructInit:
             return ir_render_struct_init(g, executable, (IrInstructionStructInit *)instruction);
         case IrInstructionIdUnionInit:
@@ -3768,7 +3705,6 @@ static LLVMValueRef pack_const_int(CodeGen *g, LLVMTypeRef big_int_type_ref, Con
         case TypeTableEntryIdNullLit:
         case TypeTableEntryIdErrorUnion:
         case TypeTableEntryIdPureError:
-        case TypeTableEntryIdEnumTag:
         case TypeTableEntryIdNamespace:
         case TypeTableEntryIdBlock:
         case TypeTableEntryIdBoundFn:
@@ -3780,7 +3716,6 @@ static LLVMValueRef pack_const_int(CodeGen *g, LLVMTypeRef big_int_type_ref, Con
             return LLVMConstInt(big_int_type_ref, const_val->data.x_bool ? 1 : 0, false);
         case TypeTableEntryIdEnum:
             {
-                assert(type_entry->data.enumeration.gen_field_count == 0);
                 assert(type_entry->data.enumeration.decl_node->data.container_decl.init_arg_expr != nullptr);
                 LLVMValueRef int_val = gen_const_val(g, const_val);
                 return LLVMConstZExt(int_val, big_int_type_ref);
@@ -3852,7 +3787,6 @@ static LLVMValueRef gen_const_val(CodeGen *g, ConstExprValue *const_val) {
 
     switch (type_entry->id) {
         case TypeTableEntryIdInt:
-        case TypeTableEntryIdEnumTag:
             return bigint_to_llvm_const(type_entry->type_ref, &const_val->data.x_bigint);
         case TypeTableEntryIdPureError:
             assert(const_val->data.x_pure_err);
@@ -4015,34 +3949,48 @@ static LLVMValueRef gen_const_val(CodeGen *g, ConstExprValue *const_val) {
                 ConstExprValue *payload_value = const_val->data.x_union.payload;
                 assert(payload_value != nullptr);
 
-                if (!type_has_bits(payload_value->type)) {
-                    return LLVMGetUndef(union_type_ref);
-                }
-
-                uint64_t field_type_bytes = LLVMStoreSizeOfType(g->target_data_ref, payload_value->type->type_ref);
-                uint64_t pad_bytes = type_entry->data.unionation.union_size_bytes - field_type_bytes;
-                LLVMValueRef correctly_typed_value = gen_const_val(g, payload_value);
-                bool make_unnamed_struct = is_llvm_value_unnamed_type(payload_value->type, correctly_typed_value) ||
-                    payload_value->type != type_entry->data.unionation.most_aligned_union_member;
-
-                LLVMValueRef union_value_ref;
-                {
-                    if (pad_bytes == 0) {
-                        union_value_ref = correctly_typed_value;
+                if (type_entry->data.unionation.gen_field_count == 0) {
+                    if (type_entry->data.unionation.gen_tag_index == SIZE_MAX) {
+                        return nullptr;
                     } else {
-                        LLVMValueRef fields[2];
-                        fields[0] = correctly_typed_value;
-                        fields[1] = LLVMGetUndef(LLVMArrayType(LLVMInt8Type(), (unsigned)pad_bytes));
-                        if (make_unnamed_struct || type_entry->data.unionation.gen_tag_index != SIZE_MAX) {
-                            union_value_ref = LLVMConstStruct(fields, 2, false);
-                        } else {
-                            union_value_ref = LLVMConstNamedStruct(union_type_ref, fields, 2);
-                        }
+                        return bigint_to_llvm_const(type_entry->data.unionation.tag_type->type_ref,
+                            &const_val->data.x_union.tag);
                     }
                 }
 
-                if (type_entry->data.unionation.gen_tag_index == SIZE_MAX) {
-                    return union_value_ref;
+                LLVMValueRef union_value_ref;
+                bool make_unnamed_struct;
+                if (!type_has_bits(payload_value->type)) {
+                    if (type_entry->data.unionation.gen_tag_index == SIZE_MAX)
+                        return LLVMGetUndef(type_entry->type_ref);
+
+                    union_value_ref = LLVMGetUndef(type_entry->data.unionation.most_aligned_union_member->type_ref);
+                    make_unnamed_struct = false;
+                } else {
+                    uint64_t field_type_bytes = LLVMStoreSizeOfType(g->target_data_ref, payload_value->type->type_ref);
+                    uint64_t pad_bytes = type_entry->data.unionation.union_size_bytes - field_type_bytes;
+                    LLVMValueRef correctly_typed_value = gen_const_val(g, payload_value);
+                    make_unnamed_struct = is_llvm_value_unnamed_type(payload_value->type, correctly_typed_value) ||
+                        payload_value->type != type_entry->data.unionation.most_aligned_union_member;
+
+                    {
+                        if (pad_bytes == 0) {
+                            union_value_ref = correctly_typed_value;
+                        } else {
+                            LLVMValueRef fields[2];
+                            fields[0] = correctly_typed_value;
+                            fields[1] = LLVMGetUndef(LLVMArrayType(LLVMInt8Type(), (unsigned)pad_bytes));
+                            if (make_unnamed_struct || type_entry->data.unionation.gen_tag_index != SIZE_MAX) {
+                                union_value_ref = LLVMConstStruct(fields, 2, false);
+                            } else {
+                                union_value_ref = LLVMConstNamedStruct(union_type_ref, fields, 2);
+                            }
+                        }
+                    }
+
+                    if (type_entry->data.unionation.gen_tag_index == SIZE_MAX) {
+                        return union_value_ref;
+                    }
                 }
 
                 LLVMValueRef tag_value = bigint_to_llvm_const(type_entry->data.unionation.tag_type->type_ref,
@@ -4059,55 +4007,9 @@ static LLVMValueRef gen_const_val(CodeGen *g, ConstExprValue *const_val) {
                 }
 
             }
+
         case TypeTableEntryIdEnum:
-            {
-                LLVMTypeRef tag_type_ref = type_entry->data.enumeration.tag_type->type_ref;
-                LLVMValueRef tag_value = bigint_to_llvm_const(tag_type_ref, &const_val->data.x_enum.tag);
-                if (type_entry->data.enumeration.gen_field_count == 0) {
-                    return tag_value;
-                } else {
-                    LLVMTypeRef union_type_ref = type_entry->data.enumeration.union_type_ref;
-                    TypeEnumField *enum_field = find_enum_field_by_tag(type_entry, &const_val->data.x_enum.tag);
-                    assert(bigint_cmp(&enum_field->value, &const_val->data.x_enum.tag) == CmpEQ);
-                    LLVMValueRef union_value;
-
-                    bool make_unnamed_struct;
-
-                    if (type_has_bits(enum_field->type_entry)) {
-                        uint64_t field_type_bytes = LLVMStoreSizeOfType(g->target_data_ref,
-                                enum_field->type_entry->type_ref);
-                        uint64_t pad_bytes = type_entry->data.enumeration.union_size_bytes - field_type_bytes;
-
-                        ConstExprValue *payload_value = const_val->data.x_enum.payload;
-                        LLVMValueRef correctly_typed_value = gen_const_val(g, payload_value);
-
-                        make_unnamed_struct = is_llvm_value_unnamed_type(payload_value->type, correctly_typed_value) ||
-                            payload_value->type != type_entry->data.enumeration.most_aligned_union_member;
-
-                        if (pad_bytes == 0) {
-                            union_value = correctly_typed_value;
-                        } else {
-                            LLVMValueRef fields[] = {
-                                correctly_typed_value,
-                                LLVMGetUndef(LLVMArrayType(LLVMInt8Type(), (unsigned)pad_bytes)),
-                            };
-                            union_value = LLVMConstStruct(fields, 2, false);
-                        }
-                    } else {
-                        make_unnamed_struct = false;
-                        union_value = LLVMGetUndef(union_type_ref);
-                    }
-                    LLVMValueRef fields[2];
-                    fields[type_entry->data.enumeration.gen_tag_index] = tag_value;
-                    fields[type_entry->data.enumeration.gen_union_index] = union_value;
-
-                    if (make_unnamed_struct) {
-                        return LLVMConstStruct(fields, 2, false);
-                    } else {
-                        return LLVMConstNamedStruct(type_entry->type_ref, fields, 2);
-                    }
-                }
-            }
+            return bigint_to_llvm_const(type_entry->type_ref, &const_val->data.x_enum_tag);
         case TypeTableEntryIdFn:
             return fn_llvm_value(g, const_val->data.x_fn.fn_entry);
         case TypeTableEntryIdPointer:
@@ -4318,9 +4220,8 @@ static void generate_enum_name_tables(CodeGen *g) {
 
 
     for (size_t enum_i = 0; enum_i < g->name_table_enums.length; enum_i += 1) {
-        TypeTableEntry *enum_tag_type = g->name_table_enums.at(enum_i);
-        assert(enum_tag_type->id == TypeTableEntryIdEnumTag);
-        TypeTableEntry *enum_type = enum_tag_type->data.enum_tag.enum_type;
+        TypeTableEntry *enum_type = g->name_table_enums.at(enum_i);
+        assert(enum_type->id == TypeTableEntryIdEnum);
 
         size_t field_count = enum_type->data.enumeration.src_field_count;
         LLVMValueRef *values = allocate<LLVMValueRef>(field_count);
@@ -4351,7 +4252,7 @@ static void generate_enum_name_tables(CodeGen *g) {
         LLVMSetGlobalConstant(name_table, true);
         LLVMSetUnnamedAddr(name_table, true);
         LLVMSetAlignment(name_table, LLVMABIAlignmentOfType(g->target_data_ref, LLVMTypeOf(name_table_init)));
-        enum_tag_type->data.enum_tag.name_table = name_table;
+        enum_type->data.enumeration.name_table = name_table;
     }
 }
 
@@ -4555,9 +4456,6 @@ static void do_code_gen(CodeGen *g) {
             } else if (instruction->id == IrInstructionIdErrWrapCode) {
                 IrInstructionErrWrapCode *err_wrap_code_instruction = (IrInstructionErrWrapCode *)instruction;
                 slot = &err_wrap_code_instruction->tmp_ptr;
-            } else if (instruction->id == IrInstructionIdInitEnum) {
-                IrInstructionInitEnum *init_enum_instruction = (IrInstructionInitEnum *)instruction;
-                slot = &init_enum_instruction->tmp_ptr;
             } else {
                 zig_unreachable();
             }
@@ -5054,7 +4952,7 @@ static void define_builtin_fns(CodeGen *g) {
     create_builtin_fn(g, BuiltinFnIdTruncate, "truncate", 2);
     create_builtin_fn(g, BuiltinFnIdCompileErr, "compileError", 1);
     create_builtin_fn(g, BuiltinFnIdCompileLog, "compileLog", SIZE_MAX);
-    create_builtin_fn(g, BuiltinFnIdIntType, "IntType", 2);
+    create_builtin_fn(g, BuiltinFnIdIntType, "IntType", 2); // TODO rename to Int
     create_builtin_fn(g, BuiltinFnIdSetDebugSafety, "setDebugSafety", 2);
     create_builtin_fn(g, BuiltinFnIdSetFloatMode, "setFloatMode", 2);
     create_builtin_fn(g, BuiltinFnIdSetGlobalSection, "setGlobalSection", 2);
@@ -5064,7 +4962,7 @@ static void define_builtin_fns(CodeGen *g) {
     create_builtin_fn(g, BuiltinFnIdBitCast, "bitCast", 2);
     create_builtin_fn(g, BuiltinFnIdIntToPtr, "intToPtr", 2);
     create_builtin_fn(g, BuiltinFnIdPtrToInt, "ptrToInt", 1);
-    create_builtin_fn(g, BuiltinFnIdEnumTagName, "enumTagName", 1); // TODO rename to memberName
+    create_builtin_fn(g, BuiltinFnIdTagName, "tagName", 1);
     create_builtin_fn(g, BuiltinFnIdEnumTagType, "EnumTagType", 1);
     create_builtin_fn(g, BuiltinFnIdFieldParentPtr, "fieldParentPtr", 3);
     create_builtin_fn(g, BuiltinFnIdOffsetOf, "offsetOf", 2);
@@ -5681,7 +5579,6 @@ static void get_c_type(CodeGen *g, TypeTableEntry *type_entry, Buf *out_buf) {
         case TypeTableEntryIdEnum:
         case TypeTableEntryIdUnion:
         case TypeTableEntryIdFn:
-        case TypeTableEntryIdEnumTag:
             zig_panic("TODO implement get_c_type for more types");
         case TypeTableEntryIdInvalid:
         case TypeTableEntryIdMetaType:
diff --git a/src/ir.cpp b/src/ir.cpp
index e51f52adae..5da59fedb7 100644
--- a/src/ir.cpp
+++ b/src/ir.cpp
@@ -223,10 +223,6 @@ static constexpr IrInstructionId ir_instruction_id(IrInstructionStructFieldPtr *
     return IrInstructionIdStructFieldPtr;
 }
 
-static constexpr IrInstructionId ir_instruction_id(IrInstructionEnumFieldPtr *) {
-    return IrInstructionIdEnumFieldPtr;
-}
-
 static constexpr IrInstructionId ir_instruction_id(IrInstructionUnionFieldPtr *) {
     return IrInstructionIdUnionFieldPtr;
 }
@@ -319,8 +315,8 @@ static constexpr IrInstructionId ir_instruction_id(IrInstructionCtz *) {
     return IrInstructionIdCtz;
 }
 
-static constexpr IrInstructionId ir_instruction_id(IrInstructionEnumTag *) {
-    return IrInstructionIdEnumTag;
+static constexpr IrInstructionId ir_instruction_id(IrInstructionUnionTag *) {
+    return IrInstructionIdUnionTag;
 }
 
 static constexpr IrInstructionId ir_instruction_id(IrInstructionImport *) {
@@ -479,10 +475,6 @@ static constexpr IrInstructionId ir_instruction_id(IrInstructionTestComptime *)
     return IrInstructionIdTestComptime;
 }
 
-static constexpr IrInstructionId ir_instruction_id(IrInstructionInitEnum *) {
-    return IrInstructionIdInitEnum;
-}
-
 static constexpr IrInstructionId ir_instruction_id(IrInstructionPtrCast *) {
     return IrInstructionIdPtrCast;
 }
@@ -913,27 +905,6 @@ static IrInstruction *ir_build_struct_field_ptr_from(IrBuilder *irb, IrInstructi
     return new_instruction;
 }
 
-static IrInstruction *ir_build_enum_field_ptr(IrBuilder *irb, Scope *scope, AstNode *source_node,
-    IrInstruction *enum_ptr, TypeEnumField *field)
-{
-    IrInstructionEnumFieldPtr *instruction = ir_build_instruction<IrInstructionEnumFieldPtr>(irb, scope, source_node);
-    instruction->enum_ptr = enum_ptr;
-    instruction->field = field;
-
-    ir_ref_instruction(enum_ptr, irb->current_basic_block);
-
-    return &instruction->base;
-}
-
-static IrInstruction *ir_build_enum_field_ptr_from(IrBuilder *irb, IrInstruction *old_instruction,
-    IrInstruction *enum_ptr, TypeEnumField *type_enum_field)
-{
-    IrInstruction *new_instruction = ir_build_enum_field_ptr(irb, old_instruction->scope,
-            old_instruction->source_node, enum_ptr, type_enum_field);
-    ir_link_new_instruction(new_instruction, old_instruction);
-    return new_instruction;
-}
-
 static IrInstruction *ir_build_union_field_ptr(IrBuilder *irb, Scope *scope, AstNode *source_node,
     IrInstruction *union_ptr, TypeUnionField *field)
 {
@@ -1528,8 +1499,8 @@ static IrInstruction *ir_build_switch_var(IrBuilder *irb, Scope *scope, AstNode
     return &instruction->base;
 }
 
-static IrInstruction *ir_build_enum_tag(IrBuilder *irb, Scope *scope, AstNode *source_node, IrInstruction *value) {
-    IrInstructionEnumTag *instruction = ir_build_instruction<IrInstructionEnumTag>(irb, scope, source_node);
+static IrInstruction *ir_build_union_tag(IrBuilder *irb, Scope *scope, AstNode *source_node, IrInstruction *value) {
+    IrInstructionUnionTag *instruction = ir_build_instruction<IrInstructionUnionTag>(irb, scope, source_node);
     instruction->value = value;
 
     ir_ref_instruction(value, irb->current_basic_block);
@@ -1537,13 +1508,6 @@ static IrInstruction *ir_build_enum_tag(IrBuilder *irb, Scope *scope, AstNode *s
     return &instruction->base;
 }
 
-static IrInstruction *ir_build_enum_tag_from(IrBuilder *irb, IrInstruction *old_instruction, IrInstruction *value) {
-    IrInstruction *new_instruction = ir_build_enum_tag(irb, old_instruction->scope,
-            old_instruction->source_node, value);
-    ir_link_new_instruction(new_instruction, old_instruction);
-    return new_instruction;
-}
-
 static IrInstruction *ir_build_import(IrBuilder *irb, Scope *scope, AstNode *source_node, IrInstruction *name) {
     IrInstructionImport *instruction = ir_build_instruction<IrInstructionImport>(irb, scope, source_node);
     instruction->name = name;
@@ -2033,28 +1997,6 @@ static IrInstruction *ir_build_test_comptime(IrBuilder *irb, Scope *scope, AstNo
     return &instruction->base;
 }
 
-static IrInstruction *ir_build_init_enum(IrBuilder *irb, Scope *scope, AstNode *source_node,
-        TypeTableEntry *enum_type, TypeEnumField *field, IrInstruction *init_value)
-{
-    IrInstructionInitEnum *instruction = ir_build_instruction<IrInstructionInitEnum>(irb, scope, source_node);
-    instruction->enum_type = enum_type;
-    instruction->field = field;
-    instruction->init_value = init_value;
-
-    ir_ref_instruction(init_value, irb->current_basic_block);
-
-    return &instruction->base;
-}
-
-static IrInstruction *ir_build_init_enum_from(IrBuilder *irb, IrInstruction *old_instruction,
-    TypeTableEntry *enum_type, TypeEnumField *field, IrInstruction *init_value)
-{
-    IrInstruction *new_instruction = ir_build_init_enum(irb, old_instruction->scope, old_instruction->source_node,
-            enum_type, field, init_value);
-    ir_link_new_instruction(new_instruction, old_instruction);
-    return new_instruction;
-}
-
 static IrInstruction *ir_build_ptr_cast(IrBuilder *irb, Scope *scope, AstNode *source_node,
         IrInstruction *dest_type, IrInstruction *ptr)
 {
@@ -2481,13 +2423,6 @@ static IrInstruction *ir_instruction_structfieldptr_get_dep(IrInstructionStructF
     }
 }
 
-static IrInstruction *ir_instruction_enumfieldptr_get_dep(IrInstructionEnumFieldPtr *instruction, size_t index) {
-    switch (index) {
-        case 0: return instruction->enum_ptr;
-        default: return nullptr;
-    }
-}
-
 static IrInstruction *ir_instruction_unionfieldptr_get_dep(IrInstructionUnionFieldPtr *instruction, size_t index) {
     switch (index) {
         case 0: return instruction->union_ptr;
@@ -2657,7 +2592,7 @@ static IrInstruction *ir_instruction_maybewrap_get_dep(IrInstructionMaybeWrap *i
     }
 }
 
-static IrInstruction *ir_instruction_enumtag_get_dep(IrInstructionEnumTag *instruction, size_t index) {
+static IrInstruction *ir_instruction_uniontag_get_dep(IrInstructionUnionTag *instruction, size_t index) {
     switch (index) {
         case 0: return instruction->value;
         default: return nullptr;
@@ -2943,13 +2878,6 @@ static IrInstruction *ir_instruction_testcomptime_get_dep(IrInstructionTestCompt
     }
 }
 
-static IrInstruction *ir_instruction_initenum_get_dep(IrInstructionInitEnum *instruction, size_t index) {
-    switch (index) {
-        case 0: return instruction->init_value;
-        default: return nullptr;
-    }
-}
-
 static IrInstruction *ir_instruction_ptrcast_get_dep(IrInstructionPtrCast *instruction,
         size_t index)
 {
@@ -3184,8 +3112,6 @@ static IrInstruction *ir_instruction_get_dep(IrInstruction *instruction, size_t
             return ir_instruction_fieldptr_get_dep((IrInstructionFieldPtr *) instruction, index);
         case IrInstructionIdStructFieldPtr:
             return ir_instruction_structfieldptr_get_dep((IrInstructionStructFieldPtr *) instruction, index);
-        case IrInstructionIdEnumFieldPtr:
-            return ir_instruction_enumfieldptr_get_dep((IrInstructionEnumFieldPtr *) instruction, index);
         case IrInstructionIdUnionFieldPtr:
             return ir_instruction_unionfieldptr_get_dep((IrInstructionUnionFieldPtr *) instruction, index);
         case IrInstructionIdElemPtr:
@@ -3234,8 +3160,8 @@ static IrInstruction *ir_instruction_get_dep(IrInstruction *instruction, size_t
             return ir_instruction_unwrapmaybe_get_dep((IrInstructionUnwrapMaybe *) instruction, index);
         case IrInstructionIdMaybeWrap:
             return ir_instruction_maybewrap_get_dep((IrInstructionMaybeWrap *) instruction, index);
-        case IrInstructionIdEnumTag:
-            return ir_instruction_enumtag_get_dep((IrInstructionEnumTag *) instruction, index);
+        case IrInstructionIdUnionTag:
+            return ir_instruction_uniontag_get_dep((IrInstructionUnionTag *) instruction, index);
         case IrInstructionIdClz:
             return ir_instruction_clz_get_dep((IrInstructionClz *) instruction, index);
         case IrInstructionIdCtz:
@@ -3312,8 +3238,6 @@ static IrInstruction *ir_instruction_get_dep(IrInstruction *instruction, size_t
             return ir_instruction_fnproto_get_dep((IrInstructionFnProto *) instruction, index);
         case IrInstructionIdTestComptime:
             return ir_instruction_testcomptime_get_dep((IrInstructionTestComptime *) instruction, index);
-        case IrInstructionIdInitEnum:
-            return ir_instruction_initenum_get_dep((IrInstructionInitEnum *) instruction, index);
         case IrInstructionIdPtrCast:
             return ir_instruction_ptrcast_get_dep((IrInstructionPtrCast *) instruction, index);
         case IrInstructionIdBitCast:
@@ -4695,14 +4619,14 @@ static IrInstruction *ir_gen_builtin_fn_call(IrBuilder *irb, Scope *scope, AstNo
 
                 return ir_build_ptr_to_int(irb, scope, node, arg0_value);
             }
-        case BuiltinFnIdEnumTagName:
+        case BuiltinFnIdTagName:
             {
                 AstNode *arg0_node = node->data.fn_call_expr.params.at(0);
                 IrInstruction *arg0_value = ir_gen_node(irb, arg0_node, scope);
                 if (arg0_value == irb->codegen->invalid_instruction)
                     return arg0_value;
 
-                IrInstruction *actual_tag = ir_build_enum_tag(irb, scope, node, arg0_value);
+                IrInstruction *actual_tag = ir_build_union_tag(irb, scope, node, arg0_value);
                 return ir_build_enum_tag_name(irb, scope, node, actual_tag);
             }
         case BuiltinFnIdEnumTagType:
@@ -8381,13 +8305,28 @@ static IrInstruction *ir_analyze_enum_to_int(IrAnalyze *ira, IrInstruction *sour
 {
     assert(wanted_type->id == TypeTableEntryIdInt);
 
+    TypeTableEntry *actual_type = target->value.type;
+    ensure_complete_type(ira->codegen, actual_type);
+    if (type_is_invalid(actual_type))
+        return ira->codegen->invalid_instruction;
+
+    if (wanted_type != actual_type->data.enumeration.tag_int_type) {
+        ir_add_error(ira, source_instr,
+                buf_sprintf("enum to integer cast to '%s' instead of its tag type, '%s'",
+                    buf_ptr(&wanted_type->name),
+                    buf_ptr(&actual_type->data.enumeration.tag_int_type->name)));
+        return ira->codegen->invalid_instruction;
+    }
+
+    assert(actual_type->id == TypeTableEntryIdEnum);
+
     if (instr_is_comptime(target)) {
         ConstExprValue *val = ir_resolve_const(ira, target, UndefBad);
         if (!val)
             return ira->codegen->invalid_instruction;
         IrInstruction *result = ir_create_const(&ira->new_irb, source_instr->scope,
                 source_instr->source_node, wanted_type);
-        init_const_bigint(&result->value, wanted_type, &val->data.x_enum.tag);
+        init_const_bigint(&result->value, wanted_type, &val->data.x_enum_tag);
         return result;
     }
 
@@ -8397,6 +8336,31 @@ static IrInstruction *ir_analyze_enum_to_int(IrAnalyze *ira, IrInstruction *sour
     return result;
 }
 
+static IrInstruction *ir_analyze_union_to_tag(IrAnalyze *ira, IrInstruction *source_instr,
+        IrInstruction *target, TypeTableEntry *wanted_type)
+{
+    assert(target->value.type->id == TypeTableEntryIdUnion);
+    assert(wanted_type->id == TypeTableEntryIdEnum);
+    assert(wanted_type == target->value.type->data.unionation.tag_type);
+
+    if (instr_is_comptime(target)) {
+        ConstExprValue *val = ir_resolve_const(ira, target, UndefBad);
+        if (!val)
+            return ira->codegen->invalid_instruction;
+        IrInstruction *result = ir_create_const(&ira->new_irb, source_instr->scope,
+                source_instr->source_node, wanted_type);
+        result->value.special = ConstValSpecialStatic;
+        result->value.type = wanted_type;
+        bigint_init_bigint(&result->value.data.x_enum_tag, &val->data.x_union.tag);
+        return result;
+    }
+
+    IrInstruction *result = ir_build_union_tag(&ira->new_irb, source_instr->scope,
+            source_instr->source_node, target);
+    result->value.type = wanted_type;
+    return result;
+}
+
 static IrInstruction *ir_analyze_undefined_to_anything(IrAnalyze *ira, IrInstruction *source_instr,
         IrInstruction *target, TypeTableEntry *wanted_type)
 {
@@ -8452,6 +8416,22 @@ static IrInstruction *ir_analyze_int_to_enum(IrAnalyze *ira, IrInstruction *sour
 {
     assert(wanted_type->id == TypeTableEntryIdEnum);
 
+    TypeTableEntry *actual_type = target->value.type;
+
+    ensure_complete_type(ira->codegen, wanted_type);
+    if (type_is_invalid(wanted_type))
+        return ira->codegen->invalid_instruction;
+
+    if (actual_type != wanted_type->data.enumeration.tag_int_type) {
+        ir_add_error(ira, source_instr,
+                buf_sprintf("integer to enum cast from '%s' instead of its tag type, '%s'",
+                    buf_ptr(&actual_type->name),
+                    buf_ptr(&wanted_type->data.enumeration.tag_int_type->name)));
+        return ira->codegen->invalid_instruction;
+    }
+
+    assert(actual_type->id == TypeTableEntryIdInt);
+
     if (instr_is_comptime(target)) {
         ConstExprValue *val = ir_resolve_const(ira, target, UndefBad);
         if (!val)
@@ -8469,7 +8449,7 @@ static IrInstruction *ir_analyze_int_to_enum(IrAnalyze *ira, IrInstruction *sour
 
         IrInstruction *result = ir_create_const(&ira->new_irb, source_instr->scope,
                 source_instr->source_node, wanted_type);
-        bigint_init_bigint(&result->value.data.x_enum.tag, &val->data.x_bigint);
+        bigint_init_bigint(&result->value.data.x_enum_tag, &val->data.x_bigint);
         return result;
     }
 
@@ -8907,39 +8887,24 @@ static IrInstruction *ir_analyze_cast(IrAnalyze *ira, IrInstruction *source_inst
     }
 
     // explicit cast from integer to enum type with no payload
-    if (actual_type->id == TypeTableEntryIdInt &&
-        wanted_type->id == TypeTableEntryIdEnum &&
-        wanted_type->data.enumeration.gen_field_count == 0)
-    {
-        ensure_complete_type(ira->codegen, wanted_type);
-        if (type_is_invalid(wanted_type))
-            return ira->codegen->invalid_instruction;
-        if (actual_type == wanted_type->data.enumeration.tag_type->data.enum_tag.int_type) {
-            return ir_analyze_int_to_enum(ira, source_instr, value, wanted_type);
-        }
-        ir_add_error(ira, source_instr,
-                buf_sprintf("integer to enum cast from '%s' instead of its tag type, '%s'",
-                    buf_ptr(&actual_type->name),
-                    buf_ptr(&wanted_type->data.enumeration.tag_type->data.enum_tag.int_type->name)));
-        return ira->codegen->invalid_instruction;
+    if (actual_type->id == TypeTableEntryIdInt && wanted_type->id == TypeTableEntryIdEnum) {
+        return ir_analyze_int_to_enum(ira, source_instr, value, wanted_type);
     }
 
     // explicit cast from enum type with no payload to integer
-    if (wanted_type->id == TypeTableEntryIdInt &&
-        actual_type->id == TypeTableEntryIdEnum &&
-        actual_type->data.enumeration.gen_field_count == 0)
-    {
-        ensure_complete_type(ira->codegen, actual_type);
+    if (wanted_type->id == TypeTableEntryIdInt && actual_type->id == TypeTableEntryIdEnum) {
+        return ir_analyze_enum_to_int(ira, source_instr, value, wanted_type);
+    }
+
+    // explicit cast from union to the enum type of the union
+    if (actual_type->id == TypeTableEntryIdUnion && wanted_type->id == TypeTableEntryIdEnum) {
+        type_ensure_zero_bits_known(ira->codegen, actual_type);
         if (type_is_invalid(actual_type))
             return ira->codegen->invalid_instruction;
-        if (wanted_type == actual_type->data.enumeration.tag_type->data.enum_tag.int_type) {
-            return ir_analyze_enum_to_int(ira, source_instr, value, wanted_type);
+
+        if (actual_type->data.unionation.tag_type == wanted_type) {
+            return ir_analyze_union_to_tag(ira, source_instr, value, wanted_type);
         }
-        ir_add_error(ira, source_instr,
-                buf_sprintf("enum to integer cast to '%s' instead of its tag type, '%s'",
-                    buf_ptr(&wanted_type->name),
-                    buf_ptr(&actual_type->data.enumeration.tag_type->data.enum_tag.int_type->name)));
-        return ira->codegen->invalid_instruction;
     }
 
     // explicit cast from undefined to anything
@@ -9148,7 +9113,7 @@ static bool ir_resolve_atomic_order(IrAnalyze *ira, IrInstruction *value, Atomic
     if (!const_val)
         return false;
 
-    *out = (AtomicOrder)bigint_as_unsigned(&const_val->data.x_enum.tag);
+    *out = (AtomicOrder)bigint_as_unsigned(&const_val->data.x_enum_tag);
     return true;
 }
 
@@ -9168,7 +9133,7 @@ static bool ir_resolve_global_linkage(IrAnalyze *ira, IrInstruction *value, Glob
     if (!const_val)
         return false;
 
-    *out = (GlobalLinkageId)bigint_as_unsigned(&const_val->data.x_enum.tag);
+    *out = (GlobalLinkageId)bigint_as_unsigned(&const_val->data.x_enum_tag);
     return true;
 }
 
@@ -9188,7 +9153,7 @@ static bool ir_resolve_float_mode(IrAnalyze *ira, IrInstruction *value, FloatMod
     if (!const_val)
         return false;
 
-    *out = (FloatMode)bigint_as_unsigned(&const_val->data.x_enum.tag);
+    *out = (FloatMode)bigint_as_unsigned(&const_val->data.x_enum_tag);
     return true;
 }
 
@@ -9400,7 +9365,7 @@ static TypeTableEntry *ir_analyze_bin_op_cmp(IrAnalyze *ira, IrInstructionBinOp
             break;
 
         case TypeTableEntryIdEnum:
-            if (!is_equality_cmp || resolved_type->data.enumeration.gen_field_count != 0) {
+            if (!is_equality_cmp) {
                 ir_add_error_node(ira, source_node,
                     buf_sprintf("operator not allowed for type '%s'", buf_ptr(&resolved_type->name)));
                 return ira->codegen->builtin_types.entry_invalid;
@@ -9419,9 +9384,6 @@ static TypeTableEntry *ir_analyze_bin_op_cmp(IrAnalyze *ira, IrInstructionBinOp
                 buf_sprintf("operator not allowed for type '%s'", buf_ptr(&resolved_type->name)));
             return ira->codegen->builtin_types.entry_invalid;
 
-        case TypeTableEntryIdEnumTag:
-            zig_panic("TODO implement comparison for enum tag type");
-
         case TypeTableEntryIdVar:
             zig_unreachable();
     }
@@ -10170,7 +10132,6 @@ static VarClassRequired get_var_class_required(TypeTableEntry *type_entry) {
         case TypeTableEntryIdVoid:
         case TypeTableEntryIdPureError:
         case TypeTableEntryIdFn:
-        case TypeTableEntryIdEnumTag:
             return VarClassRequiredAny;
         case TypeTableEntryIdNumLitFloat:
         case TypeTableEntryIdNumLitInt:
@@ -10913,7 +10874,6 @@ static TypeTableEntry *ir_analyze_unary_prefix_op_err(IrAnalyze *ira, IrInstruct
         case TypeTableEntryIdUnion:
         case TypeTableEntryIdFn:
         case TypeTableEntryIdBoundFn:
-        case TypeTableEntryIdEnumTag:
             {
                 ConstExprValue *out_val = ir_build_const_from(ira, &un_op_instruction->base);
                 TypeTableEntry *result_type = get_error_type(ira->codegen, meta_type);
@@ -11001,7 +10961,6 @@ static TypeTableEntry *ir_analyze_maybe(IrAnalyze *ira, IrInstructionUnOp *un_op
         case TypeTableEntryIdNamespace:
         case TypeTableEntryIdBlock:
         case TypeTableEntryIdBoundFn:
-        case TypeTableEntryIdEnumTag:
         case TypeTableEntryIdArgTuple:
             {
                 ConstExprValue *out_val = ir_build_const_from(ira, &un_op_instruction->base);
@@ -11662,15 +11621,8 @@ static TypeTableEntry *ir_analyze_container_field_ptr(IrAnalyze *ira, Buf *field
                 field_ptr_instruction, container_ptr, container_type);
         }
     } else if (bare_type->id == TypeTableEntryIdEnum) {
-        TypeEnumField *field = find_enum_type_field(bare_type, field_name);
-        if (field) {
-            ir_build_enum_field_ptr_from(&ira->new_irb, &field_ptr_instruction->base, container_ptr, field);
-            return get_pointer_to_type_extra(ira->codegen, field->type_entry, is_const, is_volatile,
-                    get_abi_alignment(ira->codegen, field->type_entry), 0, 0);
-        } else {
-            return ir_analyze_container_member_access_inner(ira, bare_type, field_name,
-                field_ptr_instruction, container_ptr, container_type);
-        }
+        return ir_analyze_container_member_access_inner(ira, bare_type, field_name,
+            field_ptr_instruction, container_ptr, container_type);
     } else if (bare_type->id == TypeTableEntryIdUnion) {
         TypeUnionField *field = find_union_type_field(bare_type, field_name);
         if (field) {
@@ -11841,20 +11793,27 @@ static TypeTableEntry *ir_analyze_instruction_field_ptr(IrAnalyze *ira, IrInstru
 
                 TypeEnumField *field = find_enum_type_field(child_type, field_name);
                 if (field) {
-                    if (field->type_entry->id == TypeTableEntryIdVoid) {
-                        bool ptr_is_const = true;
-                        bool ptr_is_volatile = false;
-                        return ir_analyze_const_ptr(ira, &field_ptr_instruction->base,
-                                create_const_enum_tag(child_type, &field->value), child_type,
-                                ConstPtrMutComptimeConst, ptr_is_const, ptr_is_volatile);
-                    } else {
-                        bool ptr_is_const = true;
-                        bool ptr_is_volatile = false;
-                        return ir_analyze_const_ptr(ira, &field_ptr_instruction->base,
-                            create_const_bigint(child_type->data.enumeration.tag_type, &field->value),
-                            child_type->data.enumeration.tag_type,
+                    bool ptr_is_const = true;
+                    bool ptr_is_volatile = false;
+                    return ir_analyze_const_ptr(ira, &field_ptr_instruction->base,
+                            create_const_enum(child_type, &field->value), child_type,
+                            ConstPtrMutComptimeConst, ptr_is_const, ptr_is_volatile);
+                }
+            } else if (child_type->id == TypeTableEntryIdUnion &&
+                    (child_type->data.unionation.decl_node->data.container_decl.init_arg_expr != nullptr ||
+                    child_type->data.unionation.decl_node->data.container_decl.auto_enum))
+            {
+                ensure_complete_type(ira->codegen, child_type);
+                if (type_is_invalid(child_type))
+                    return ira->codegen->builtin_types.entry_invalid;
+                TypeUnionField *field = find_union_type_field(child_type, field_name);
+                if (field) {
+                    TypeTableEntry *enum_type = child_type->data.unionation.tag_type;
+                    bool ptr_is_const = true;
+                    bool ptr_is_volatile = false;
+                    return ir_analyze_const_ptr(ira, &field_ptr_instruction->base,
+                            create_const_enum(enum_type, &field->enum_field->value), enum_type,
                             ConstPtrMutComptimeConst, ptr_is_const, ptr_is_volatile);
-                    }
                 }
             }
             ScopeDecls *container_scope = get_container_scope(child_type);
@@ -12163,7 +12122,6 @@ static TypeTableEntry *ir_analyze_instruction_typeof(IrAnalyze *ira, IrInstructi
         case TypeTableEntryIdEnum:
         case TypeTableEntryIdUnion:
         case TypeTableEntryIdFn:
-        case TypeTableEntryIdEnumTag:
         case TypeTableEntryIdArgTuple:
         case TypeTableEntryIdOpaque:
             {
@@ -12511,7 +12469,6 @@ static TypeTableEntry *ir_analyze_instruction_slice_type(IrAnalyze *ira,
         case TypeTableEntryIdFn:
         case TypeTableEntryIdNamespace:
         case TypeTableEntryIdBoundFn:
-        case TypeTableEntryIdEnumTag:
             {
                 type_ensure_zero_bits_known(ira->codegen, child_type);
                 TypeTableEntry *slice_ptr_type = get_pointer_to_type_extra(ira->codegen, child_type,
@@ -12620,7 +12577,6 @@ static TypeTableEntry *ir_analyze_instruction_array_type(IrAnalyze *ira,
         case TypeTableEntryIdFn:
         case TypeTableEntryIdNamespace:
         case TypeTableEntryIdBoundFn:
-        case TypeTableEntryIdEnumTag:
             {
                 TypeTableEntry *result_type = get_array_type(ira->codegen, child_type, size);
                 ConstExprValue *out_val = ir_build_const_from(ira, &array_type_instruction->base);
@@ -12671,7 +12627,6 @@ static TypeTableEntry *ir_analyze_instruction_size_of(IrAnalyze *ira,
         case TypeTableEntryIdPureError:
         case TypeTableEntryIdEnum:
         case TypeTableEntryIdUnion:
-        case TypeTableEntryIdEnumTag:
         case TypeTableEntryIdFn:
             {
                 uint64_t size_in_bytes = type_size(ira->codegen, type_entry);
@@ -12824,17 +12779,22 @@ static TypeTableEntry *ir_analyze_instruction_clz(IrAnalyze *ira, IrInstructionC
     }
 }
 
-static IrInstruction *ir_analyze_enum_tag(IrAnalyze *ira, IrInstruction *source_instr, IrInstruction *value) {
+static IrInstruction *ir_analyze_union_tag(IrAnalyze *ira, IrInstruction *source_instr, IrInstruction *value) {
     if (type_is_invalid(value->value.type))
         return ira->codegen->invalid_instruction;
 
-    if (value->value.type->id != TypeTableEntryIdEnum) {
+    if (value->value.type->id == TypeTableEntryIdEnum) {
+        return value;
+    }
+
+    if (value->value.type->id != TypeTableEntryIdUnion) {
         ir_add_error(ira, source_instr,
-            buf_sprintf("expected enum type, found '%s'", buf_ptr(&value->value.type->name)));
+            buf_sprintf("expected enum or union type, found '%s'", buf_ptr(&value->value.type->name)));
         return ira->codegen->invalid_instruction;
     }
 
-    TypeTableEntry *tag_type = value->value.type->data.enumeration.tag_type;
+    TypeTableEntry *tag_type = value->value.type->data.unionation.tag_type;
+    assert(tag_type->id == TypeTableEntryIdEnum);
 
     if (instr_is_comptime(value)) {
         ConstExprValue *val = ir_resolve_const(ira, value, UndefBad);
@@ -12845,11 +12805,11 @@ static IrInstruction *ir_analyze_enum_tag(IrAnalyze *ira, IrInstruction *source_
                 source_instr->scope, source_instr->source_node);
         const_instruction->base.value.type = tag_type;
         const_instruction->base.value.special = ConstValSpecialStatic;
-        bigint_init_bigint(&const_instruction->base.value.data.x_bigint, &val->data.x_enum.tag);
+        bigint_init_bigint(&const_instruction->base.value.data.x_enum_tag, &val->data.x_union.tag);
         return &const_instruction->base;
     }
 
-    IrInstruction *result = ir_build_enum_tag(&ira->new_irb, source_instr->scope, source_instr->source_node, value);
+    IrInstruction *result = ir_build_union_tag(&ira->new_irb, source_instr->scope, source_instr->source_node, value);
     result->value.type = tag_type;
     return result;
 }
@@ -12880,7 +12840,7 @@ static TypeTableEntry *ir_analyze_instruction_switch_br(IrAnalyze *ira,
                 return ir_unreach_error(ira);
 
             if (case_value->value.type->id == TypeTableEntryIdEnum) {
-                case_value = ir_analyze_enum_tag(ira, &switch_br_instruction->base, case_value);
+                case_value = ir_analyze_union_tag(ira, &switch_br_instruction->base, case_value);
                 if (type_is_invalid(case_value->value.type))
                     return ir_unreach_error(ira);
             }
@@ -12927,7 +12887,7 @@ static TypeTableEntry *ir_analyze_instruction_switch_br(IrAnalyze *ira,
             continue;
 
         if (new_value->value.type->id == TypeTableEntryIdEnum) {
-            new_value = ir_analyze_enum_tag(ira, &switch_br_instruction->base, new_value);
+            new_value = ir_analyze_union_tag(ira, &switch_br_instruction->base, new_value);
             if (type_is_invalid(new_value->value.type))
                 continue;
         }
@@ -13009,34 +12969,54 @@ static TypeTableEntry *ir_analyze_instruction_switch_target(IrAnalyze *ira,
 
             ir_build_load_ptr_from(&ira->new_irb, &switch_target_instruction->base, target_value_ptr);
             return target_type;
-        case TypeTableEntryIdEnum:
-            {
-                TypeTableEntry *tag_type = target_type->data.enumeration.tag_type;
-                assert(tag_type != nullptr);
-                if (pointee_val) {
-                    ConstExprValue *out_val = ir_build_const_from(ira, &switch_target_instruction->base);
-                    bigint_init_bigint(&out_val->data.x_bigint, &pointee_val->data.x_enum.tag);
-                    return tag_type;
-                }
-
-                IrInstruction *enum_value = ir_build_load_ptr(&ira->new_irb, switch_target_instruction->base.scope,
-                    switch_target_instruction->base.source_node, target_value_ptr);
-                enum_value->value.type = target_type;
-                ir_build_enum_tag_from(&ira->new_irb, &switch_target_instruction->base, enum_value);
+        case TypeTableEntryIdUnion: {
+            if (target_type->data.unionation.gen_tag_index == SIZE_MAX) {
+                ErrorMsg *msg = ir_add_error(ira, target_value_ptr,
+                    buf_sprintf("switch on union which has no attached enum"));
+                add_error_note(ira->codegen, msg, target_type->data.unionation.decl_node,
+                        buf_sprintf("union declared here"));
+                return ira->codegen->builtin_types.entry_invalid;
+            }
+            TypeTableEntry *tag_type = target_type->data.unionation.tag_type;
+            assert(tag_type != nullptr);
+            if (pointee_val) {
+                ConstExprValue *out_val = ir_build_const_from(ira, &switch_target_instruction->base);
+                bigint_init_bigint(&out_val->data.x_enum_tag, &pointee_val->data.x_union.tag);
                 return tag_type;
             }
+
+            IrInstruction *union_value = ir_build_load_ptr(&ira->new_irb, switch_target_instruction->base.scope,
+                switch_target_instruction->base.source_node, target_value_ptr);
+            union_value->value.type = target_type;
+
+            IrInstruction *union_tag_inst = ir_build_union_tag(&ira->new_irb, switch_target_instruction->base.scope,
+                    switch_target_instruction->base.source_node, union_value);
+            union_tag_inst->value.type = tag_type;
+            ir_link_new_instruction(union_tag_inst, &switch_target_instruction->base);
+            return tag_type;
+        }
+        case TypeTableEntryIdEnum: {
+            if (pointee_val) {
+                ConstExprValue *out_val = ir_build_const_from(ira, &switch_target_instruction->base);
+                bigint_init_bigint(&out_val->data.x_enum_tag, &pointee_val->data.x_enum_tag);
+                return target_type;
+            }
+
+            IrInstruction *enum_value = ir_build_load_ptr(&ira->new_irb, switch_target_instruction->base.scope,
+                switch_target_instruction->base.source_node, target_value_ptr);
+            enum_value->value.type = target_type;
+            ir_link_new_instruction(enum_value, &switch_target_instruction->base);
+            return target_type;
+        }
         case TypeTableEntryIdErrorUnion:
-            // see https://github.com/andrewrk/zig/issues/83
+            // see https://github.com/andrewrk/zig/issues/632
             zig_panic("TODO switch on error union");
-        case TypeTableEntryIdEnumTag:
-            zig_panic("TODO switch on enum tag type");
         case TypeTableEntryIdUnreachable:
         case TypeTableEntryIdArray:
         case TypeTableEntryIdStruct:
         case TypeTableEntryIdUndefLit:
         case TypeTableEntryIdNullLit:
         case TypeTableEntryIdMaybe:
-        case TypeTableEntryIdUnion:
         case TypeTableEntryIdBlock:
         case TypeTableEntryIdBoundFn:
         case TypeTableEntryIdArgTuple:
@@ -13059,19 +13039,13 @@ static TypeTableEntry *ir_analyze_instruction_switch_var(IrAnalyze *ira, IrInstr
 
     assert(target_value_ptr->value.type->id == TypeTableEntryIdPointer);
     TypeTableEntry *target_type = target_value_ptr->value.type->data.pointer.child_type;
-    if (target_type->id == TypeTableEntryIdEnum) {
+    if (target_type->id == TypeTableEntryIdUnion) {
         ConstExprValue *prong_val = ir_resolve_const(ira, prong_value, UndefBad);
         if (!prong_val)
             return ira->codegen->builtin_types.entry_invalid;
 
-        TypeEnumField *field;
-        if (prong_value->value.type->id == TypeTableEntryIdEnumTag) {
-            field = find_enum_field_by_tag(target_type, &prong_val->data.x_bigint);
-        } else if (prong_value->value.type->id == TypeTableEntryIdEnum) {
-            field = find_enum_field_by_tag(target_type, &prong_val->data.x_enum.tag);
-        } else {
-            zig_unreachable();
-        }
+        assert(prong_value->value.type->id == TypeTableEntryIdEnum);
+        TypeUnionField *field = find_union_field_by_tag(target_type, &prong_val->data.x_enum_tag);
 
         if (instr_is_comptime(target_value_ptr)) {
             ConstExprValue *target_val_ptr = ir_resolve_const(ira, target_value_ptr, UndefBad);
@@ -13082,11 +13056,11 @@ static TypeTableEntry *ir_analyze_instruction_switch_var(IrAnalyze *ira, IrInstr
             ConstExprValue *out_val = ir_build_const_from(ira, &instruction->base);
             out_val->data.x_ptr.special = ConstPtrSpecialRef;
             out_val->data.x_ptr.mut = target_val_ptr->data.x_ptr.mut;
-            out_val->data.x_ptr.data.ref.pointee = pointee_val->data.x_enum.payload;
+            out_val->data.x_ptr.data.ref.pointee = pointee_val->data.x_union.payload;
             return get_pointer_to_type(ira->codegen, field->type_entry, target_val_ptr->type->data.pointer.is_const);
         }
 
-        ir_build_enum_field_ptr_from(&ira->new_irb, &instruction->base, target_value_ptr, field);
+        ir_build_union_field_ptr_from(&ira->new_irb, &instruction->base, target_value_ptr, field);
         return get_pointer_to_type(ira->codegen, field->type_entry,
                 target_value_ptr->value.type->data.pointer.is_const);
     } else {
@@ -13096,10 +13070,10 @@ static TypeTableEntry *ir_analyze_instruction_switch_var(IrAnalyze *ira, IrInstr
     }
 }
 
-static TypeTableEntry *ir_analyze_instruction_enum_tag(IrAnalyze *ira, IrInstructionEnumTag *enum_tag_instruction) {
-    IrInstruction *value = enum_tag_instruction->value->other;
-    IrInstruction *new_instruction = ir_analyze_enum_tag(ira, &enum_tag_instruction->base, value);
-    ir_link_new_instruction(new_instruction, &enum_tag_instruction->base);
+static TypeTableEntry *ir_analyze_instruction_union_tag(IrAnalyze *ira, IrInstructionUnionTag *instruction) {
+    IrInstruction *value = instruction->value->other;
+    IrInstruction *new_instruction = ir_analyze_union_tag(ira, &instruction->base, value);
+    ir_link_new_instruction(new_instruction, &instruction->base);
     return new_instruction->value.type;
 }
 
@@ -13255,7 +13229,7 @@ static TypeTableEntry *ir_analyze_container_init_fields_union(IrAnalyze *ira, Ir
 
         ConstExprValue *out_val = ir_build_const_from(ira, instruction);
         out_val->data.x_union.payload = field_val;
-        out_val->data.x_union.tag = type_field->value;
+        out_val->data.x_union.tag = type_field->enum_field->value;
 
         ConstParent *parent = get_const_val_parent(ira->codegen, field_val);
         if (parent != nullptr) {
@@ -13502,46 +13476,9 @@ static TypeTableEntry *ir_analyze_instruction_container_init_list(IrAnalyze *ira
                     buf_ptr(&container_type->name)));
             return ira->codegen->builtin_types.entry_invalid;
         }
-    } else if (container_type_value->value.type->id == TypeTableEntryIdEnumTag) {
-        if (elem_count != 1) {
-            ir_add_error(ira, &instruction->base, buf_sprintf("enum initialization requires exactly one element"));
-            return ira->codegen->builtin_types.entry_invalid;
-        }
-        ConstExprValue *tag_value = ir_resolve_const(ira, container_type_value, UndefBad);
-        if (!tag_value)
-            return ira->codegen->builtin_types.entry_invalid;
-
-        TypeTableEntry *enum_type = container_type_value->value.type->data.enum_tag.enum_type;
-
-        TypeEnumField *field = find_enum_field_by_tag(enum_type, &tag_value->data.x_bigint);
-        assert(field != nullptr);
-        TypeTableEntry *this_field_type = field->type_entry;
-
-        IrInstruction *init_value = instruction->items[0]->other;
-        if (type_is_invalid(init_value->value.type))
-            return ira->codegen->builtin_types.entry_invalid;
-
-        IrInstruction *casted_init_value = ir_implicit_cast(ira, init_value, this_field_type);
-        if (casted_init_value == ira->codegen->invalid_instruction)
-            return ira->codegen->builtin_types.entry_invalid;
-
-        if (instr_is_comptime(casted_init_value)) {
-            ConstExprValue *init_val = ir_resolve_const(ira, casted_init_value, UndefOk);
-            if (!init_val)
-                return ira->codegen->builtin_types.entry_invalid;
-            ConstExprValue *out_val = ir_build_const_from(ira, &instruction->base);
-            bigint_init_bigint(&out_val->data.x_enum.tag, &tag_value->data.x_bigint);
-            out_val->data.x_enum.payload = init_val;
-            return enum_type;
-        }
-
-        IrInstruction *new_instruction = ir_build_init_enum_from(&ira->new_irb, &instruction->base,
-                enum_type, field, casted_init_value);
-        ir_add_alloca(ira, new_instruction, enum_type);
-        return enum_type;
     } else {
         ir_add_error(ira, container_type_value,
-            buf_sprintf("expected type, found '%s'", buf_ptr(&container_type_value->value.type->name)));
+            buf_sprintf("expected type, found '%s' value", buf_ptr(&container_type_value->value.type->name)));
         return ira->codegen->builtin_types.entry_invalid;
     }
 }
@@ -13584,8 +13521,8 @@ static TypeTableEntry *ir_analyze_min_max(IrAnalyze *ira, IrInstruction *source_
                 eval_min_max_value(ira->codegen, target_type, out_val, is_max);
                 return target_type;
             }
-        case TypeTableEntryIdEnumTag:
-            zig_panic("TODO min/max value for enum tag type");
+        case TypeTableEntryIdEnum:
+            zig_panic("TODO min/max value for enum type");
         case TypeTableEntryIdVar:
         case TypeTableEntryIdMetaType:
         case TypeTableEntryIdUnreachable:
@@ -13599,7 +13536,6 @@ static TypeTableEntry *ir_analyze_min_max(IrAnalyze *ira, IrInstruction *source_
         case TypeTableEntryIdMaybe:
         case TypeTableEntryIdErrorUnion:
         case TypeTableEntryIdPureError:
-        case TypeTableEntryIdEnum:
         case TypeTableEntryIdUnion:
         case TypeTableEntryIdFn:
         case TypeTableEntryIdNamespace:
@@ -13707,20 +13643,18 @@ static TypeTableEntry *ir_analyze_instruction_enum_tag_name(IrAnalyze *ira, IrIn
     if (type_is_invalid(target->value.type))
         return ira->codegen->builtin_types.entry_invalid;
 
-    assert(target->value.type->id == TypeTableEntryIdEnumTag);
+    assert(target->value.type->id == TypeTableEntryIdEnum);
 
     if (instr_is_comptime(target)) {
-        TypeTableEntry *enum_type = target->value.type->data.enum_tag.enum_type;
-        uint64_t tag_value = bigint_as_unsigned(&target->value.data.x_bigint);
-        TypeEnumField *field = &enum_type->data.enumeration.fields[tag_value];
+        TypeEnumField *field = find_enum_field_by_tag(target->value.type, &target->value.data.x_bigint);
         ConstExprValue *array_val = create_const_str_lit(ira->codegen, field->name);
         ConstExprValue *out_val = ir_build_const_from(ira, &instruction->base);
         init_const_slice(ira->codegen, out_val, array_val, 0, buf_len(field->name), true);
         return out_val->type;
     }
 
-    if (!target->value.type->data.enum_tag.generate_name_table) {
-        target->value.type->data.enum_tag.generate_name_table = true;
+    if (!target->value.type->data.enumeration.generate_name_table) {
+        target->value.type->data.enumeration.generate_name_table = true;
         ira->codegen->name_table_enums.append(target->value.type);
     }
 
@@ -13869,7 +13803,7 @@ static TypeTableEntry *ir_analyze_instruction_type_id(IrAnalyze *ira,
     TypeTableEntry *result_type = var_value->data.x_type;
 
     ConstExprValue *out_val = ir_build_const_from(ira, &instruction->base);
-    bigint_init_unsigned(&out_val->data.x_enum.tag, type_id_index(type_entry->id));
+    bigint_init_unsigned(&out_val->data.x_enum_tag, type_id_index(type_entry->id));
     return result_type;
 }
 
@@ -14698,14 +14632,14 @@ static TypeTableEntry *ir_analyze_instruction_member_type(IrAnalyze *ira, IrInst
         ConstExprValue *out_val = ir_build_const_from(ira, &instruction->base);
         out_val->data.x_type = field->type_entry;
         return ira->codegen->builtin_types.entry_type;
-    } else if (container_type->id == TypeTableEntryIdEnum) {
-        if (member_index >= container_type->data.enumeration.src_field_count) {
+    } else if (container_type->id == TypeTableEntryIdUnion) {
+        if (member_index >= container_type->data.unionation.src_field_count) {
             ir_add_error(ira, index_value,
                 buf_sprintf("member index %" ZIG_PRI_u64 " out of bounds; '%s' has %" PRIu32 " members",
-                    member_index, buf_ptr(&container_type->name), container_type->data.enumeration.src_field_count));
+                    member_index, buf_ptr(&container_type->name), container_type->data.unionation.src_field_count));
             return ira->codegen->builtin_types.entry_invalid;
         }
-        TypeEnumField *field = &container_type->data.enumeration.fields[member_index];
+        TypeUnionField *field = &container_type->data.unionation.fields[member_index];
 
         ConstExprValue *out_val = ir_build_const_from(ira, &instruction->base);
         out_val->data.x_type = field->type_entry;
@@ -14749,6 +14683,18 @@ static TypeTableEntry *ir_analyze_instruction_member_name(IrAnalyze *ira, IrInst
         }
         TypeEnumField *field = &container_type->data.enumeration.fields[member_index];
 
+        ConstExprValue *out_val = ir_build_const_from(ira, &instruction->base);
+        init_const_str_lit(ira->codegen, out_val, field->name);
+        return out_val->type;
+    } else if (container_type->id == TypeTableEntryIdUnion) {
+        if (member_index >= container_type->data.unionation.src_field_count) {
+            ir_add_error(ira, index_value,
+                buf_sprintf("member index %" ZIG_PRI_u64 " out of bounds; '%s' has %" PRIu32 " members",
+                    member_index, buf_ptr(&container_type->name), container_type->data.unionation.src_field_count));
+            return ira->codegen->builtin_types.entry_invalid;
+        }
+        TypeUnionField *field = &container_type->data.unionation.fields[member_index];
+
         ConstExprValue *out_val = ir_build_const_from(ira, &instruction->base);
         init_const_str_lit(ira->codegen, out_val, field->name);
         return out_val->type;
@@ -14819,7 +14765,6 @@ static TypeTableEntry *ir_analyze_instruction_align_of(IrAnalyze *ira, IrInstruc
         case TypeTableEntryIdErrorUnion:
         case TypeTableEntryIdPureError:
         case TypeTableEntryIdEnum:
-        case TypeTableEntryIdEnumTag:
         case TypeTableEntryIdUnion:
         case TypeTableEntryIdFn:
             {
@@ -15125,10 +15070,9 @@ static TypeTableEntry *ir_analyze_instruction_check_switch_prongs(IrAnalyze *ira
     if (type_is_invalid(switch_type))
         return ira->codegen->builtin_types.entry_invalid;
 
-    if (switch_type->id == TypeTableEntryIdEnumTag) {
-        TypeTableEntry *enum_type = switch_type->data.enum_tag.enum_type;
+    if (switch_type->id == TypeTableEntryIdEnum) {
         HashMap<BigInt, AstNode *, bigint_hash, bigint_eql> field_prev_uses = {};
-        field_prev_uses.init(enum_type->data.enumeration.src_field_count);
+        field_prev_uses.init(switch_type->data.enumeration.src_field_count);
 
         for (size_t range_i = 0; range_i < instruction->range_count; range_i += 1) {
             IrInstructionCheckSwitchProngsRange *range = &instruction->ranges[range_i];
@@ -15141,22 +15085,13 @@ static TypeTableEntry *ir_analyze_instruction_check_switch_prongs(IrAnalyze *ira
             if (type_is_invalid(end_value->value.type))
                 return ira->codegen->builtin_types.entry_invalid;
 
+            assert(start_value->value.type->id == TypeTableEntryIdEnum);
             BigInt start_index;
+            bigint_init_bigint(&start_index, &start_value->value.data.x_enum_tag);
+
+            assert(end_value->value.type->id == TypeTableEntryIdEnum);
             BigInt end_index;
-            if (start_value->value.type->id == TypeTableEntryIdEnumTag) {
-                bigint_init_bigint(&start_index, &start_value->value.data.x_bigint);
-            } else if (start_value->value.type->id == TypeTableEntryIdEnum) {
-                bigint_init_bigint(&start_index, &start_value->value.data.x_enum.tag);
-            } else {
-                zig_unreachable();
-            }
-            if (end_value->value.type->id == TypeTableEntryIdEnumTag) {
-                bigint_init_bigint(&end_index, &end_value->value.data.x_bigint);
-            } else if (end_value->value.type->id == TypeTableEntryIdEnum) {
-                bigint_init_bigint(&end_index, &end_value->value.data.x_enum.tag);
-            } else {
-                zig_unreachable();
-            }
+            bigint_init_bigint(&end_index, &end_value->value.data.x_enum_tag);
 
             BigInt field_index;
             bigint_init_bigint(&field_index, &start_index);
@@ -15168,10 +15103,10 @@ static TypeTableEntry *ir_analyze_instruction_check_switch_prongs(IrAnalyze *ira
                 auto entry = field_prev_uses.put_unique(field_index, start_value->source_node);
                 if (entry) {
                     AstNode *prev_node = entry->value;
-                    TypeEnumField *enum_field = find_enum_field_by_tag(enum_type, &field_index);
+                    TypeEnumField *enum_field = find_enum_field_by_tag(switch_type, &field_index);
                     assert(enum_field != nullptr);
                     ErrorMsg *msg = ir_add_error(ira, start_value,
-                        buf_sprintf("duplicate switch value: '%s.%s'", buf_ptr(&enum_type->name),
+                        buf_sprintf("duplicate switch value: '%s.%s'", buf_ptr(&switch_type->name),
                             buf_ptr(enum_field->name)));
                     add_error_note(ira->codegen, msg, prev_node, buf_sprintf("other value is here"));
                 }
@@ -15179,13 +15114,13 @@ static TypeTableEntry *ir_analyze_instruction_check_switch_prongs(IrAnalyze *ira
             }
         }
         if (!instruction->have_else_prong) {
-            for (uint32_t i = 0; i < enum_type->data.enumeration.src_field_count; i += 1) {
-                TypeEnumField *enum_field = &enum_type->data.enumeration.fields[i];
+            for (uint32_t i = 0; i < switch_type->data.enumeration.src_field_count; i += 1) {
+                TypeEnumField *enum_field = &switch_type->data.enumeration.fields[i];
 
                 auto entry = field_prev_uses.maybe_get(enum_field->value);
                 if (!entry) {
                     ir_add_error(ira, &instruction->base,
-                        buf_sprintf("enumeration value '%s.%s' not handled in switch", buf_ptr(&enum_type->name),
+                        buf_sprintf("enumeration value '%s.%s' not handled in switch", buf_ptr(&switch_type->name),
                             buf_ptr(enum_field->name)));
                 }
             }
@@ -15481,8 +15416,6 @@ static void buf_write_value_bytes(CodeGen *codegen, uint8_t *buf, ConstExprValue
             zig_panic("TODO buf_write_value_bytes pure error type");
         case TypeTableEntryIdEnum:
             zig_panic("TODO buf_write_value_bytes enum type");
-        case TypeTableEntryIdEnumTag:
-            zig_panic("TODO buf_write_value_bytes enum tag type");
         case TypeTableEntryIdFn:
             zig_panic("TODO buf_write_value_bytes fn type");
         case TypeTableEntryIdUnion:
@@ -15541,8 +15474,6 @@ static void buf_read_value_bytes(CodeGen *codegen, uint8_t *buf, ConstExprValue
             zig_panic("TODO buf_read_value_bytes pure error type");
         case TypeTableEntryIdEnum:
             zig_panic("TODO buf_read_value_bytes enum type");
-        case TypeTableEntryIdEnumTag:
-            zig_panic("TODO buf_read_value_bytes enum tag type");
         case TypeTableEntryIdFn:
             zig_panic("TODO buf_read_value_bytes fn type");
         case TypeTableEntryIdUnion:
@@ -15920,11 +15851,8 @@ static TypeTableEntry *ir_analyze_instruction_enum_tag_type(IrAnalyze *ira, IrIn
     if (type_is_invalid(enum_type))
         return ira->codegen->builtin_types.entry_invalid;
 
-    TypeTableEntry *non_int_tag_type = enum_type->data.enumeration.tag_type;
-    assert(non_int_tag_type->id == TypeTableEntryIdEnumTag);
-
     ConstExprValue *out_val = ir_build_const_from(ira, &instruction->base);
-    out_val->data.x_type = non_int_tag_type->data.enum_tag.int_type;
+    out_val->data.x_type = enum_type->data.enumeration.tag_int_type;
     return ira->codegen->builtin_types.entry_type;
 }
 
@@ -15938,9 +15866,7 @@ static TypeTableEntry *ir_analyze_instruction_nocast(IrAnalyze *ira, IrInstructi
         case IrInstructionIdStructInit:
         case IrInstructionIdUnionInit:
         case IrInstructionIdStructFieldPtr:
-        case IrInstructionIdEnumFieldPtr:
         case IrInstructionIdUnionFieldPtr:
-        case IrInstructionIdInitEnum:
         case IrInstructionIdMaybeWrap:
         case IrInstructionIdErrWrapCode:
         case IrInstructionIdErrWrapPayload:
@@ -16012,8 +15938,8 @@ static TypeTableEntry *ir_analyze_instruction_nocast(IrAnalyze *ira, IrInstructi
             return ir_analyze_instruction_switch_target(ira, (IrInstructionSwitchTarget *)instruction);
         case IrInstructionIdSwitchVar:
             return ir_analyze_instruction_switch_var(ira, (IrInstructionSwitchVar *)instruction);
-        case IrInstructionIdEnumTag:
-            return ir_analyze_instruction_enum_tag(ira, (IrInstructionEnumTag *)instruction);
+        case IrInstructionIdUnionTag:
+            return ir_analyze_instruction_union_tag(ira, (IrInstructionUnionTag *)instruction);
         case IrInstructionIdImport:
             return ir_analyze_instruction_import(ira, (IrInstructionImport *)instruction);
         case IrInstructionIdArrayLen:
@@ -16260,7 +16186,6 @@ bool ir_has_side_effects(IrInstruction *instruction) {
         case IrInstructionIdPtrTypeChild:
         case IrInstructionIdArrayLen:
         case IrInstructionIdStructFieldPtr:
-        case IrInstructionIdEnumFieldPtr:
         case IrInstructionIdUnionFieldPtr:
         case IrInstructionIdArrayType:
         case IrInstructionIdSliceType:
@@ -16271,7 +16196,7 @@ bool ir_has_side_effects(IrInstruction *instruction) {
         case IrInstructionIdCtz:
         case IrInstructionIdSwitchVar:
         case IrInstructionIdSwitchTarget:
-        case IrInstructionIdEnumTag:
+        case IrInstructionIdUnionTag:
         case IrInstructionIdRef:
         case IrInstructionIdMinValue:
         case IrInstructionIdMaxValue:
@@ -16293,7 +16218,6 @@ bool ir_has_side_effects(IrInstruction *instruction) {
         case IrInstructionIdErrWrapPayload:
         case IrInstructionIdFnProto:
         case IrInstructionIdTestComptime:
-        case IrInstructionIdInitEnum:
         case IrInstructionIdPtrCast:
         case IrInstructionIdBitCast:
         case IrInstructionIdWidenOrShorten:
diff --git a/src/ir_print.cpp b/src/ir_print.cpp
index 0e06d1b563..99617056de 100644
--- a/src/ir_print.cpp
+++ b/src/ir_print.cpp
@@ -291,7 +291,7 @@ static void ir_print_struct_init(IrPrint *irp, IrInstructionStructInit *instruct
 }
 
 static void ir_print_union_init(IrPrint *irp, IrInstructionUnionInit *instruction) {
-    Buf *field_name = instruction->field->name;
+    Buf *field_name = instruction->field->enum_field->name;
 
     fprintf(irp->f, "%s {", buf_ptr(&instruction->union_type->name));
     fprintf(irp->f, ".%s = ", buf_ptr(field_name));
@@ -361,17 +361,10 @@ static void ir_print_struct_field_ptr(IrPrint *irp, IrInstructionStructFieldPtr
     fprintf(irp->f, ")");
 }
 
-static void ir_print_enum_field_ptr(IrPrint *irp, IrInstructionEnumFieldPtr *instruction) {
-    fprintf(irp->f, "@EnumFieldPtr(&");
-    ir_print_other_instruction(irp, instruction->enum_ptr);
-    fprintf(irp->f, ".%s", buf_ptr(instruction->field->name));
-    fprintf(irp->f, ")");
-}
-
 static void ir_print_union_field_ptr(IrPrint *irp, IrInstructionUnionFieldPtr *instruction) {
     fprintf(irp->f, "@UnionFieldPtr(&");
     ir_print_other_instruction(irp, instruction->union_ptr);
-    fprintf(irp->f, ".%s", buf_ptr(instruction->field->name));
+    fprintf(irp->f, ".%s", buf_ptr(instruction->field->enum_field->name));
     fprintf(irp->f, ")");
 }
 
@@ -509,8 +502,8 @@ static void ir_print_switch_target(IrPrint *irp, IrInstructionSwitchTarget *inst
     ir_print_other_instruction(irp, instruction->target_value_ptr);
 }
 
-static void ir_print_enum_tag(IrPrint *irp, IrInstructionEnumTag *instruction) {
-    fprintf(irp->f, "enumtag ");
+static void ir_print_union_tag(IrPrint *irp, IrInstructionUnionTag *instruction) {
+    fprintf(irp->f, "uniontag ");
     ir_print_other_instruction(irp, instruction->value);
 }
 
@@ -799,12 +792,6 @@ static void ir_print_test_comptime(IrPrint *irp, IrInstructionTestComptime *inst
     fprintf(irp->f, ")");
 }
 
-static void ir_print_init_enum(IrPrint *irp, IrInstructionInitEnum *instruction) {
-    fprintf(irp->f, "%s.%s {", buf_ptr(&instruction->enum_type->name), buf_ptr(instruction->field->name));
-    ir_print_other_instruction(irp, instruction->init_value);
-    fprintf(irp->f, "}");
-}
-
 static void ir_print_ptr_cast(IrPrint *irp, IrInstructionPtrCast *instruction) {
     fprintf(irp->f, "@ptrCast(");
     if (instruction->dest_type) {
@@ -1078,9 +1065,6 @@ static void ir_print_instruction(IrPrint *irp, IrInstruction *instruction) {
         case IrInstructionIdStructFieldPtr:
             ir_print_struct_field_ptr(irp, (IrInstructionStructFieldPtr *)instruction);
             break;
-        case IrInstructionIdEnumFieldPtr:
-            ir_print_enum_field_ptr(irp, (IrInstructionEnumFieldPtr *)instruction);
-            break;
         case IrInstructionIdUnionFieldPtr:
             ir_print_union_field_ptr(irp, (IrInstructionUnionFieldPtr *)instruction);
             break;
@@ -1123,8 +1107,8 @@ static void ir_print_instruction(IrPrint *irp, IrInstruction *instruction) {
         case IrInstructionIdSwitchTarget:
             ir_print_switch_target(irp, (IrInstructionSwitchTarget *)instruction);
             break;
-        case IrInstructionIdEnumTag:
-            ir_print_enum_tag(irp, (IrInstructionEnumTag *)instruction);
+        case IrInstructionIdUnionTag:
+            ir_print_union_tag(irp, (IrInstructionUnionTag *)instruction);
             break;
         case IrInstructionIdImport:
             ir_print_import(irp, (IrInstructionImport *)instruction);
@@ -1237,9 +1221,6 @@ static void ir_print_instruction(IrPrint *irp, IrInstruction *instruction) {
         case IrInstructionIdTestComptime:
             ir_print_test_comptime(irp, (IrInstructionTestComptime *)instruction);
             break;
-        case IrInstructionIdInitEnum:
-            ir_print_init_enum(irp, (IrInstructionInitEnum *)instruction);
-            break;
         case IrInstructionIdPtrCast:
             ir_print_ptr_cast(irp, (IrInstructionPtrCast *)instruction);
             break;
diff --git a/src/parser.cpp b/src/parser.cpp
index c36434b521..26ca7da31a 100644
--- a/src/parser.cpp
+++ b/src/parser.cpp
@@ -2377,7 +2377,9 @@ static AstNode *ast_parse_use(ParseContext *pc, size_t *token_index, VisibMod vi
 }
 
 /*
-ContainerDecl = option("extern" | "packed") ("struct" | "union" | ("enum" option(GroupedExpression))) "{" many(ContainerMember) "}"
+ContainerDecl = option("extern" | "packed")
+  ("struct" option(GroupedExpression) | "union" option("enum" option(GroupedExpression) | GroupedExpression) | ("enum" option(GroupedExpression)))
+  "{" many(ContainerMember) "}"
 ContainerMember = (ContainerField | FnDef | GlobalVarDecl)
 ContainerField = Symbol option(":" PrefixOpExpression option("=" PrefixOpExpression ","
 */
@@ -2414,7 +2416,28 @@ static AstNode *ast_parse_container_decl(ParseContext *pc, size_t *token_index,
     AstNode *node = ast_create_node(pc, NodeTypeContainerDecl, first_token);
     node->data.container_decl.layout = layout;
     node->data.container_decl.kind = kind;
-    node->data.container_decl.init_arg_expr = ast_parse_grouped_expr(pc, token_index, false);
+
+    if (kind == ContainerKindUnion) {
+        Token *lparen_token = &pc->tokens->at(*token_index);
+        if (lparen_token->id == TokenIdLParen) {
+            Token *enum_token = &pc->tokens->at(*token_index + 1);
+            if (enum_token->id == TokenIdKeywordEnum) {
+                Token *paren_token = &pc->tokens->at(*token_index + 2);
+                if (paren_token->id == TokenIdLParen) {
+                    node->data.container_decl.auto_enum = true;
+                    *token_index += 2;
+                    node->data.container_decl.init_arg_expr = ast_parse_grouped_expr(pc, token_index, true);
+                    ast_eat_token(pc, token_index, TokenIdRParen);
+                } else if (paren_token->id == TokenIdRParen) {
+                    node->data.container_decl.auto_enum = true;
+                    *token_index += 3;
+                }
+            }
+        }
+    }
+    if (!node->data.container_decl.auto_enum) {
+        node->data.container_decl.init_arg_expr = ast_parse_grouped_expr(pc, token_index, false);
+    }
 
     ast_eat_token(pc, token_index, TokenIdLBrace);
 
@@ -2461,8 +2484,6 @@ static AstNode *ast_parse_container_decl(ParseContext *pc, size_t *token_index,
             if (colon_token->id == TokenIdColon) {
                 *token_index += 1;
                 field_node->data.struct_field.type = ast_parse_prefix_op_expr(pc, token_index, true);
-            } else {
-                field_node->data.struct_field.type = ast_create_void_type_node(pc, colon_token);
             }
             Token *eq_token = &pc->tokens->at(*token_index);
             if (eq_token->id == TokenIdEq) {
diff --git a/std/build.zig b/std/build.zig
index 009295c6ad..9bdc4b3076 100644
--- a/std/build.zig
+++ b/std/build.zig
@@ -69,8 +69,8 @@ pub const Builder = struct {
         used: bool,
     };
 
-    const UserValue = enum {
-        Flag,
+    const UserValue = union(enum) {
+        Flag: void,
         Scalar: []const u8,
         List: ArrayList([]const u8),
     };
@@ -450,7 +450,7 @@ pub const Builder = struct {
     pub fn addUserInputOption(self: &Builder, name: []const u8, value: []const u8) -> bool {
         if (%%self.user_input_options.put(name, UserInputOption {
             .name = name,
-            .value = UserValue.Scalar { value },
+            .value = UserValue { .Scalar = value },
             .used = false,
         })) |*prev_value| {
             // option already exists
@@ -462,7 +462,7 @@ pub const Builder = struct {
                     %%list.append(value);
                     _ = %%self.user_input_options.put(name, UserInputOption {
                         .name = name,
-                        .value = UserValue.List { list },
+                        .value = UserValue { .List = list },
                         .used = false,
                     });
                 },
@@ -471,7 +471,7 @@ pub const Builder = struct {
                     %%list.append(value);
                     _ = %%self.user_input_options.put(name, UserInputOption {
                         .name = name,
-                        .value = UserValue.List { *list },
+                        .value = UserValue { .List = *list },
                         .used = false,
                     });
                 },
@@ -487,7 +487,7 @@ pub const Builder = struct {
     pub fn addUserInputFlag(self: &Builder, name: []const u8) -> bool {
         if (%%self.user_input_options.put(name, UserInputOption {
             .name = name,
-            .value = UserValue.Flag,
+            .value = UserValue {.Flag = {} },
             .used = false,
         })) |*prev_value| {
             switch (prev_value.value) {
@@ -685,8 +685,8 @@ const CrossTarget = struct {
     environ: builtin.Environ,
 };
 
-const Target = enum {
-    Native,
+const Target = union(enum) {
+    Native: void,
     Cross: CrossTarget,
 
     pub fn oFileExt(self: &const Target) -> []const u8 {
@@ -844,7 +844,7 @@ pub const LibExeObjStep = struct {
             .kind = kind,
             .root_src = root_src,
             .name = name,
-            .target = Target.Native,
+            .target = Target { .Native = {} },
             .linker_script = null,
             .link_libs = BufSet.init(builder.allocator),
             .frameworks = BufSet.init(builder.allocator),
@@ -879,7 +879,7 @@ pub const LibExeObjStep = struct {
             .kind = kind,
             .version = *version,
             .static = static,
-            .target = Target.Native,
+            .target = Target { .Native = {} },
             .cflags = ArrayList([]const u8).init(builder.allocator),
             .source_files = ArrayList([]const u8).init(builder.allocator),
             .object_files = ArrayList([]const u8).init(builder.allocator),
@@ -948,8 +948,8 @@ pub const LibExeObjStep = struct {
     pub fn setTarget(self: &LibExeObjStep, target_arch: builtin.Arch, target_os: builtin.Os,
         target_environ: builtin.Environ)
     {
-        self.target = Target.Cross {
-            CrossTarget {
+        self.target = Target {
+            .Cross = CrossTarget {
                 .arch = target_arch,
                 .os = target_os,
                 .environ = target_environ,
@@ -1186,13 +1186,13 @@ pub const LibExeObjStep = struct {
             Target.Native => {},
             Target.Cross => |cross_target| {
                 %%zig_args.append("--target-arch");
-                %%zig_args.append(@enumTagName(cross_target.arch));
+                %%zig_args.append(@tagName(cross_target.arch));
 
                 %%zig_args.append("--target-os");
-                %%zig_args.append(@enumTagName(cross_target.os));
+                %%zig_args.append(@tagName(cross_target.os));
 
                 %%zig_args.append("--target-environ");
-                %%zig_args.append(@enumTagName(cross_target.environ));
+                %%zig_args.append(@tagName(cross_target.environ));
             },
         }
 
@@ -1553,7 +1553,7 @@ pub const TestStep = struct {
             .name_prefix = "",
             .filter = null,
             .link_libs = BufSet.init(builder.allocator),
-            .target = Target.Native,
+            .target = Target { .Native = {} },
             .exec_cmd_args = null,
         }
     }
@@ -1581,8 +1581,8 @@ pub const TestStep = struct {
     pub fn setTarget(self: &TestStep, target_arch: builtin.Arch, target_os: builtin.Os,
         target_environ: builtin.Environ)
     {
-        self.target = Target.Cross {
-            CrossTarget {
+        self.target = Target {
+            .Cross = CrossTarget {
                 .arch = target_arch,
                 .os = target_os,
                 .environ = target_environ,
@@ -1620,13 +1620,13 @@ pub const TestStep = struct {
             Target.Native => {},
             Target.Cross => |cross_target| {
                 %%zig_args.append("--target-arch");
-                %%zig_args.append(@enumTagName(cross_target.arch));
+                %%zig_args.append(@tagName(cross_target.arch));
 
                 %%zig_args.append("--target-os");
-                %%zig_args.append(@enumTagName(cross_target.os));
+                %%zig_args.append(@tagName(cross_target.os));
 
                 %%zig_args.append("--target-environ");
-                %%zig_args.append(@enumTagName(cross_target.environ));
+                %%zig_args.append(@tagName(cross_target.environ));
             },
         }
 
diff --git a/std/debug.zig b/std/debug.zig
index 50322024c3..5bfa436614 100644
--- a/std/debug.zig
+++ b/std/debug.zig
@@ -280,7 +280,7 @@ const AbbrevAttr = struct {
     form_id: u64,
 };
 
-const FormValue = enum {
+const FormValue = union(enum) {
     Address: u64,
     Block: []u8,
     Const: Constant,
@@ -475,7 +475,7 @@ fn readAllocBytes(allocator: &mem.Allocator, in_stream: &io.InStream, size: usiz
 
 fn parseFormValueBlockLen(allocator: &mem.Allocator, in_stream: &io.InStream, size: usize) -> %FormValue {
     const buf = %return readAllocBytes(allocator, in_stream, size);
-    return FormValue.Block { buf };
+    return FormValue { .Block = buf };
 }
 
 fn parseFormValueBlock(allocator: &mem.Allocator, in_stream: &io.InStream, size: usize) -> %FormValue {
@@ -484,7 +484,7 @@ fn parseFormValueBlock(allocator: &mem.Allocator, in_stream: &io.InStream, size:
 }
 
 fn parseFormValueConstant(allocator: &mem.Allocator, in_stream: &io.InStream, signed: bool, size: usize) -> %FormValue {
-    FormValue.Const { Constant {
+    FormValue { .Const = Constant {
         .signed = signed,
         .payload = %return readAllocBytes(allocator, in_stream, size),
     }}
@@ -510,7 +510,7 @@ fn parseFormValueTargetAddrSize(in_stream: &io.InStream) -> %u64 {
 
 fn parseFormValueRefLen(allocator: &mem.Allocator, in_stream: &io.InStream, size: usize) -> %FormValue {
     const buf = %return readAllocBytes(allocator, in_stream, size);
-    return FormValue.Ref { buf };
+    return FormValue { .Ref = buf };
 }
 
 fn parseFormValueRef(allocator: &mem.Allocator, in_stream: &io.InStream, comptime T: type) -> %FormValue {
@@ -520,7 +520,7 @@ fn parseFormValueRef(allocator: &mem.Allocator, in_stream: &io.InStream, comptim
 
 fn parseFormValue(allocator: &mem.Allocator, in_stream: &io.InStream, form_id: u64, is_64: bool) -> %FormValue {
     return switch (form_id) {
-        DW.FORM_addr => FormValue.Address { %return parseFormValueTargetAddrSize(in_stream) },
+        DW.FORM_addr => FormValue { .Address = %return parseFormValueTargetAddrSize(in_stream) },
         DW.FORM_block1 => parseFormValueBlock(allocator, in_stream, 1),
         DW.FORM_block2 => parseFormValueBlock(allocator, in_stream, 2),
         DW.FORM_block4 => parseFormValueBlock(allocator, in_stream, 4),
@@ -540,13 +540,11 @@ fn parseFormValue(allocator: &mem.Allocator, in_stream: &io.InStream, form_id: u
         DW.FORM_exprloc => {
             const size = %return readULeb128(in_stream);
             const buf = %return readAllocBytes(allocator, in_stream, size);
-            return FormValue.ExprLoc { buf };
-        },
-        DW.FORM_flag => FormValue.Flag { (%return in_stream.readByte()) != 0 },
-        DW.FORM_flag_present => FormValue.Flag { true },
-        DW.FORM_sec_offset => FormValue.SecOffset {
-            %return parseFormValueDwarfOffsetSize(in_stream, is_64)
+            return FormValue { .ExprLoc = buf };
         },
+        DW.FORM_flag => FormValue { .Flag = (%return in_stream.readByte()) != 0 },
+        DW.FORM_flag_present => FormValue { .Flag = true },
+        DW.FORM_sec_offset => FormValue { .SecOffset = %return parseFormValueDwarfOffsetSize(in_stream, is_64) },
 
         DW.FORM_ref1 => parseFormValueRef(allocator, in_stream, u8),
         DW.FORM_ref2 => parseFormValueRef(allocator, in_stream, u16),
@@ -557,11 +555,11 @@ fn parseFormValue(allocator: &mem.Allocator, in_stream: &io.InStream, form_id: u
             parseFormValueRefLen(allocator, in_stream, ref_len)
         },
 
-        DW.FORM_ref_addr => FormValue.RefAddr { %return parseFormValueDwarfOffsetSize(in_stream, is_64) },
-        DW.FORM_ref_sig8 => FormValue.RefSig8 { %return in_stream.readIntLe(u64) },
+        DW.FORM_ref_addr => FormValue { .RefAddr = %return parseFormValueDwarfOffsetSize(in_stream, is_64) },
+        DW.FORM_ref_sig8 => FormValue { .RefSig8 = %return in_stream.readIntLe(u64) },
 
-        DW.FORM_string => FormValue.String { %return readStringRaw(allocator, in_stream) },
-        DW.FORM_strp => FormValue.StrPtr { %return parseFormValueDwarfOffsetSize(in_stream, is_64) },
+        DW.FORM_string => FormValue { .String = %return readStringRaw(allocator, in_stream) },
+        DW.FORM_strp => FormValue { .StrPtr = %return parseFormValueDwarfOffsetSize(in_stream, is_64) },
         DW.FORM_indirect => {
             const child_form_id = %return readULeb128(in_stream);
             parseFormValue(allocator, in_stream, child_form_id, is_64)
diff --git a/std/os/child_process.zig b/std/os/child_process.zig
index 005d9772e4..3a4c9410c5 100644
--- a/std/os/child_process.zig
+++ b/std/os/child_process.zig
@@ -58,7 +58,7 @@ pub const ChildProcess = struct {
     err_pipe: if (is_windows) void else [2]i32,
     llnode: if (is_windows) void else LinkedList(&ChildProcess).Node,
 
-    pub const Term = enum {
+    pub const Term = union(enum) {
         Exited: i32,
         Signal: i32,
         Stopped: i32,
@@ -281,13 +281,13 @@ pub const ChildProcess = struct {
 
     fn statusToTerm(status: i32) -> Term {
         return if (posix.WIFEXITED(status)) {
-            Term.Exited { posix.WEXITSTATUS(status) }
+            Term { .Exited = posix.WEXITSTATUS(status) }
         } else if (posix.WIFSIGNALED(status)) {
-            Term.Signal { posix.WTERMSIG(status) }
+            Term { .Signal = posix.WTERMSIG(status) }
         } else if (posix.WIFSTOPPED(status)) {
-            Term.Stopped { posix.WSTOPSIG(status) }
+            Term { .Stopped = posix.WSTOPSIG(status) }
         } else {
-            Term.Unknown { status }
+            Term { .Unknown = status }
         };
     }
 
diff --git a/std/os/path.zig b/std/os/path.zig
index a372b5b077..3fd7b5e2db 100644
--- a/std/os/path.zig
+++ b/std/os/path.zig
@@ -1016,7 +1016,7 @@ pub fn real(allocator: &Allocator, pathname: []const u8) -> %[]u8 {
 
             return os.readLink(allocator, proc_path);
         },
-        else => @compileError("TODO implement os.path.real for " ++ @enumTagName(builtin.os)),
+        else => @compileError("TODO implement os.path.real for " ++ @tagName(builtin.os)),
     }
 }
 
diff --git a/test/cases/bugs/394.zig b/test/cases/bugs/394.zig
index aaa7de6f88..071619d59c 100644
--- a/test/cases/bugs/394.zig
+++ b/test/cases/bugs/394.zig
@@ -1,9 +1,9 @@
-const E = enum { A: [9]u8, B: u64, };
+const E = union(enum) { A: [9]u8, B: u64, };
 const S = struct { x: u8, y: E, };
 
 const assert = @import("std").debug.assert;
 
 test "bug 394 fixed" {
-    const x = S { .x = 3, .y = E.B {1} };
+    const x = S { .x = 3, .y = E {.B = 1 } };
     assert(x.x == 3);
 }
diff --git a/test/cases/enum.zig b/test/cases/enum.zig
index eda3cf6376..f3240045df 100644
--- a/test/cases/enum.zig
+++ b/test/cases/enum.zig
@@ -2,8 +2,8 @@ const assert = @import("std").debug.assert;
 const mem = @import("std").mem;
 
 test "enum type" {
-    const foo1 = Foo.One {13};
-    const foo2 = Foo.Two { Point { .x = 1234, .y = 5678, }};
+    const foo1 = Foo{ .One = 13};
+    const foo2 = Foo{. Two = Point { .x = 1234, .y = 5678, }};
     const bar = Bar.B;
 
     assert(bar == Bar.B);
@@ -24,12 +24,12 @@ const Point = struct {
     x: u64,
     y: u64,
 };
-const Foo = enum {
+const Foo = union(enum) {
     One: i32,
     Two: Point,
     Three: void,
 };
-const FooNoVoid = enum {
+const FooNoVoid = union(enum) {
     One: i32,
     Two: Point,
 };
@@ -41,13 +41,13 @@ const Bar = enum {
 };
 
 fn returnAnInt(x: i32) -> Foo {
-    Foo.One { x }
+    Foo { .One = x }
 }
 
 
 test "constant enum with payload" {
-    var empty = AnEnumWithPayload.Empty;
-    var full = AnEnumWithPayload.Full {13};
+    var empty = AnEnumWithPayload {.Empty = {}};
+    var full = AnEnumWithPayload {.Full = 13};
     shouldBeEmpty(empty);
     shouldBeNotEmpty(full);
 }
@@ -66,8 +66,8 @@ fn shouldBeNotEmpty(x: &const AnEnumWithPayload) {
     }
 }
 
-const AnEnumWithPayload = enum {
-    Empty,
+const AnEnumWithPayload = union(enum) {
+    Empty: void,
     Full: i32,
 };
 
@@ -109,13 +109,13 @@ const IntToEnumNumber = enum {
 };
 
 
-test "@enumTagName" {
+test "@tagName" {
     assert(mem.eql(u8, testEnumTagNameBare(BareNumber.Three), "Three"));
     comptime assert(mem.eql(u8, testEnumTagNameBare(BareNumber.Three), "Three"));
 }
 
 fn testEnumTagNameBare(n: BareNumber) -> []const u8 {
-    return @enumTagName(n);
+    return @tagName(n);
 }
 
 const BareNumber = enum {
@@ -132,7 +132,7 @@ test "enum alignment" {
     }
 }
 
-const AlignTestEnum = enum {
+const AlignTestEnum = union(enum) {
     A: [9]u8,
     B: u64,
 };
diff --git a/test/cases/enum_with_members.zig b/test/cases/enum_with_members.zig
index 67ed0410bc..ae48a266d0 100644
--- a/test/cases/enum_with_members.zig
+++ b/test/cases/enum_with_members.zig
@@ -2,7 +2,7 @@ const assert = @import("std").debug.assert;
 const mem = @import("std").mem;
 const fmt = @import("std").fmt;
 
-const ET = enum {
+const ET = union(enum) {
     SINT: i32,
     UINT: u32,
 
@@ -15,8 +15,8 @@ const ET = enum {
 };
 
 test "enum with members" {
-    const a = ET.SINT { -42 };
-    const b = ET.UINT { 42 };
+    const a = ET { .SINT = -42 };
+    const b = ET { .UINT = 42 };
     var buf: [20]u8 = undefined;
 
     assert(%%a.print(buf[0..]) == 3);
diff --git a/test/cases/misc.zig b/test/cases/misc.zig
index 76e1b829ee..9f4f064f6b 100644
--- a/test/cases/misc.zig
+++ b/test/cases/misc.zig
@@ -324,8 +324,8 @@ test "constant enum initialization with differing sizes" {
     test3_1(test3_foo);
     test3_2(test3_bar);
 }
-const Test3Foo = enum {
-    One,
+const Test3Foo = union(enum) {
+    One: void,
     Two: f32,
     Three: Test3Point,
 };
@@ -333,8 +333,8 @@ const Test3Point = struct {
     x: i32,
     y: i32,
 };
-const test3_foo = Test3Foo.Three{Test3Point {.x = 3, .y = 4}};
-const test3_bar = Test3Foo.Two{13};
+const test3_foo = Test3Foo { .Three = Test3Point {.x = 3, .y = 4}};
+const test3_bar = Test3Foo { .Two = 13};
 fn test3_1(f: &const Test3Foo) {
     switch (*f) {
         Test3Foo.Three => |pt| {
@@ -449,7 +449,8 @@ fn testArray2DConstDoublePtr(ptr: &const f32) {
 const Tid = builtin.TypeId;
 const AStruct = struct { x: i32, };
 const AnEnum = enum { One, Two, };
-const AnEnumWithPayload = enum { One: i32, Two, };
+const AUnionEnum = union(enum) { One: i32, Two: void, };
+const AUnion = union { One: void, Two: void };
 
 test "@typeId" {
     comptime {
@@ -474,8 +475,9 @@ test "@typeId" {
         assert(@typeId(%i32) == Tid.ErrorUnion);
         assert(@typeId(error) == Tid.Error);
         assert(@typeId(AnEnum) == Tid.Enum);
-        assert(@typeId(@typeOf(AnEnumWithPayload.One)) == Tid.EnumTag);
-        // TODO  union
+        assert(@typeId(@typeOf(AUnionEnum.One)) == Tid.Enum);
+        assert(@typeId(AUnionEnum) == Tid.Union);
+        assert(@typeId(AUnion) == Tid.Union);
         assert(@typeId(fn()) == Tid.Fn);
         assert(@typeId(@typeOf(builtin)) == Tid.Namespace);
         assert(@typeId(@typeOf({this})) == Tid.Block);
diff --git a/test/cases/reflection.zig b/test/cases/reflection.zig
index 4227f79a04..cbd98d212f 100644
--- a/test/cases/reflection.zig
+++ b/test/cases/reflection.zig
@@ -62,8 +62,8 @@ const Foo = struct {
     three: void,
 };
 
-const Bar = enum {
-    One,
+const Bar = union(enum) {
+    One: void,
     Two: i32,
     Three: bool,
     Four: f64,
diff --git a/test/cases/switch.zig b/test/cases/switch.zig
index 9ec50c7e25..11c2178427 100644
--- a/test/cases/switch.zig
+++ b/test/cases/switch.zig
@@ -83,14 +83,14 @@ const SwitchStatmentFoo = enum {
 
 
 test "switch prong with variable" {
-    switchProngWithVarFn(SwitchProngWithVarEnum.One {13});
-    switchProngWithVarFn(SwitchProngWithVarEnum.Two {13.0});
-    switchProngWithVarFn(SwitchProngWithVarEnum.Meh);
+    switchProngWithVarFn(SwitchProngWithVarEnum { .One = 13});
+    switchProngWithVarFn(SwitchProngWithVarEnum { .Two = 13.0});
+    switchProngWithVarFn(SwitchProngWithVarEnum { .Meh = {}});
 }
-const SwitchProngWithVarEnum = enum {
+const SwitchProngWithVarEnum = union(enum) {
     One: i32,
     Two: f32,
-    Meh,
+    Meh: void,
 };
 fn switchProngWithVarFn(a: &const SwitchProngWithVarEnum) {
     switch(*a) {
@@ -112,7 +112,7 @@ test "switch on enum using pointer capture" {
 }
 
 fn testSwitchEnumPtrCapture() {
-    var value = SwitchProngWithVarEnum.One { 1234 };
+    var value = SwitchProngWithVarEnum { .One = 1234 };
     switch (value) {
         SwitchProngWithVarEnum.One => |*x| *x += 1,
         else => unreachable,
@@ -136,13 +136,13 @@ fn returnsFive() -> i32 {
 }
 
 
-const Number = enum {
+const Number = union(enum) {
     One: u64,
     Two: u8,
     Three: f32,
 };
 
-const number = Number.Three { 1.23 };
+const number = Number { .Three = 1.23 };
 
 fn returnsFalse() -> bool {
     switch (number) {
diff --git a/test/cases/switch_prong_err_enum.zig b/test/cases/switch_prong_err_enum.zig
index d565de8c71..21f6b04037 100644
--- a/test/cases/switch_prong_err_enum.zig
+++ b/test/cases/switch_prong_err_enum.zig
@@ -9,14 +9,14 @@ fn readOnce() -> %u64 {
 
 error InvalidDebugInfo;
 
-const FormValue = enum {
+const FormValue = union(enum) {
     Address: u64,
     Other: bool,
 };
 
 fn doThing(form_id: u64) -> %FormValue {
     return switch (form_id) {
-        17 => FormValue.Address { %return readOnce() },
+        17 => FormValue { .Address = %return readOnce() },
         else => error.InvalidDebugInfo,
     }
 }
diff --git a/test/cases/switch_prong_implicit_cast.zig b/test/cases/switch_prong_implicit_cast.zig
index 131c5e1c80..e7fe53b841 100644
--- a/test/cases/switch_prong_implicit_cast.zig
+++ b/test/cases/switch_prong_implicit_cast.zig
@@ -1,7 +1,7 @@
 const assert = @import("std").debug.assert;
 
-const FormValue = enum {
-    One,
+const FormValue = union(enum) {
+    One: void,
     Two: bool,
 };
 
@@ -9,8 +9,8 @@ error Whatever;
 
 fn foo(id: u64) -> %FormValue {
     switch (id) {
-        2 => FormValue.Two { true },
-        1 => FormValue.One,
+        2 => FormValue { .Two = true },
+        1 => FormValue { .One = {} },
         else => return error.Whatever,
     }
 }
diff --git a/test/cases/union.zig b/test/cases/union.zig
index 4044721582..ec050a20d5 100644
--- a/test/cases/union.zig
+++ b/test/cases/union.zig
@@ -1,6 +1,6 @@
 const assert = @import("std").debug.assert;
 
-const Value = enum {
+const Value = union(enum) {
     Int: u64,
     Array: [9]u8,
 };
@@ -10,8 +10,8 @@ const Agg = struct {
     val2: Value,
 };
 
-const v1 = Value.Int { 1234 };
-const v2 = Value.Array { []u8{3} ** 9 };
+const v1 = Value { .Int = 1234 };
+const v2 = Value { .Array = []u8{3} ** 9 };
 
 const err = (%Agg)(Agg {
     .val1 = v1,
@@ -75,3 +75,32 @@ test "basic extern unions" {
     assert(foo.float == 12.34);
 }
 
+
+const Letter = enum {
+    A,
+    B,
+    C,
+};
+const Payload = union(Letter) {
+    A: i32,
+    B: f64,
+    C: bool,
+};
+
+test "union with specified enum tag" {
+    doTest();
+    comptime doTest();
+}
+
+fn doTest() {
+    assert(bar(Payload {.A = 1234}) == -10);
+}
+
+fn bar(value: &const Payload) -> i32 {
+    assert(Letter(*value) == Letter.A);
+    return switch (*value) {
+        Payload.A => |x| return x - 1244,
+        Payload.B => |x| if (x == 12.34) i32(20) else 21,
+        Payload.C => |x| if (x) i32(30) else 31,
+    };
+}
diff --git a/test/compile_errors.zig b/test/compile_errors.zig
index e1de167ac5..ac06b5aa2a 100644
--- a/test/compile_errors.zig
+++ b/test/compile_errors.zig
@@ -930,8 +930,8 @@ pub fn addCases(cases: &tests.CompileErrorContext) {
         \\fn bad_eql_1(a: []u8, b: []u8) -> bool {
         \\    a == b
         \\}
-        \\const EnumWithData = enum {
-        \\    One,
+        \\const EnumWithData = union(enum) {
+        \\    One: void,
         \\    Two: i32,
         \\};
         \\fn bad_eql_2(a: &const EnumWithData, b: &const EnumWithData) -> bool {
@@ -1145,19 +1145,19 @@ pub fn addCases(cases: &tests.CompileErrorContext) {
         \\const JasonHM = u8;
         \\const JasonList = &JsonNode;
         \\
-        \\const JsonOA = enum {
+        \\const JsonOA = union(enum) {
         \\    JSONArray: JsonList,
         \\    JSONObject: JasonHM,
         \\};
         \\
-        \\const JsonType = enum {
+        \\const JsonType = union(enum) {
         \\    JSONNull: void,
         \\    JSONInteger: isize,
         \\    JSONDouble: f64,
         \\    JSONBool: bool,
         \\    JSONString: []u8,
-        \\    JSONArray,
-        \\    JSONObject,
+        \\    JSONArray: void,
+        \\    JSONObject: void,
         \\};
         \\
         \\pub const JsonNode = struct {
@@ -2138,7 +2138,7 @@ pub fn addCases(cases: &tests.CompileErrorContext) {
         \\
         \\const MdText = ArrayList(u8);
         \\
-        \\const MdNode = enum {
+        \\const MdNode = union(enum) {
         \\    Header: struct {
         \\        text: MdText,
         \\        weight: HeaderValue,
@@ -2297,6 +2297,14 @@ pub fn addCases(cases: &tests.CompileErrorContext) {
     ,
         ".tmp_source.zig:2:21: error: type 'i32' does not support @memberType");
 
+    cases.add("@memberType on enum",
+        \\comptime {
+        \\    _ = @memberType(Foo, 0);
+        \\}
+        \\const Foo = enum {A,};
+    ,
+        ".tmp_source.zig:2:21: error: type 'Foo' does not support @memberType");
+
     cases.add("@memberType struct out of bounds",
         \\comptime {
         \\    _ = @memberType(Foo, 0);
@@ -2305,11 +2313,11 @@ pub fn addCases(cases: &tests.CompileErrorContext) {
     ,
         ".tmp_source.zig:2:26: error: member index 0 out of bounds; 'Foo' has 0 members");
 
-    cases.add("@memberType enum out of bounds",
+    cases.add("@memberType union out of bounds",
         \\comptime {
         \\    _ = @memberType(Foo, 1);
         \\}
-        \\const Foo = enum {A,};
+        \\const Foo = union {A: void,};
     ,
         ".tmp_source.zig:2:26: error: member index 1 out of bounds; 'Foo' has 1 members");
 
@@ -2336,6 +2344,14 @@ pub fn addCases(cases: &tests.CompileErrorContext) {
     ,
         ".tmp_source.zig:2:26: error: member index 1 out of bounds; 'Foo' has 1 members");
 
+    cases.add("@memberName union out of bounds",
+        \\comptime {
+        \\    _ = @memberName(Foo, 1);
+        \\}
+        \\const Foo = union {A:i32,};
+    ,
+        ".tmp_source.zig:2:26: error: member index 1 out of bounds; 'Foo' has 1 members");
+
     cases.add("calling var args extern function, passing array instead of pointer",
         \\export fn entry() {
         \\    foo("hello");
@@ -2466,7 +2482,8 @@ pub fn addCases(cases: &tests.CompileErrorContext) {
         \\        var x: MultipleChoice = undefined;
         \\}
     ,
-        ".tmp_source.zig:2:14: error: enums, not unions, support field assignment");
+        ".tmp_source.zig:2:14: error: non-enum union field assignment",
+        ".tmp_source.zig:1:24: note: consider 'union(enum)' here");
 
     cases.add("enum with 0 fields",
         \\const Foo = enum {};
@@ -2490,4 +2507,21 @@ pub fn addCases(cases: &tests.CompileErrorContext) {
     ,
         ".tmp_source.zig:6:9: error: enum tag value 60 already taken",
         ".tmp_source.zig:4:9: note: other occurrence here");
+
+    cases.add("union with specified enum omits field",
+        \\const Letter = enum {
+        \\    A,
+        \\    B,
+        \\    C,
+        \\};
+        \\const Payload = union(Letter) {
+        \\    A: i32,
+        \\    B: f64,
+        \\};
+        \\export fn entry() -> usize {
+        \\    return @sizeOf(Payload);
+        \\}
+    ,
+        ".tmp_source.zig:6:17: error: enum field missing: 'C'",
+        ".tmp_source.zig:4:5: note: declared here");
 }
diff --git a/test/tests.zig b/test/tests.zig
index 73d9646552..e74afa1755 100644
--- a/test/tests.zig
+++ b/test/tests.zig
@@ -150,8 +150,8 @@ pub fn addPkgTests(b: &build.Builder, test_filter: ?[]const u8, root_src: []cons
                     continue;
                 }
                 const these_tests = b.addTest(root_src);
-                these_tests.setNamePrefix(b.fmt("{}-{}-{}-{}-{} ", name, @enumTagName(test_target.os),
-                    @enumTagName(test_target.arch), @enumTagName(mode), if (link_libc) "c" else "bare"));
+                these_tests.setNamePrefix(b.fmt("{}-{}-{}-{}-{} ", name, @tagName(test_target.os),
+                    @tagName(test_target.arch), @tagName(mode), if (link_libc) "c" else "bare"));
                 these_tests.setFilter(test_filter);
                 these_tests.setBuildMode(mode);
                 if (!is_native) {
@@ -428,7 +428,7 @@ pub const CompareOutputContext = struct {
             Special.None => {
                 for ([]Mode{Mode.Debug, Mode.ReleaseSafe, Mode.ReleaseFast}) |mode| {
                     const annotated_case_name = %%fmt.allocPrint(self.b.allocator, "{} {} ({})",
-                        "compare-output", case.name, @enumTagName(mode));
+                        "compare-output", case.name, @tagName(mode));
                     if (self.test_filter) |filter| {
                         if (mem.indexOf(u8, annotated_case_name, filter) == null)
                             continue;
@@ -682,7 +682,7 @@ pub const CompileErrorContext = struct {
 
         for ([]Mode{Mode.Debug, Mode.ReleaseSafe, Mode.ReleaseFast}) |mode| {
             const annotated_case_name = %%fmt.allocPrint(self.b.allocator, "compile-error {} ({})",
-                case.name, @enumTagName(mode));
+                case.name, @tagName(mode));
             if (self.test_filter) |filter| {
                 if (mem.indexOf(u8, annotated_case_name, filter) == null)
                     continue;
@@ -750,7 +750,7 @@ pub const BuildExamplesContext = struct {
 
         for ([]Mode{Mode.Debug, Mode.ReleaseSafe, Mode.ReleaseFast}) |mode| {
             const annotated_case_name = %%fmt.allocPrint(self.b.allocator, "build {} ({})",
-                root_src, @enumTagName(mode));
+                root_src, @tagName(mode));
             if (self.test_filter) |filter| {
                 if (mem.indexOf(u8, annotated_case_name, filter) == null)
                     continue;