diff --git a/src/AstGen.zig b/src/AstGen.zig
index d4b942ecbc..a44ac04557 100644
--- a/src/AstGen.zig
+++ b/src/AstGen.zig
@@ -2464,6 +2464,7 @@ fn unusedResultExpr(gz: *GenZir, scope: *Scope, statement: Ast.Node.Index) Inner
             .repeat,
             .repeat_inline,
             .panic,
+            .panic_comptime,
             => {
                 noreturn_src_node = statement;
                 break :b true;
@@ -7358,7 +7359,7 @@ fn builtinCall(
         .bool_to_int           => return simpleUnOp(gz, scope, rl, node, bool_rl,                             params[0], .bool_to_int),
         .embed_file            => return simpleUnOp(gz, scope, rl, node, .{ .ty = .const_slice_u8_type },     params[0], .embed_file),
         .error_name            => return simpleUnOp(gz, scope, rl, node, .{ .ty = .anyerror_type },           params[0], .error_name),
-        .panic                 => return simpleUnOp(gz, scope, rl, node, .{ .ty = .const_slice_u8_type },     params[0], .panic),
+        .panic                 => return simpleUnOp(gz, scope, rl, node, .{ .ty = .const_slice_u8_type },     params[0], if (gz.force_comptime) .panic_comptime else .panic),
         .set_cold              => return simpleUnOp(gz, scope, rl, node, bool_rl,                             params[0], .set_cold),
         .set_runtime_safety    => return simpleUnOp(gz, scope, rl, node, bool_rl,                             params[0], .set_runtime_safety),
         .sqrt                  => return simpleUnOp(gz, scope, rl, node, .none,                               params[0], .sqrt),
diff --git a/src/Sema.zig b/src/Sema.zig
index 49878397e9..db222b698e 100644
--- a/src/Sema.zig
+++ b/src/Sema.zig
@@ -903,7 +903,8 @@ fn analyzeBodyInner(
             .ret_load       => break sema.zirRetLoad(block, inst),
             .ret_err_value  => break sema.zirRetErrValue(block, inst),
             .@"unreachable" => break sema.zirUnreachable(block, inst),
-            .panic          => break sema.zirPanic(block, inst),
+            .panic          => break sema.zirPanic(block, inst, false),
+            .panic_comptime => break sema.zirPanic(block, inst, true),
             // zig fmt: on
 
             .extended => ext: {
@@ -3870,10 +3871,26 @@ fn zirValidateDeref(sema: *Sema, block: *Block, inst: Zir.Inst.Index) CompileErr
         .Slice => return sema.fail(block, src, "index syntax required for slice type '{}'", .{operand_ty.fmt(sema.mod)}),
     }
 
+    const elem_ty = operand_ty.elemType2();
     if (try sema.resolveMaybeUndefVal(block, operand_src, operand)) |val| {
         if (val.isUndef()) {
             return sema.fail(block, src, "cannot dereference undefined value", .{});
         }
+    } else if (!(try sema.validateRunTimeType(block, src, elem_ty, false))) {
+        const msg = msg: {
+            const msg = try sema.errMsg(
+                block,
+                src,
+                "values of type '{}' must be comptime known, but operand value is runtime known",
+                .{elem_ty.fmt(sema.mod)},
+            );
+            errdefer msg.destroy(sema.gpa);
+
+            const src_decl = sema.mod.declPtr(block.src_decl);
+            try sema.explainWhyTypeIsComptime(block, src, msg, src.toSrcLoc(src_decl), elem_ty);
+            break :msg msg;
+        };
+        return sema.failWithOwnedErrorMsg(block, msg);
     }
 }
 
@@ -4308,11 +4325,15 @@ fn zirCompileLog(
     return Air.Inst.Ref.void_value;
 }
 
-fn zirPanic(sema: *Sema, block: *Block, inst: Zir.Inst.Index) CompileError!Zir.Inst.Index {
+fn zirPanic(sema: *Sema, block: *Block, inst: Zir.Inst.Index, force_comptime: bool) CompileError!Zir.Inst.Index {
     const inst_data = sema.code.instructions.items(.data)[inst].un_node;
-    const src: LazySrcLoc = inst_data.src();
+    const src = inst_data.src();
     const msg_inst = try sema.resolveInst(inst_data.operand);
 
+    if (block.is_comptime or force_comptime) {
+        return sema.fail(block, src, "encountered @panic at comptime", .{});
+    }
+    try sema.requireRuntimeBlock(block, src);
     return sema.panicWithMsg(block, src, msg_inst);
 }
 
@@ -8357,7 +8378,7 @@ fn zirSwitchBlock(sema: *Sema, block: *Block, inst: Zir.Inst.Index) CompileError
                     return sema.fail(
                         block,
                         src,
-                        "switch must handle all possibilities",
+                        "else prong required when switching on type 'anyerror'",
                         .{},
                     );
                 }
@@ -17416,7 +17437,7 @@ fn zirVarExtended(
     const extra = sema.code.extraData(Zir.Inst.ExtendedVar, extended.operand);
     const src = sema.src;
     const ty_src: LazySrcLoc = src; // TODO add a LazySrcLoc that points at type
-    const mut_src: LazySrcLoc = src; // TODO add a LazySrcLoc that points at mut token
+    const name_src: LazySrcLoc = src; // TODO add a LazySrcLoc that points at the name token
     const init_src: LazySrcLoc = src; // TODO add a LazySrcLoc that points at init expr
     const small = @bitCast(Zir.Inst.ExtendedVar.Small, extended.small);
 
@@ -17460,7 +17481,7 @@ fn zirVarExtended(
             return sema.failWithNeededComptime(block, init_src);
     } else Value.initTag(.unreachable_value);
 
-    try sema.validateVarType(block, mut_src, var_ty, small.is_extern);
+    try sema.validateVarType(block, name_src, var_ty, small.is_extern);
 
     const new_var = try sema.gpa.create(Module.Var);
     errdefer sema.gpa.destroy(new_var);
@@ -17957,6 +17978,9 @@ fn validateVarType(
 
         const src_decl = mod.declPtr(block.src_decl);
         try sema.explainWhyTypeIsComptime(block, src, msg, src.toSrcLoc(src_decl), var_ty);
+        if (var_ty.zigTypeTag() == .ComptimeInt or var_ty.zigTypeTag() == .ComptimeFloat) {
+            try sema.errNote(block, src, msg, "to modify this variable at runtime, it must be given an explicit fixed-size number type", .{});
+        }
 
         break :msg msg;
     };
@@ -20041,6 +20065,15 @@ fn coerce(
                         });
                         return sema.addConstant(dest_ty, slice_val);
                     }
+
+                    if (inst_ty.zigTypeTag() == .Array) {
+                        return sema.fail(
+                            block,
+                            inst_src,
+                            "array literal requires address-of operator (&) to coerce to slice type '{}'",
+                            .{dest_ty.fmt(sema.mod)},
+                        );
+                    }
                 },
                 .Many => p: {
                     if (!inst_ty.isSlice()) break :p;
@@ -24433,6 +24466,10 @@ fn semaUnionFields(block: *Block, mod: *Module, union_obj: *Module.Union) Compil
         } else {
             // The provided type is the enum tag type.
             union_obj.tag_ty = try provided_ty.copy(decl_arena_allocator);
+            if (union_obj.tag_ty.zigTypeTag() != .Enum) {
+                const tag_ty_src = src; // TODO better source location
+                return sema.fail(block, tag_ty_src, "expected enum tag type, found '{}'", .{union_obj.tag_ty.fmt(sema.mod)});
+            }
             // The fields of the union must match the enum exactly.
             // Store a copy of the enum field names so we can check for
             // missing or extraneous fields later.
diff --git a/src/Zir.zig b/src/Zir.zig
index 9448b54346..8735ba636d 100644
--- a/src/Zir.zig
+++ b/src/Zir.zig
@@ -798,6 +798,8 @@ pub const Inst = struct {
         error_name,
         /// Implement builtin `@panic`. Uses `un_node`.
         panic,
+        /// Same as `panic` but forces comptime.
+        panic_comptime,
         /// Implement builtin `@setCold`. Uses `un_node`.
         set_cold,
         /// Implement builtin `@setRuntimeSafety`. Uses `un_node`.
@@ -1260,6 +1262,7 @@ pub const Inst = struct {
                 .repeat,
                 .repeat_inline,
                 .panic,
+                .panic_comptime,
                 => true,
             };
         }
@@ -1533,6 +1536,7 @@ pub const Inst = struct {
                 .repeat,
                 .repeat_inline,
                 .panic,
+                .panic_comptime,
                 .@"try",
                 .try_ptr,
                 //.try_inline,
@@ -1726,6 +1730,7 @@ pub const Inst = struct {
                 .embed_file = .un_node,
                 .error_name = .un_node,
                 .panic = .un_node,
+                .panic_comptime = .un_node,
                 .set_cold = .un_node,
                 .set_runtime_safety = .un_node,
                 .sqrt = .un_node,
diff --git a/src/print_zir.zig b/src/print_zir.zig
index 6cd289c092..2fcd82baf3 100644
--- a/src/print_zir.zig
+++ b/src/print_zir.zig
@@ -197,6 +197,7 @@ const Writer = struct {
             .embed_file,
             .error_name,
             .panic,
+            .panic_comptime,
             .set_cold,
             .set_runtime_safety,
             .sqrt,
diff --git a/test/cases/compile_errors/stage1/obj/method_call_with_first_arg_type_primitive.zig b/test/cases/compile_errors/method_call_with_first_arg_type_primitive.zig
similarity index 70%
rename from test/cases/compile_errors/stage1/obj/method_call_with_first_arg_type_primitive.zig
rename to test/cases/compile_errors/method_call_with_first_arg_type_primitive.zig
index 316be2d899..e93d1e717d 100644
--- a/test/cases/compile_errors/stage1/obj/method_call_with_first_arg_type_primitive.zig
+++ b/test/cases/compile_errors/method_call_with_first_arg_type_primitive.zig
@@ -15,7 +15,7 @@ export fn f() void {
 }
 
 // error
-// backend=stage1
+// backend=stage2
 // target=native
 //
-// tmp.zig:14:5: error: expected type 'i32', found 'Foo'
+// :14:9: error: type 'tmp.Foo' has no field or member function named 'init'
diff --git a/test/cases/compile_errors/stage1/obj/method_call_with_first_arg_type_wrong_container.zig b/test/cases/compile_errors/method_call_with_first_arg_type_wrong_container.zig
similarity index 82%
rename from test/cases/compile_errors/stage1/obj/method_call_with_first_arg_type_wrong_container.zig
rename to test/cases/compile_errors/method_call_with_first_arg_type_wrong_container.zig
index 04ab89fb6c..0820151ff1 100644
--- a/test/cases/compile_errors/stage1/obj/method_call_with_first_arg_type_wrong_container.zig
+++ b/test/cases/compile_errors/method_call_with_first_arg_type_wrong_container.zig
@@ -24,7 +24,7 @@ export fn foo() void {
 }
 
 // error
-// backend=stage1
+// backend=llvm
 // target=native
 //
-// tmp.zig:23:5: error: expected type '*Allocator', found '*List'
+// :23:6: error: type 'tmp.List' has no field or member function named 'init'
diff --git a/test/cases/compile_errors/stage1/obj/missing_const_in_slice_with_nested_array_type.zig b/test/cases/compile_errors/missing_const_in_slice_with_nested_array_type.zig
similarity index 71%
rename from test/cases/compile_errors/stage1/obj/missing_const_in_slice_with_nested_array_type.zig
rename to test/cases/compile_errors/missing_const_in_slice_with_nested_array_type.zig
index af9d690d43..1ef986935b 100644
--- a/test/cases/compile_errors/stage1/obj/missing_const_in_slice_with_nested_array_type.zig
+++ b/test/cases/compile_errors/missing_const_in_slice_with_nested_array_type.zig
@@ -12,7 +12,7 @@ export fn entry() void {
 }
 
 // error
-// backend=stage1
+// backend=llvm
 // target=native
 //
-// tmp.zig:4:30: error: array literal requires address-of operator (&) to coerce to slice type '[][2]f32'
+// :4:30: error: array literal requires address-of operator (&) to coerce to slice type '[][2]f32'
diff --git a/test/cases/compile_errors/mul_overflow_in_function_evaluation.zig b/test/cases/compile_errors/mul_overflow_in_function_evaluation.zig
new file mode 100644
index 0000000000..6be57f770e
--- /dev/null
+++ b/test/cases/compile_errors/mul_overflow_in_function_evaluation.zig
@@ -0,0 +1,14 @@
+const y = mul(300, 6000);
+fn mul(a: u16, b: u16) u16 {
+    return a * b;
+}
+
+export fn entry() usize { return @sizeOf(@TypeOf(&y)); }
+
+// error
+// backend=stage2
+// target=native
+//
+// :3:14: error: overflow of integer type 'u16' with value '1800000'
+// :1:14: note: called from here
+
diff --git a/test/cases/compile_errors/negation_overflow_in_function_evaluation.zig b/test/cases/compile_errors/negation_overflow_in_function_evaluation.zig
new file mode 100644
index 0000000000..abd8549fd2
--- /dev/null
+++ b/test/cases/compile_errors/negation_overflow_in_function_evaluation.zig
@@ -0,0 +1,13 @@
+const y = neg(-128);
+fn neg(x: i8) i8 {
+    return -x;
+}
+
+export fn entry() usize { return @sizeOf(@TypeOf(&y)); }
+
+// error
+// backend=stage2
+// target=native
+//
+// :3:12: error: overflow of integer type 'i8' with value '128'
+// :1:14: note: called from here
diff --git a/test/cases/compile_errors/stage1/obj/no_else_prong_on_switch_on_global_error_set.zig b/test/cases/compile_errors/no_else_prong_on_switch_on_global_error_set.zig
similarity index 61%
rename from test/cases/compile_errors/stage1/obj/no_else_prong_on_switch_on_global_error_set.zig
rename to test/cases/compile_errors/no_else_prong_on_switch_on_global_error_set.zig
index 39f4399caf..6c13611d64 100644
--- a/test/cases/compile_errors/stage1/obj/no_else_prong_on_switch_on_global_error_set.zig
+++ b/test/cases/compile_errors/no_else_prong_on_switch_on_global_error_set.zig
@@ -8,7 +8,7 @@ fn foo(a: anyerror) void {
 }
 
 // error
-// backend=stage1
+// backend=stage2
 // target=native
 //
-// tmp.zig:5:5: error: else prong required when switching on type 'anyerror'
+// :5:5: error: else prong required when switching on type 'anyerror'
diff --git a/test/cases/compile_errors/non-const_variables_of_things_that_require_const_variables.zig b/test/cases/compile_errors/non-const_variables_of_things_that_require_const_variables.zig
new file mode 100644
index 0000000000..c6713b755a
--- /dev/null
+++ b/test/cases/compile_errors/non-const_variables_of_things_that_require_const_variables.zig
@@ -0,0 +1,44 @@
+export fn entry1() void {
+   var m2 = &2;
+   _ = m2;
+}
+export fn entry2() void {
+   var a = undefined;
+   _ = a;
+}
+export fn entry3() void {
+   var b = 1;
+   _ = b;
+}
+export fn entry4() void {
+   var c = 1.0;
+   _ = c;
+}
+export fn entry5() void {
+   var d = null;
+   _ = d;
+}
+export fn entry6(opaque_: *Opaque) void {
+   var e = opaque_.*;
+   _ = e;
+}
+export fn entry7() void {
+   var f = i32;
+   _ = f;
+}
+const Opaque = opaque {};
+
+// error
+// backend=stage2
+// target=native
+//
+// :2:8: error: variable of type '*const comptime_int' must be const or comptime
+// :6:8: error: variable of type '@TypeOf(undefined)' must be const or comptime
+// :10:8: error: variable of type 'comptime_int' must be const or comptime
+// :10:8: note: to modify this variable at runtime, it must be given an explicit fixed-size number type
+// :14:8: error: variable of type 'comptime_float' must be const or comptime
+// :14:8: note: to modify this variable at runtime, it must be given an explicit fixed-size number type
+// :18:8: error: variable of type '@TypeOf(null)' must be const or comptime
+// :22:19: error: values of type 'tmp.Opaque' must be comptime known, but operand value is runtime known
+// :26:8: error: variable of type 'type' must be const or comptime
+// :26:8: note: types are not available at runtime
diff --git a/test/cases/compile_errors/non_error_sets_used_in_merge_error_sets_operator.zig b/test/cases/compile_errors/non_error_sets_used_in_merge_error_sets_operator.zig
new file mode 100644
index 0000000000..eca7ba698c
--- /dev/null
+++ b/test/cases/compile_errors/non_error_sets_used_in_merge_error_sets_operator.zig
@@ -0,0 +1,15 @@
+export fn foo() void {
+    const Errors = u8 || u16;
+    _ = Errors;
+}
+export fn bar() void {
+    const Errors = error{} || u16;
+    _ = Errors;
+}
+
+// error
+// backend=stage2
+// target=native
+//
+// :2:20: error: expected error set type, found 'u8'
+// :6:31: error: expected error set type, found 'u16'
diff --git a/test/cases/compile_errors/stage1/obj/panic_called_at_compile_time.zig b/test/cases/compile_errors/panic_called_at_compile_time.zig
similarity index 57%
rename from test/cases/compile_errors/stage1/obj/panic_called_at_compile_time.zig
rename to test/cases/compile_errors/panic_called_at_compile_time.zig
index 7c16932913..220161930b 100644
--- a/test/cases/compile_errors/stage1/obj/panic_called_at_compile_time.zig
+++ b/test/cases/compile_errors/panic_called_at_compile_time.zig
@@ -5,7 +5,7 @@ export fn entry() void {
 }
 
 // error
-// backend=stage1
+// backend=stage2
 // target=native
 //
-// tmp.zig:3:9: error: encountered @panic at compile-time
+// :3:9: error: encountered @panic at comptime
diff --git a/test/cases/compile_errors/stage1/obj/non_compile_time_array_concatenation.zig b/test/cases/compile_errors/stage1/non_compile_time_array_concatenation.zig
similarity index 100%
rename from test/cases/compile_errors/stage1/obj/non_compile_time_array_concatenation.zig
rename to test/cases/compile_errors/stage1/non_compile_time_array_concatenation.zig
diff --git a/test/cases/compile_errors/stage1/obj/mul_overflow_in_function_evaluation.zig b/test/cases/compile_errors/stage1/obj/mul_overflow_in_function_evaluation.zig
deleted file mode 100644
index 8209b33f94..0000000000
--- a/test/cases/compile_errors/stage1/obj/mul_overflow_in_function_evaluation.zig
+++ /dev/null
@@ -1,12 +0,0 @@
-const y = mul(300, 6000);
-fn mul(a: u16, b: u16) u16 {
-    return a * b;
-}
-
-export fn entry() usize { return @sizeOf(@TypeOf(y)); }
-
-// error
-// backend=stage1
-// target=native
-//
-// tmp.zig:3:14: error: operation caused overflow
diff --git a/test/cases/compile_errors/stage1/obj/negation_overflow_in_function_evaluation.zig b/test/cases/compile_errors/stage1/obj/negation_overflow_in_function_evaluation.zig
deleted file mode 100644
index f83e0de6ae..0000000000
--- a/test/cases/compile_errors/stage1/obj/negation_overflow_in_function_evaluation.zig
+++ /dev/null
@@ -1,12 +0,0 @@
-const y = neg(-128);
-fn neg(x: i8) i8 {
-    return -x;
-}
-
-export fn entry() usize { return @sizeOf(@TypeOf(y)); }
-
-// error
-// backend=stage1
-// target=native
-//
-// tmp.zig:3:12: error: negation caused overflow
diff --git a/test/cases/compile_errors/stage1/obj/non-const_variables_of_things_that_require_const_variables.zig b/test/cases/compile_errors/stage1/obj/non-const_variables_of_things_that_require_const_variables.zig
deleted file mode 100644
index 73ef29aa54..0000000000
--- a/test/cases/compile_errors/stage1/obj/non-const_variables_of_things_that_require_const_variables.zig
+++ /dev/null
@@ -1,51 +0,0 @@
-export fn entry1() void {
-   var m2 = &2;
-   _ = m2;
-}
-export fn entry2() void {
-   var a = undefined;
-   _ = a;
-}
-export fn entry3() void {
-   var b = 1;
-   _ = b;
-}
-export fn entry4() void {
-   var c = 1.0;
-   _ = c;
-}
-export fn entry5() void {
-   var d = null;
-   _ = d;
-}
-export fn entry6(opaque_: *Opaque) void {
-   var e = opaque_.*;
-   _ = e;
-}
-export fn entry7() void {
-   var f = i32;
-   _ = f;
-}
-export fn entry8() void {
-   var h = (Foo {}).bar;
-   _ = h;
-}
-const Opaque = opaque {};
-const Foo = struct {
-    fn bar(self: *const Foo) void {_ = self;}
-};
-
-// error
-// backend=stage1
-// target=native
-//
-// tmp.zig:2:4: error: variable of type '*const comptime_int' must be const or comptime
-// tmp.zig:6:4: error: variable of type '@Type(.Undefined)' must be const or comptime
-// tmp.zig:10:4: error: variable of type 'comptime_int' must be const or comptime
-// tmp.zig:10:4: note: to modify this variable at runtime, it must be given an explicit fixed-size number type
-// tmp.zig:14:4: error: variable of type 'comptime_float' must be const or comptime
-// tmp.zig:14:4: note: to modify this variable at runtime, it must be given an explicit fixed-size number type
-// tmp.zig:18:4: error: variable of type '@Type(.Null)' must be const or comptime
-// tmp.zig:22:4: error: variable of type 'Opaque' not allowed
-// tmp.zig:26:4: error: variable of type 'type' must be const or comptime
-// tmp.zig:30:4: error: variable of type '(bound fn(*const Foo) void)' must be const or comptime
diff --git a/test/cases/compile_errors/stage1/obj/non_error_sets_used_in_merge_error_sets_operator.zig b/test/cases/compile_errors/stage1/obj/non_error_sets_used_in_merge_error_sets_operator.zig
deleted file mode 100644
index fc3431820c..0000000000
--- a/test/cases/compile_errors/stage1/obj/non_error_sets_used_in_merge_error_sets_operator.zig
+++ /dev/null
@@ -1,17 +0,0 @@
-export fn foo() void {
-    const Errors = u8 || u16;
-    _ = Errors;
-}
-export fn bar() void {
-    const Errors = error{} || u16;
-    _ = Errors;
-}
-
-// error
-// backend=stage1
-// target=native
-//
-// tmp.zig:2:20: error: expected error set type, found type 'u8'
-// tmp.zig:2:23: note: `||` merges error sets; `or` performs boolean OR
-// tmp.zig:6:31: error: expected error set type, found type 'u16'
-// tmp.zig:6:28: note: `||` merges error sets; `or` performs boolean OR
diff --git a/test/cases/compile_errors/use_invalid_number_literal_as_array_index.zig b/test/cases/compile_errors/use_invalid_number_literal_as_array_index.zig
index 6fab70abf8..120ba5c588 100644
--- a/test/cases/compile_errors/use_invalid_number_literal_as_array_index.zig
+++ b/test/cases/compile_errors/use_invalid_number_literal_as_array_index.zig
@@ -9,3 +9,4 @@ export fn entry() void {
 // target=native
 //
 // :1:1: error: variable of type 'comptime_int' must be const or comptime
+// :1:1: note: to modify this variable at runtime, it must be given an explicit fixed-size number type