Merge pull request #11010 from mitchellh/peer-errors

stage2: peer resolve error sets and error unions

src/Sema.zig

@@ -5141,15 +5141,15 @@ fn zirErrorUnionType(sema: *Sema, block: *Block, inst: Zir.Inst.Index) CompileEr
     const extra = sema.code.extraData(Zir.Inst.Bin, inst_data.payload_index).data;
     const lhs_src: LazySrcLoc = .{ .node_offset_bin_lhs = inst_data.src_node };
     const rhs_src: LazySrcLoc = .{ .node_offset_bin_rhs = inst_data.src_node };
-    const error_union = try sema.resolveType(block, lhs_src, extra.lhs);
+    const error_set = try sema.resolveType(block, lhs_src, extra.lhs);
     const payload = try sema.resolveType(block, rhs_src, extra.rhs);
 
-    if (error_union.zigTypeTag() != .ErrorSet) {
+    if (error_set.zigTypeTag() != .ErrorSet) {
         return sema.fail(block, lhs_src, "expected error set type, found {}", .{
-            error_union.elemType(),
+            error_set,
         });
     }
-    const err_union_ty = try Module.errorUnionType(sema.arena, error_union, payload);
+    const err_union_ty = try Module.errorUnionType(sema.arena, error_set, payload);
     return sema.addType(err_union_ty);
 }
 
@@ -5281,31 +5281,7 @@ fn zirMergeErrorSets(sema: *Sema, block: *Block, inst: Zir.Inst.Index) CompileEr
         }
     }
 
-    // Resolve both error sets now.
-    const lhs_names = lhs_ty.errorSetNames();
-    const rhs_names = rhs_ty.errorSetNames();
-
-    // TODO do we really want to create a Decl for this?
-    // The reason we do it right now is for memory management.
-    var anon_decl = try block.startAnonDecl(src);
-    defer anon_decl.deinit();
-
-    var names = Module.ErrorSet.NameMap{};
-    // TODO: Guess is an upper bound, but maybe this needs to be reduced by computing the exact size first.
-    try names.ensureUnusedCapacity(anon_decl.arena(), @intCast(u32, lhs_names.len + rhs_names.len));
-    for (lhs_names) |name| {
-        names.putAssumeCapacityNoClobber(name, {});
-    }
-    for (rhs_names) |name| {
-        names.putAssumeCapacity(name, {});
-    }
-
-    const err_set_ty = try Type.Tag.error_set_merged.create(anon_decl.arena(), names);
-    const err_set_decl = try anon_decl.finish(
-        Type.type,
-        try Value.Tag.ty.create(anon_decl.arena(), err_set_ty),
-    );
-    try sema.mod.declareDeclDependency(sema.owner_decl, err_set_decl);
+    const err_set_ty = try lhs_ty.errorSetMerge(sema.arena, rhs_ty);
     return sema.addType(err_set_ty);
 }
 
@@ -6858,9 +6834,7 @@ fn zirSwitchBlock(sema: *Sema, block: *Block, inst: Zir.Inst.Index) CompileError
                 }
             }
 
-            if (operand_ty.castTag(.error_set_inferred)) |inferred| {
-                try sema.resolveInferredErrorSet(inferred.data);
-            }
+            try sema.resolveInferredErrorSetTy(operand_ty);
 
             if (operand_ty.isAnyError()) {
                 if (special_prong != .@"else") {
@@ -10361,9 +10335,7 @@ fn zirTypeInfo(sema: *Sema, block: *Block, inst: Zir.Inst.Index) CompileError!Ai
             };
 
             // If the error set is inferred it has to be resolved at this point
-            if (ty.castTag(.error_set_inferred)) |payload| {
-                try sema.resolveInferredErrorSet(payload.data);
-            }
+            try sema.resolveInferredErrorSetTy(ty);
 
             // Build our list of Error values
             // Optional value is only null if anyerror
@@ -17610,30 +17582,25 @@ fn resolvePeerTypes(
     const target = sema.mod.getTarget();
 
     var chosen = instructions[0];
+    // If this is non-null then it does the following thing, depending on the chosen zigTypeTag().
+    //  * ErrorSet: this is an override
+    //  * ErrorUnion: this is an override of the error set only
+    //  * other: at the end we make an ErrorUnion with the other thing and this
+    var err_set_ty: ?Type = null;
     var any_are_null = false;
-    var make_the_slice_const = false;
+    var seen_const = false;
     var convert_to_slice = false;
     var chosen_i: usize = 0;
     for (instructions[1..]) |candidate, candidate_i| {
         const candidate_ty = sema.typeOf(candidate);
         const chosen_ty = sema.typeOf(chosen);
+
+        const candidate_ty_tag = try candidate_ty.zigTypeTagOrPoison();
+        const chosen_ty_tag = try chosen_ty.zigTypeTagOrPoison();
+
         if (candidate_ty.eql(chosen_ty))
             continue;
 
-        // If the candidate can coerce into our chosen type, we're done.
-        // If the chosen type can coerce into the candidate, use that.
-        if ((try sema.coerceInMemoryAllowed(block, chosen_ty, candidate_ty, false, target, src, src)) == .ok) {
-            continue;
-        }
-        if ((try sema.coerceInMemoryAllowed(block, candidate_ty, chosen_ty, false, target, src, src)) == .ok) {
-            chosen = candidate;
-            chosen_i = candidate_i + 1;
-            continue;
-        }
-
-        const candidate_ty_tag = candidate_ty.zigTypeTag();
-        const chosen_ty_tag = chosen_ty.zigTypeTag();
-
         switch (candidate_ty_tag) {
             .NoReturn, .Undefined => continue,
 
@@ -17711,29 +17678,121 @@ fn resolvePeerTypes(
                 },
                 else => {},
             },
-            .ErrorUnion => {
-                const payload_ty = candidate_ty.errorUnionPayload();
-                if (chosen_ty_tag == .Pointer and
-                    chosen_ty.ptrSize() == .One and
-                    chosen_ty.childType().zigTypeTag() == .Array and
-                    payload_ty.isSlice())
-                {
-                    const chosen_child_ty = chosen_ty.childType();
-                    const chosen_elem_ty = chosen_child_ty.elemType2();
-                    const candidate_elem_ty = payload_ty.elemType2();
-                    if ((try sema.coerceInMemoryAllowed(block, candidate_elem_ty, chosen_elem_ty, false, target, src, src)) == .ok) {
-                        chosen = candidate;
-                        chosen_i = candidate_i + 1;
-
-                        convert_to_slice = false; // it already is a slice
-
-                        // If the prev pointer is const then we need to const
-                        if (chosen_child_ty.isConstPtr())
-                            make_the_slice_const = true;
+            .ErrorSet => switch (chosen_ty_tag) {
+                .ErrorSet => {
+                    // If chosen is superset of candidate, keep it.
+                    // If candidate is superset of chosen, switch it.
+                    // If neither is a superset, merge errors.
+                    const chosen_set_ty = err_set_ty orelse chosen_ty;
 
+                    if (.ok == try sema.coerceInMemoryAllowedErrorSets(chosen_set_ty, candidate_ty)) {
                         continue;
                     }
-                }
+                    if (.ok == try sema.coerceInMemoryAllowedErrorSets(candidate_ty, chosen_set_ty)) {
+                        err_set_ty = null;
+                        chosen = candidate;
+                        chosen_i = candidate_i + 1;
+                        continue;
+                    }
+
+                    err_set_ty = try chosen_set_ty.errorSetMerge(sema.arena, candidate_ty);
+                    continue;
+                },
+                .ErrorUnion => {
+                    const chosen_set_ty = err_set_ty orelse chosen_ty.errorUnionSet();
+
+                    if (.ok == try sema.coerceInMemoryAllowedErrorSets(chosen_set_ty, candidate_ty)) {
+                        continue;
+                    }
+                    if (.ok == try sema.coerceInMemoryAllowedErrorSets(candidate_ty, chosen_set_ty)) {
+                        err_set_ty = candidate_ty;
+                        continue;
+                    }
+
+                    err_set_ty = try chosen_set_ty.errorSetMerge(sema.arena, candidate_ty);
+                    continue;
+                },
+                else => {
+                    if (err_set_ty) |chosen_set_ty| {
+                        if (.ok == try sema.coerceInMemoryAllowedErrorSets(chosen_set_ty, candidate_ty)) {
+                            continue;
+                        }
+                        if (.ok == try sema.coerceInMemoryAllowedErrorSets(candidate_ty, chosen_set_ty)) {
+                            err_set_ty = candidate_ty;
+                            continue;
+                        }
+
+                        err_set_ty = try chosen_set_ty.errorSetMerge(sema.arena, candidate_ty);
+                        continue;
+                    } else {
+                        err_set_ty = candidate_ty;
+                        continue;
+                    }
+                },
+            },
+            .ErrorUnion => switch (chosen_ty_tag) {
+                .ErrorSet => {
+                    const chosen_set_ty = err_set_ty orelse chosen_ty;
+                    const candidate_set_ty = candidate_ty.errorUnionSet();
+
+                    if (.ok == try sema.coerceInMemoryAllowedErrorSets(chosen_set_ty, candidate_set_ty)) {
+                        err_set_ty = chosen_set_ty;
+                    } else if (.ok == try sema.coerceInMemoryAllowedErrorSets(candidate_set_ty, chosen_set_ty)) {
+                        err_set_ty = null;
+                    } else {
+                        err_set_ty = try chosen_set_ty.errorSetMerge(sema.arena, candidate_set_ty);
+                    }
+                    chosen = candidate;
+                    chosen_i = candidate_i + 1;
+                    continue;
+                },
+
+                .ErrorUnion => {
+                    const chosen_payload_ty = chosen_ty.errorUnionPayload();
+                    const candidate_payload_ty = candidate_ty.errorUnionPayload();
+
+                    const coerce_chosen = (try sema.coerceInMemoryAllowed(block, chosen_payload_ty, candidate_payload_ty, false, target, src, src)) == .ok;
+                    const coerce_candidate = (try sema.coerceInMemoryAllowed(block, candidate_payload_ty, chosen_payload_ty, false, target, src, src)) == .ok;
+
+                    if (coerce_chosen or coerce_candidate) {
+                        // If we can coerce to the candidate, we switch to that
+                        // type. This is the same logic as the bare (non-union)
+                        // coercion check we do at the top of this func.
+                        if (coerce_candidate) {
+                            chosen = candidate;
+                            chosen_i = candidate_i + 1;
+                        }
+
+                        const chosen_set_ty = err_set_ty orelse chosen_ty.errorUnionSet();
+                        const candidate_set_ty = chosen_ty.errorUnionSet();
+
+                        if (.ok == try sema.coerceInMemoryAllowedErrorSets(chosen_set_ty, candidate_set_ty)) {
+                            err_set_ty = chosen_set_ty;
+                        } else if (.ok == try sema.coerceInMemoryAllowedErrorSets(candidate_set_ty, chosen_set_ty)) {
+                            err_set_ty = candidate_set_ty;
+                        } else {
+                            err_set_ty = try chosen_set_ty.errorSetMerge(sema.arena, candidate_set_ty);
+                        }
+                        continue;
+                    }
+                },
+
+                else => {
+                    if (err_set_ty) |chosen_set_ty| {
+                        const candidate_set_ty = candidate_ty.errorUnionSet();
+                        if (.ok == try sema.coerceInMemoryAllowedErrorSets(chosen_set_ty, candidate_set_ty)) {
+                            err_set_ty = chosen_set_ty;
+                        } else if (.ok == try sema.coerceInMemoryAllowedErrorSets(candidate_set_ty, chosen_set_ty)) {
+                            err_set_ty = null;
+                        } else {
+                            err_set_ty = try chosen_set_ty.errorSetMerge(sema.arena, candidate_set_ty);
+                        }
+                    }
+                    seen_const = seen_const or chosen_ty.isConstPtr();
+                    chosen = candidate;
+                    chosen_i = candidate_i + 1;
+                    continue;
+                },
             },
             .Pointer => {
                 if (candidate_ty.ptrSize() == .C) {
@@ -17759,7 +17818,7 @@ fn resolvePeerTypes(
                     convert_to_slice = false;
 
                     if (chosen_ty.childType().isConstPtr() and !candidate_ty.childType().isConstPtr())
-                        make_the_slice_const = true;
+                        seen_const = true;
 
                     continue;
                 }
@@ -17771,7 +17830,7 @@ fn resolvePeerTypes(
                     chosen_ty.ptrSize() == .Many)
                 {
                     if (candidate_ty.childType().isConstPtr() and !chosen_ty.childType().isConstPtr())
-                        make_the_slice_const = true;
+                        seen_const = true;
 
                     continue;
                 }
@@ -17792,7 +17851,7 @@ fn resolvePeerTypes(
 
                         // If the pointer is const then we need to const
                         if (candidate_ty.childType().isConstPtr())
-                            make_the_slice_const = true;
+                            seen_const = true;
 
                         continue;
                     }
@@ -17815,7 +17874,7 @@ fn resolvePeerTypes(
 
                         // If the prev pointer is const then we need to const
                         if (chosen_child_ty.isConstPtr())
-                            make_the_slice_const = true;
+                            seen_const = true;
 
                         continue;
                     }
@@ -17851,7 +17910,7 @@ fn resolvePeerTypes(
                             // If one of the pointers is to const data, the slice
                             // must also be const.
                             if (candidate_child_ty.isConstPtr() or chosen_child_ty.isConstPtr())
-                                make_the_slice_const = true;
+                                seen_const = true;
 
                             continue;
                         }
@@ -17899,9 +17958,26 @@ fn resolvePeerTypes(
                     continue;
                 }
             },
+            .ErrorUnion => {
+                const payload_ty = chosen_ty.errorUnionPayload();
+                if ((try sema.coerceInMemoryAllowed(block, payload_ty, candidate_ty, false, target, src, src)) == .ok) {
+                    continue;
+                }
+            },
             else => {},
         }
 
+        // If the candidate can coerce into our chosen type, we're done.
+        // If the chosen type can coerce into the candidate, use that.
+        if ((try sema.coerceInMemoryAllowed(block, chosen_ty, candidate_ty, false, target, src, src)) == .ok) {
+            continue;
+        }
+        if ((try sema.coerceInMemoryAllowed(block, candidate_ty, chosen_ty, false, target, src, src)) == .ok) {
+            chosen = candidate;
+            chosen_i = candidate_i + 1;
+            continue;
+        }
+
         // At this point, we hit a compile error. We need to recover
         // the source locations.
         const chosen_src = candidate_srcs.resolve(
@@ -17945,23 +18021,49 @@ fn resolvePeerTypes(
         var info = chosen_ty.ptrInfo();
         info.data.sentinel = chosen_child_ty.sentinel();
         info.data.size = .Slice;
-        info.data.mutable = chosen_child_ty.isConstPtr() or make_the_slice_const;
+        info.data.mutable = seen_const or chosen_child_ty.isConstPtr();
         info.data.pointee_type = switch (chosen_child_ty.tag()) {
             .array => chosen_child_ty.elemType2(),
             .array_u8, .array_u8_sentinel_0 => Type.initTag(.u8),
             else => unreachable,
         };
 
-        return Type.ptr(sema.arena, target, info.data);
+        const new_ptr_ty = try Type.ptr(sema.arena, target, info.data);
+        const set_ty = err_set_ty orelse return new_ptr_ty;
+        return try Module.errorUnionType(sema.arena, set_ty, new_ptr_ty);
     }
 
-    if (make_the_slice_const) {
+    if (seen_const) {
         // turn []T => []const T
-        var info = chosen_ty.ptrInfo();
-        info.data.mutable = false;
-        return Type.ptr(sema.arena, target, info.data);
+        switch (chosen_ty.zigTypeTag()) {
+            .ErrorUnion => {
+                const ptr_ty = chosen_ty.errorUnionPayload();
+                var info = ptr_ty.ptrInfo();
+                info.data.mutable = false;
+                const new_ptr_ty = try Type.ptr(sema.arena, target, info.data);
+                const set_ty = err_set_ty orelse chosen_ty.errorUnionSet();
+                return try Module.errorUnionType(sema.arena, set_ty, new_ptr_ty);
+            },
+            .Pointer => {
+                var info = chosen_ty.ptrInfo();
+                info.data.mutable = false;
+                const new_ptr_ty = try Type.ptr(sema.arena, target, info.data);
+                const set_ty = err_set_ty orelse return new_ptr_ty;
+                return try Module.errorUnionType(sema.arena, set_ty, new_ptr_ty);
+            },
+            else => return chosen_ty,
+        }
     }
 
+    if (err_set_ty) |ty| switch (chosen_ty.zigTypeTag()) {
+        .ErrorSet => return ty,
+        .ErrorUnion => {
+            const payload_ty = chosen_ty.errorUnionPayload();
+            return try Module.errorUnionType(sema.arena, ty, payload_ty);
+        },
+        else => return try Module.errorUnionType(sema.arena, ty, chosen_ty),
+    };
+
     return chosen_ty;
 }
 
@@ -18248,6 +18350,12 @@ fn resolveInferredErrorSet(sema: *Sema, inferred_error_set: *Module.Fn.InferredE
     inferred_error_set.is_resolved = true;
 }
 
+fn resolveInferredErrorSetTy(sema: *Sema, ty: Type) CompileError!void {
+    if (ty.castTag(.error_set_inferred)) |inferred| {
+        try sema.resolveInferredErrorSet(inferred.data);
+    }
+}
+
 fn semaStructFields(
     mod: *Module,
     struct_obj: *Module.Struct,

src/type.zig

@@ -4216,6 +4216,23 @@ pub const Type = extern union {
         };
     }
 
+    /// Merge lhs with rhs.
+    /// Asserts that lhs and rhs are both error sets and are resolved.
+    pub fn errorSetMerge(lhs: Type, arena: Allocator, rhs: Type) !Type {
+        const lhs_names = lhs.errorSetNames();
+        const rhs_names = rhs.errorSetNames();
+        var names: Module.ErrorSet.NameMap = .{};
+        try names.ensureUnusedCapacity(arena, lhs_names.len);
+        for (lhs_names) |name| {
+            names.putAssumeCapacityNoClobber(name, {});
+        }
+        for (rhs_names) |name| {
+            try names.put(arena, name, {});
+        }
+
+        return try Tag.error_set_merged.create(arena, names);
+    }
+
     pub fn enumFields(ty: Type) Module.EnumFull.NameMap {
         return switch (ty.tag()) {
             .enum_full, .enum_nonexhaustive => ty.cast(Payload.EnumFull).?.data.fields,

test/behavior/cast.zig

@@ -591,7 +591,10 @@ test "@floatCast cast down" {
 }
 
 test "peer type resolution: unreachable, error set, unreachable" {
-    if (builtin.zig_backend != .stage1) return error.SkipZigTest; // TODO
+    if (builtin.zig_backend == .stage2_wasm) return error.SkipZigTest; // TODO
+    if (builtin.zig_backend == .stage2_x86_64) return error.SkipZigTest; // TODO
+    if (builtin.zig_backend == .stage2_aarch64) return error.SkipZigTest; // TODO
+    if (builtin.zig_backend == .stage2_arm) return error.SkipZigTest; // TODO
 
     const Error = error{
         FileDescriptorAlreadyPresentInSet,
@@ -615,6 +618,147 @@ test "peer type resolution: unreachable, error set, unreachable" {
     try expect(transformed_err == error.SystemResources);
 }
 
+test "peer cast: error set any anyerror" {
+    const a: error{ One, Two } = undefined;
+    const b: anyerror = undefined;
+    try expect(@TypeOf(a, b) == anyerror);
+    try expect(@TypeOf(b, a) == anyerror);
+}
+
+test "peer type resolution: error set supersets" {
+    if (builtin.zig_backend == .stage2_x86_64) return error.SkipZigTest; // TODO
+    if (builtin.zig_backend == .stage2_aarch64) return error.SkipZigTest; // TODO
+    if (builtin.zig_backend == .stage2_arm) return error.SkipZigTest; // TODO
+
+    const a: error{ One, Two } = undefined;
+    const b: error{One} = undefined;
+
+    // A superset of B
+    {
+        const ty = @TypeOf(a, b);
+        const error_set_info = @typeInfo(ty);
+        try expect(error_set_info == .ErrorSet);
+        try expect(error_set_info.ErrorSet.?.len == 2);
+        try expect(mem.eql(u8, error_set_info.ErrorSet.?[0].name, "One"));
+        try expect(mem.eql(u8, error_set_info.ErrorSet.?[1].name, "Two"));
+    }
+
+    // B superset of A
+    {
+        const ty = @TypeOf(b, a);
+        const error_set_info = @typeInfo(ty);
+        try expect(error_set_info == .ErrorSet);
+        try expect(error_set_info.ErrorSet.?.len == 2);
+        try expect(mem.eql(u8, error_set_info.ErrorSet.?[0].name, "One"));
+        try expect(mem.eql(u8, error_set_info.ErrorSet.?[1].name, "Two"));
+    }
+}
+
+test "peer type resolution: disjoint error sets" {
+    if (builtin.zig_backend == .stage1) {
+        // stage1 gets the order of the error names wrong after merging the sets.
+        return error.SkipZigTest;
+    }
+
+    if (builtin.zig_backend == .stage2_x86_64) return error.SkipZigTest; // TODO
+    if (builtin.zig_backend == .stage2_arm) return error.SkipZigTest; // TODO
+    if (builtin.zig_backend == .stage2_aarch64) return error.SkipZigTest; // TODO
+
+    const a: error{ One, Two } = undefined;
+    const b: error{Three} = undefined;
+
+    {
+        const ty = @TypeOf(a, b);
+        const error_set_info = @typeInfo(ty);
+        try expect(error_set_info == .ErrorSet);
+        try expect(error_set_info.ErrorSet.?.len == 3);
+        try expect(mem.eql(u8, error_set_info.ErrorSet.?[0].name, "One"));
+        try expect(mem.eql(u8, error_set_info.ErrorSet.?[1].name, "Two"));
+        try expect(mem.eql(u8, error_set_info.ErrorSet.?[2].name, "Three"));
+    }
+
+    {
+        const ty = @TypeOf(b, a);
+        const error_set_info = @typeInfo(ty);
+        try expect(error_set_info == .ErrorSet);
+        try expect(error_set_info.ErrorSet.?.len == 3);
+        try expect(mem.eql(u8, error_set_info.ErrorSet.?[0].name, "Three"));
+        try expect(mem.eql(u8, error_set_info.ErrorSet.?[1].name, "One"));
+        try expect(mem.eql(u8, error_set_info.ErrorSet.?[2].name, "Two"));
+    }
+}
+
+test "peer type resolution: error union and error set" {
+    if (builtin.zig_backend == .stage1) {
+        // stage1 gets the order of the error names wrong after merging the sets.
+        return error.SkipZigTest;
+    }
+
+    if (builtin.zig_backend == .stage2_x86_64) return error.SkipZigTest; // TODO
+    if (builtin.zig_backend == .stage2_arm) return error.SkipZigTest; // TODO
+    if (builtin.zig_backend == .stage2_aarch64) return error.SkipZigTest; // TODO
+
+    const a: error{Three} = undefined;
+    const b: error{ One, Two }!u32 = undefined;
+
+    {
+        const ty = @TypeOf(a, b);
+        const info = @typeInfo(ty);
+        try expect(info == .ErrorUnion);
+
+        const error_set_info = @typeInfo(info.ErrorUnion.error_set);
+        try expect(error_set_info.ErrorSet.?.len == 3);
+        try expect(mem.eql(u8, error_set_info.ErrorSet.?[0].name, "Three"));
+        try expect(mem.eql(u8, error_set_info.ErrorSet.?[1].name, "One"));
+        try expect(mem.eql(u8, error_set_info.ErrorSet.?[2].name, "Two"));
+    }
+
+    {
+        const ty = @TypeOf(b, a);
+        const info = @typeInfo(ty);
+        try expect(info == .ErrorUnion);
+
+        const error_set_info = @typeInfo(info.ErrorUnion.error_set);
+        try expect(error_set_info.ErrorSet.?.len == 3);
+        try expect(mem.eql(u8, error_set_info.ErrorSet.?[0].name, "One"));
+        try expect(mem.eql(u8, error_set_info.ErrorSet.?[1].name, "Two"));
+        try expect(mem.eql(u8, error_set_info.ErrorSet.?[2].name, "Three"));
+    }
+}
+
+test "peer type resolution: error union after non-error" {
+    if (builtin.zig_backend == .stage2_x86_64) return error.SkipZigTest; // TODO
+    if (builtin.zig_backend == .stage2_arm) return error.SkipZigTest; // TODO
+    if (builtin.zig_backend == .stage2_aarch64) return error.SkipZigTest; // TODO
+
+    const a: u32 = undefined;
+    const b: error{ One, Two }!u32 = undefined;
+
+    {
+        const ty = @TypeOf(a, b);
+        const info = @typeInfo(ty);
+        try expect(info == .ErrorUnion);
+        try expect(info.ErrorUnion.payload == u32);
+
+        const error_set_info = @typeInfo(info.ErrorUnion.error_set);
+        try expect(error_set_info.ErrorSet.?.len == 2);
+        try expect(mem.eql(u8, error_set_info.ErrorSet.?[0].name, "One"));
+        try expect(mem.eql(u8, error_set_info.ErrorSet.?[1].name, "Two"));
+    }
+
+    {
+        const ty = @TypeOf(b, a);
+        const info = @typeInfo(ty);
+        try expect(info == .ErrorUnion);
+        try expect(info.ErrorUnion.payload == u32);
+
+        const error_set_info = @typeInfo(info.ErrorUnion.error_set);
+        try expect(error_set_info.ErrorSet.?.len == 2);
+        try expect(mem.eql(u8, error_set_info.ErrorSet.?[0].name, "One"));
+        try expect(mem.eql(u8, error_set_info.ErrorSet.?[1].name, "Two"));
+    }
+}
+
 test "peer cast *[0]T to E![]const T" {
     if (builtin.zig_backend == .stage2_aarch64) return error.SkipZigTest;
     if (builtin.zig_backend == .stage2_c) return error.SkipZigTest; // TODO

test/behavior/error.zig

@@ -264,7 +264,8 @@ fn testErrToIntWithOnePossibleValue(
 }
 
 test "error union peer type resolution" {
-    if (builtin.zig_backend != .stage1) return error.SkipZigTest; // TODO
+    if (builtin.zig_backend == .stage2_arm) return error.SkipZigTest; // TODO
+    if (builtin.zig_backend == .stage2_aarch64) return error.SkipZigTest; // TODO
 
     try testErrorUnionPeerTypeResolution(1);
 }