add implicit cast from enum to union

when the enum is the tag type of the union and is comptime known
to be of a void field of the union

See #642

src/analyze.cpp

@@ -2244,12 +2244,10 @@ static void resolve_union_zero_bits(CodeGen *g, TypeTableEntry *union_type) {
         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;
@@ -3319,7 +3317,7 @@ TypeStructField *find_struct_type_field(TypeTableEntry *type_entry, Buf *name) {
 
 TypeUnionField *find_union_type_field(TypeTableEntry *type_entry, Buf *name) {
     assert(type_entry->id == TypeTableEntryIdUnion);
-    assert(type_entry->data.unionation.complete);
+    assert(type_entry->data.unionation.zero_bits_known);
     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->enum_field->name, name)) {
@@ -3331,7 +3329,7 @@ TypeUnionField *find_union_type_field(TypeTableEntry *type_entry, Buf *name) {
 
 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.zero_bits_known);
     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];
@@ -3888,14 +3886,21 @@ bool handle_is_ptr(TypeTableEntry *type_entry) {
              return false;
         case TypeTableEntryIdArray:
         case TypeTableEntryIdStruct:
-        case TypeTableEntryIdUnion:
              return type_has_bits(type_entry);
         case TypeTableEntryIdErrorUnion:
              return type_has_bits(type_entry->data.error.child_type);
         case TypeTableEntryIdMaybe:
              return type_has_bits(type_entry->data.maybe.child_type) &&
                     type_entry->data.maybe.child_type->id != TypeTableEntryIdPointer &&
                     type_entry->data.maybe.child_type->id != TypeTableEntryIdFn;
+        case TypeTableEntryIdUnion:
+             assert(type_entry->data.unionation.complete);
+             if (type_entry->data.unionation.gen_field_count == 0)
+                 return false;
+             if (!type_has_bits(type_entry))
+                 return false;
+             return true;
+
     }
     zig_unreachable();
 }

src/codegen.cpp

@@ -3946,8 +3946,6 @@ static LLVMValueRef gen_const_val(CodeGen *g, ConstExprValue *const_val) {
         case TypeTableEntryIdUnion:
             {
                 LLVMTypeRef union_type_ref = type_entry->data.unionation.union_type_ref;
-                ConstExprValue *payload_value = const_val->data.x_union.payload;
-                assert(payload_value != nullptr);
 
                 if (type_entry->data.unionation.gen_field_count == 0) {
                     if (type_entry->data.unionation.gen_tag_index == SIZE_MAX) {
@@ -3960,7 +3958,8 @@ static LLVMValueRef gen_const_val(CodeGen *g, ConstExprValue *const_val) {
 
                 LLVMValueRef union_value_ref;
                 bool make_unnamed_struct;
-                if (!type_has_bits(payload_value->type)) {
+                ConstExprValue *payload_value = const_val->data.x_union.payload;
+                if (payload_value == nullptr || !type_has_bits(payload_value->type)) {
                     if (type_entry->data.unionation.gen_tag_index == SIZE_MAX)
                         return LLVMGetUndef(type_entry->type_ref);
 

src/ir.cpp

@@ -7468,6 +7468,17 @@ static ImplicitCastMatchResult ir_types_match_with_implicit_cast(IrAnalyze *ira,
         }
     }
 
+    // implicit enum to union which has the enum as the tag type
+    if (expected_type->id == TypeTableEntryIdUnion && actual_type->id == TypeTableEntryIdEnum &&
+        (expected_type->data.unionation.decl_node->data.container_decl.auto_enum ||
+        expected_type->data.unionation.decl_node->data.container_decl.init_arg_expr != nullptr))
+    {
+        type_ensure_zero_bits_known(ira->codegen, expected_type);
+        if (expected_type->data.unionation.tag_type == actual_type) {
+            return ImplicitCastMatchResultYes;
+        }
+    }
+
     // implicit undefined literal to anything
     if (actual_type->id == TypeTableEntryIdUndefLit) {
         return ImplicitCastMatchResultYes;
@@ -8370,6 +8381,63 @@ static IrInstruction *ir_analyze_undefined_to_anything(IrAnalyze *ira, IrInstruc
     return result;
 }
 
+static IrInstruction *ir_analyze_enum_to_union(IrAnalyze *ira, IrInstruction *source_instr,
+        IrInstruction *target, TypeTableEntry *wanted_type)
+{
+    assert(wanted_type->id == TypeTableEntryIdUnion);
+    assert(target->value.type->id == TypeTableEntryIdEnum);
+
+    if (instr_is_comptime(target)) {
+        ConstExprValue *val = ir_resolve_const(ira, target, UndefBad);
+        if (!val)
+            return ira->codegen->invalid_instruction;
+        TypeUnionField *union_field = find_union_field_by_tag(wanted_type, &val->data.x_enum_tag);
+        assert(union_field != nullptr);
+        type_ensure_zero_bits_known(ira->codegen, union_field->type_entry);
+        if (!union_field->type_entry->zero_bits) {
+            AstNode *field_node = wanted_type->data.unionation.decl_node->data.container_decl.fields.at(
+                    union_field->enum_field->decl_index);
+            ErrorMsg *msg = ir_add_error(ira, source_instr,
+                    buf_sprintf("cast to union '%s' must initialize '%s' field '%s'",
+                        buf_ptr(&wanted_type->name),
+                        buf_ptr(&union_field->type_entry->name),
+                        buf_ptr(union_field->name)));
+            add_error_note(ira->codegen, msg, field_node,
+                    buf_sprintf("field '%s' declared here", buf_ptr(union_field->name)));
+            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_union.tag, &val->data.x_enum_tag);
+        return result;
+    }
+
+    // if the union has all fields 0 bits, we can do it
+    // and in fact it's a noop cast because the union value is just the enum value
+    if (wanted_type->data.unionation.gen_field_count == 0) {
+        IrInstruction *result = ir_build_cast(&ira->new_irb, target->scope, target->source_node, wanted_type, target, CastOpNoop);
+        result->value.type = wanted_type;
+        return result;
+    }
+
+    ErrorMsg *msg = ir_add_error(ira, source_instr,
+            buf_sprintf("runtime cast to union '%s' which has non-void fields",
+                buf_ptr(&wanted_type->name)));
+    for (uint32_t i = 0; i < wanted_type->data.unionation.src_field_count; i += 1) {
+        TypeUnionField *union_field = &wanted_type->data.unionation.fields[i];
+        if (type_has_bits(union_field->type_entry)) {
+            AstNode *field_node = wanted_type->data.unionation.decl_node->data.container_decl.fields.at(i);
+            add_error_note(ira->codegen, msg, field_node,
+                    buf_sprintf("field '%s' has type '%s'",
+                        buf_ptr(union_field->name),
+                        buf_ptr(&union_field->type_entry->name)));
+        }
+    }
+    return ira->codegen->invalid_instruction;
+}
+
 static IrInstruction *ir_analyze_widen_or_shorten(IrAnalyze *ira, IrInstruction *source_instr,
         IrInstruction *target, TypeTableEntry *wanted_type)
 {
@@ -8919,6 +8987,17 @@ static IrInstruction *ir_analyze_cast(IrAnalyze *ira, IrInstruction *source_inst
         }
     }
 
+    // explicit enum to union which has the enum as the tag type
+    if (wanted_type->id == TypeTableEntryIdUnion && actual_type->id == TypeTableEntryIdEnum &&
+        (wanted_type->data.unionation.decl_node->data.container_decl.auto_enum ||
+        wanted_type->data.unionation.decl_node->data.container_decl.init_arg_expr != nullptr))
+    {
+        type_ensure_zero_bits_known(ira->codegen, wanted_type);
+        if (wanted_type->data.unionation.tag_type == actual_type) {
+            return ir_analyze_enum_to_union(ira, source_instr, value, wanted_type);
+        }
+    }
+
     // explicit cast from undefined to anything
     if (actual_type->id == TypeTableEntryIdUndefLit) {
         return ir_analyze_undefined_to_anything(ira, source_instr, value, wanted_type);

test/cases/union.zig

@@ -190,3 +190,10 @@ test "cast union to tag type of union" {
 fn testCastUnionToTagType(x: &const TheUnion) {
     assert(TheTag(*x) == TheTag.B);
 }
+
+test "cast tag type of union to union" {
+    var x: Value2 = Letter2.B;
+    assert(Letter2(x) == Letter2.B);
+}
+const Letter2 = enum { A, B, C };
+const Value2 = union(Letter2) { A: i32, B, C, };

test/compile_errors.zig

@@ -2696,4 +2696,35 @@ pub fn addCases(cases: &tests.CompileErrorContext) {
     ,
         ".tmp_source.zig:6:16: error: enum 'Foo' has no tag matching integer value 0",
         ".tmp_source.zig:1:13: note: 'Foo' declared here");
+
+    cases.add("comptime cast enum to union but field has payload",
+        \\const Letter = enum { A, B, C };
+        \\const Value = union(Letter) {
+        \\    A: i32,
+        \\    B,
+        \\    C,
+        \\};
+        \\export fn entry() {
+        \\    var x: Value = Letter.A;
+        \\}
+    ,
+        ".tmp_source.zig:8:26: error: cast to union 'Value' must initialize 'i32' field 'A'",
+        ".tmp_source.zig:3:5: note: field 'A' declared here");
+
+    cases.add("runtime cast to union which has non-void fields",
+        \\const Letter = enum { A, B, C };
+        \\const Value = union(Letter) {
+        \\    A: i32,
+        \\    B,
+        \\    C,
+        \\};
+        \\export fn entry() {
+        \\    foo(Letter.A);
+        \\}
+        \\fn foo(l: Letter) {
+        \\    var x: Value = l;
+        \\}
+    ,
+        ".tmp_source.zig:11:20: error: runtime cast to union 'Value' which has non-void fields",
+        ".tmp_source.zig:3:5: note: field 'A' has type 'i32'");
 }