fix tagged union initialization with a runtime void

closes #1328

src/ir.cpp

@@ -9628,6 +9628,9 @@ static ConstExprValue *ir_resolve_const(IrAnalyze *ira, IrInstruction *value, Un
         case ConstValSpecialStatic:
             return &value->value;
         case ConstValSpecialRuntime:
+            if (!type_has_bits(value->value.type)) {
+                return &value->value;
+            }
             ir_add_error(ira, value, buf_sprintf("unable to evaluate constant expression"));
             return nullptr;
         case ConstValSpecialUndef:
@@ -16129,8 +16132,14 @@ static TypeTableEntry *ir_analyze_container_init_fields_union(IrAnalyze *ira, Ir
     if (casted_field_value == ira->codegen->invalid_instruction)
         return ira->codegen->builtin_types.entry_invalid;
 
+    type_ensure_zero_bits_known(ira->codegen, casted_field_value->value.type);
+    if (type_is_invalid(casted_field_value->value.type))
+        return ira->codegen->builtin_types.entry_invalid;
+
     bool is_comptime = ir_should_inline(ira->new_irb.exec, instruction->scope);
-    if (is_comptime || casted_field_value->value.special != ConstValSpecialRuntime) {
+    if (is_comptime || casted_field_value->value.special != ConstValSpecialRuntime ||
+        !type_has_bits(casted_field_value->value.type))
+    {
         ConstExprValue *field_val = ir_resolve_const(ira, casted_field_value, UndefOk);
         if (!field_val)
             return ira->codegen->builtin_types.entry_invalid;

test/cases/union.zig

@@ -297,3 +297,17 @@ test "access a member of tagged union with conflicting enum tag name" {
 
     comptime assert(Bar.A == u8);
 }
+
+test "tagged union initialization with runtime void" {
+    assert(testTaggedUnionInit({}));
+}
+
+const TaggedUnionWithAVoid = union(enum) {
+    A,
+    B: i32,
+};
+
+fn testTaggedUnionInit(x: var) bool {
+    const y = TaggedUnionWithAVoid{ .A = x };
+    return @TagType(TaggedUnionWithAVoid)(y) == TaggedUnionWithAVoid.A;
+}