message: Disallow defaults on structs.
  Context:
  Tested: on Linux.
diff: @@ -648,8 +648,9 @@ CheckedError Parser::ParseField(StructDef &struct_def) {
  if (token_ == '=') {
  NEXT();
  - if (!IsScalar(type.base_type))
  - return Error("default values currently only supported for scalars");
  + if (!IsScalar(type.base_type) || struct_def.fixed)
  + return Error(
  + "default values currently only supported for scalars in tables");
  ECHECK(ParseSingleValue(field->value));
  }
  if (IsFloat(field->value.type.base_type)) {
  