message: fixes return type in lambda
diff: @@ -382,7 +382,7 @@ struct dimensions {
  dimensions parse_dims(rapidjson::Value& test_case)
  {
  auto get_optional = [](rapidjson::Value& obj, const char* name,
  - gko::size_type default_value) {
  + gko::size_type default_value) -> gko::size_type {
  if (obj.HasMember(name)) {
  return obj[name].GetUint64();
  } else {
  