Parse to custom class from unordered_json breaks on G++11.2.0 with C++20 #3312
Description
What is the issue you have?
Parsing a custom class with a std::string member in it using nlohmann::ordered_json with C++20 starting with v3.10.4 results in a compilation error.
Using v3.10.3 compiles fine
Probably related issue: #3207
Please describe the steps to reproduce the issue.
Godbolt link with example showing that g++ 11.2.0 fails to compile when C++20 is enabled
https://godbolt.org/z/MsYe45qP9
- Add custom class containing a
std::string(custom class with just an int works) - Add custom
from_jsonfunction for custom class - convert from
ordered_jsonto class (normal unordered json works) - get compilation error
Example code, which I'd like to add to unittests, but don't know where. (for the error message I added it to the bottom of unit-deserialization.cpp)
struct MyClass {
std::string name;
};
void from_json(const nlohmann::json &j, MyClass &obj)
{
j.at("name").get_to(obj.name);
}
TEST_CASE("custom class parsing")
{
SECTION("parse class directly")
{
nlohmann::ordered_json j = nlohmann::ordered_json::parse("{\"name\": \"class\"}");
MyClass obj;
j.get_to(obj);
CHECK(obj.name == "class");
}
}Can you provide a small but working code example?
see above
What is the expected behavior?
Compile without error
And what is the actual behavior instead?
Compillation error:
In file included from /home/nero/repos/external/nlohmann-json/test/src/unit-deserialization.cpp:32:
/home/nero/repos/external/nlohmann-json/single_include/nlohmann/json.hpp: In instantiation of 'void nlohmann::detail::from_json(const BasicJsonType&, ConstructibleStringType&) [with BasicJsonType = nlohmann::basic_json<nlohmann::ordered_map>; ConstructibleStringType = MyClass; typename std::enable_if<(nlohmann::detail::is_constructible_string_type<BasicJsonType, ConstructibleStringType>::value && (! std::is_same<typename BasicJsonType::string_t, ConstructibleStringType>::value)), int>::type <anonymous> = 0]':
/home/nero/repos/external/nlohmann-json/single_include/nlohmann/json.hpp:4273:25: required from 'decltype (nlohmann::detail::from_json(j, forward<T>(val))) nlohmann::detail::from_json_fn::operator()(const BasicJsonType&, T&&) const [with BasicJsonType = nlohmann::basic_json<nlohmann::ordered_map>; T = MyClass&; decltype (nlohmann::detail::from_json(j, forward<T>(val))) = void]'
/home/nero/repos/external/nlohmann-json/single_include/nlohmann/json.hpp:4934:30: required from 'static decltype ((nlohmann::{anonymous}::from_json(forward<BasicJsonType>(j), val), void())) nlohmann::adl_serializer<T, SFINAE>::from_json(BasicJsonType&&, TargetType&) [with BasicJsonType = const nlohmann::basic_json<nlohmann::ordered_map>&; TargetType = MyClass; ValueType = MyClass; <template-parameter-1-2> = void; decltype ((nlohmann::{anonymous}::from_json(forward<BasicJsonType>(j), val), void())) = void]'
/home/nero/repos/external/nlohmann-json/single_include/nlohmann/json.hpp:18955:45: required from 'ValueType& nlohmann::basic_json<ObjectType, ArrayType, StringType, BooleanType, NumberIntegerType, NumberUnsignedType, NumberFloatType, AllocatorType, JSONSerializer, BinaryType>::get_to(ValueType&) const [with ValueType = MyClass; typename std::enable_if<((! nlohmann::detail::is_basic_json<BasicJsonType>::value) && nlohmann::detail::has_from_json<nlohmann::basic_json<ObjectType, ArrayType, StringType, BooleanType, NumberIntegerType, NumberUnsignedType, NumberFloatType, AllocatorType, JSONSerializer, BinaryType>, ValueType>::value), int>::type <anonymous> = 0; ObjectType = nlohmann::ordered_map; ArrayType = std::vector; StringType = std::__cxx11::basic_string<char>; BooleanType = bool; NumberIntegerType = long int; NumberUnsignedType = long unsigned int; NumberFloatType = double; AllocatorType = std::allocator; JSONSerializer = nlohmann::adl_serializer; BinaryType = std::vector<unsigned char>]'
/home/nero/repos/external/nlohmann-json/test/src/unit-deserialization.cpp:1143:17: required from here
/home/nero/repos/external/nlohmann-json/single_include/nlohmann/json.hpp:3914:7: error: no match for 'operator=' (operand types are 'MyClass' and 'const string_t' {aka 'const std::__cxx11::basic_string<char>'})
3914 | s = *j.template get_ptr<const typename BasicJsonType::string_t*>();
| ~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/home/nero/repos/external/nlohmann-json/test/src/unit-deserialization.cpp:1127:8: note: candidate: 'MyClass& MyClass::operator=(const MyClass&)'
1127 | struct MyClass {
| ^~~~~~~
/home/nero/repos/external/nlohmann-json/test/src/unit-deserialization.cpp:1127:8: note: no known conversion for argument 1 from 'const string_t' {aka 'const std::__cxx11::basic_string<char>'} to 'const MyClass&'
/home/nero/repos/external/nlohmann-json/test/src/unit-deserialization.cpp:1127:8: note: candidate: 'MyClass& MyClass::operator=(MyClass&&)'
/home/nero/repos/external/nlohmann-json/test/src/unit-deserialization.cpp:1127:8: note: no known conversion for argument 1 from 'const string_t' {aka 'const std::__cxx11::basic_string<char>'} to 'MyClass&&'
Error message when compiling with Visual Studio 16 2019 in C++20 mode:
Error C2679 binary '=': no operator found which takes a right-hand operand of type 'const std::basic_string<char,std::char_traits<char>,std::allocator<char>>' (or there is no acceptable conversion) test C:\Software\vcpkg\installed\x64-windows\include\nlohmann\detail\conversions\from_json.hpp 121
Which compiler and operating system are you using?
- Compiler: g++ 11.2.0 (only when setting C++20 mode)
- Operating system: Ubuntu 21.10
Also on Visual Studio 16 2019 in C++20 mode
Which version of the library did you use?
- latest release version 3.10.5
- other release: 3.10.4
- other release: 3.10.3 (not affected by bug, working)
- the
developbranch
If you experience a compilation error: can you compile and run the unit tests?
- yes
- no - please copy/paste the error message below
"test-comparison" start time: Feb 01 15:05 UTC
Output:
----------------------------------------------------------
[doctest] doctest version is "2.4.6"
[doctest] run with "--help" for options
===============================================================================
/home/nero/repos/external/nlohmann-json/test/src/unit-comparison.cpp:46:
TEST CASE: lexicographical comparison operators
values
comparison: less
/home/nero/repos/external/nlohmann-json/test/src/unit-comparison.cpp:213: ERROR: CHECK( (j_values[i] < j_values[j]) == expected[i][j] ) is NOT correct!
values: CHECK( false == 1 )
logged: i := 12
j := 13
j_values[i] := [1,2,3]
j_values[j] := ["one","two","three"]
/home/nero/repos/external/nlohmann-json/test/src/unit-comparison.cpp:213: ERROR: CHECK( (j_values[i] < j_values[j]) == expected[i][j] ) is NOT correct!
values: CHECK( true == 0 )
logged: i := 13
j := 12
j_values[i] := ["one","two","three"]
j_values[j] := [1,2,3]
===============================================================================
[doctest] test cases: 1 | 0 passed | 1 failed | 0 skipped
[doctest] assertions: 2220 | 2218 passed | 2 failed |
[doctest] Status: FAILURE!
edit: the unittest test-comparison fails in C++20 mode, as reported in issue: #3207