[MSVC][std:c++latest] JSON failed with error C2678: binary '==': no operator found which takes a left-hand operand of type 'nlohmann::json_abi_v3_11_2::json'

[QuellaZhang]

Description

After the MSVC team implemented P2468R2 The Equality Operator You Are Looking For, we got compiler errors with /std:c++latest like below. The way this fails is that the != operator here: https://github.com/nlohmann/json/blob/develop/include/nlohmann/json.hpp#L3704 actually suppresses the rewritten operator candidates and stops the code from compiling because the rewritten candidates are discarded. Could you please look this issue? Thanks.

Note: this issue will be reproduced on next release version of VS(17.6 or later).

Reproduction steps

1. open VS2019 x64 tools command (Or VS2022)
2. git clone https://github.com/nlohmann/json F:\gitP\nlohmann\json
3. mkdir F:\gitP\nlohmann\json\build_amd64
4. cd F:\gitP\nlohmann\json\build_amd64
5. cmake -G "Visual Studio 16 2019" -A x64 -DCMAKE_SYSTEM_VERSION=10.0.18362.0 -DJSON_BuildTests=On ..
6. set CL= /std:c++latest
7. msbuild /m /p:Platform=x64 /p:Configuration=Release nlohmann_json.sln /t:Rebuild

Expected vs. actual results

Expected Result:
The build was successful.

Actual Result:
build.log: JSON_amd64_build.log

unit-constructor1.cpp
F:\gitP\nlohmann\json\tests\src\unit-constructor1.cpp(282,13): error C2088: '==': illegal for class

unit-iterators2.cpp
F:\gitP\nlohmann\json\tests\src\unit-iterators2.cpp(946,17): error C2678: binary '==': no operator found which takes a left-hand operand of type 'nlohmann::json_abi_v3_11_2::json' (or there is no acceptable conversion)
C:\Program Files (x86)\Microsoft Visual Studio\2019\Enterprise\VC\Tools\MSVC\14.29.30133\include\thread(242,36): note: could be 'bool std::operator ==(std::thread::id,std::thread::id) noexcept' [found using argument-dependent lookup]
F:\gitP\nlohmann\json\include\nlohmann/json.hpp(3680,10): note: or 'bool nlohmann::json_abi_v3_11_2::basic_json<std::map,std::vector,std::string,bool,int64_t,uint64_t,double,std::allocator,nlohmann::json_abi_v3_11_2::adl_serializer,std::vector<uint8_t,std::allocator<uint8_t>>,void>::operator ==(const nlohmann::json_abi_v3_11_2::basic_json<std::map,std::vector,std::string,bool,int64_t,uint64_t,double,std::allocator,nlohmann::json_abi_v3_11_2::adl_serializer,std::vector<uint8_t,std::allocator<uint8_t>>,void> &) noexcept const' [synthesized expression 'y == x']
C:\Program Files (x86)\Microsoft Visual Studio\2019\Enterprise\VC\Tools\MSVC\14.29.30133\include\thread(242,36): note: or 'bool std::operator ==(std::thread::id,std::thread::id) noexcept' [synthesized expression 'y == x']
F:\gitP\nlohmann\json\tests\thirdparty\doctest\doctest.h(1238,9): note: or       'unknown-type doctest::detail::Expression_lhs<std::ranges::reverse_view<std::ranges::ref_view<nlohmann::json_abi_v3_11_2::basic_json<std::map,std::vector,std::string,bool,int64_t,uint64_t,double,std::allocator,nlohmann::json_abi_v3_11_2::adl_serializer,std::vector<uint8_t,std::allocator<uint8_t>>,void>>> &>::operator ==(R &&)'
F:\gitP\nlohmann\json\tests\src\unit-iterators2.cpp(946,17): note: Failed to specialize function template 'unknown-type doctest::detail::Expression_lhs<std::ranges::reverse_view<std::ranges::ref_view<nlohmann::json_abi_v3_11_2::basic_json<std::map,std::vector,std::string,bool,int64_t,uint64_t,double,std::allocator,nlohmann::json_abi_v3_11_2::adl_serializer,std::vector<uint8_t,std::allocator<uint8_t>>,void>>> &>::operator ==(R &&)'
F:\gitP\nlohmann\json\tests\src\unit-iterators2.cpp(946,17): note: With the following template arguments:
F:\gitP\nlohmann\json\tests\src\unit-iterators2.cpp(946,17): note: 'R=nlohmann::json_abi_v3_11_2::json &'
F:\gitP\nlohmann\json\include\nlohmann/json.hpp(3697,10): note: or 'bool nlohmann::json_abi_v3_11_2::basic_json<std::map,std::vector,std::string,bool,int64_t,uint64_t,double,std::allocator,nlohmann::json_abi_v3_11_2::adl_serializer,std::vector<uint8_t,std::allocator<uint8_t>>,void>::operator ==(ScalarType) noexcept const' [synthesized expression 'y == x']
F:\gitP\nlohmann\json\tests\src\unit-iterators2.cpp(946,17): note: the associated constraints are not satisfied
F:\gitP\nlohmann\json\include\nlohmann/json.hpp(3696,14): note: the constraint was not satisfied
C:\Program Files (x86)\Microsoft Visual Studio\2019\Enterprise\VC\Tools\MSVC\14.29.30133\include\utility(367,27): note: or       'bool std::operator ==(const std::pair<_Ty1,_Ty2> &,const std::pair<_Ty1,_Ty2> &)'
F:\gitP\nlohmann\json\tests\src\unit-iterators2.cpp(946,17): note: 'bool std::operator ==(const std::pair<_Ty1,_Ty2> &,const std::pair<_Ty1,_Ty2> &)': could not deduce template argument for 'const std::pair<_Ty1,_Ty2> &' from 
F:\gitP\nlohmann\json\tests\src\unit-iterators2.cpp(946,17): error C2660: 'doctest::detail::decomp_assert': function does not take 4 arguments
F:\gitP\nlohmann\json\tests\thirdparty\doctest\doctest.h(1425,28): note: see declaration of 'doctest::detail::decomp_assert'
F:\gitP\nlohmann\json\tests\src\unit-iterators2.cpp(946,17): note: while trying to match the argument list '(doctest::assertType::Enum, const char [52], int, const char [23])'

Minimal code example

No response

Error messages

No response

Compiler and operating system

Internal unreleased MSVC

Library version

develop branch of 8f83153

Validation

• The bug also occurs if the latest version from the develop branch is used.
• I can successfully compile and run the unit tests.

[dnlbaines]

Can confirm I am hitting this in Visual Studio 2022 preview, so looks like this will be an issue in the next update.

[nlohmann]

Issue #3979 is a duplicate of this. So it's not MSVC specific.

[nlohmann]

I still struggle to find a solution here. Help greatly appreciated!

[cardinotGV]

I had the same issue using the SDK 10.0.22000.0

The workaround was to disable JSON_HAS_THREE_WAY_COMPARISON

[jakob-stark-viavisolutions]

I guess a fix would be to remove the operator!=() completely and let the compiler default it to !operator==(). Interestingly this is not what the library currently is doing: For the unordered comparisons (NaN and discarded), the libraries current comparisons yield false for both the operator!=() and operator==(). For example:

json::value_t::discarded == json::value_t::null;  // false 
json::value_t::discarded != json::value_t::null;  // false as well !!

Thus removing the operator!=() in favor of a compiler generated one breaks the API slightly. Is there a specific reason that discarded should compare unequal and equal as false or should this by itself be considered a bug?