Conversion operator for nlohmann::json is not SFINAE friendly

[ldionne]

• What is the issue you have?

nlohmann::json's conversion operator is mis-behaved. It pretends to be convertible to things that it is not actually convertible to, which causes errors when using generic libraries that rely on these checks.

• Please describe the steps to reproduce the issue. Can you provide a small but working code example?

In the following program, the static_assert passes because std::is_convertible checks whether there's a conversion operator for the given type. However, when actually trying to instantiate the conversion operator, we fail because no viable get function can be found:

#include <nlohmann/json.hpp>
#include <type_traits>

struct MyType { };

static_assert(std::is_convertible<nlohmann::json, MyType>::value, "");
nlohmann::json f();
MyType foo = f();

The error message is

In file included from ../test/src/unit-json_conversion_operator.cpp:30:
../single_include/nlohmann/json.hpp:13914:16: error: no matching member function for call to 'get'
        return get<ValueType>();
               ^~~~~~~~~~~~~~
../test/src/unit-json_conversion_operator.cpp:37:14: note: in instantiation of function template specialization 'nlohmann::basic_json<std::map, std::vector, std::__1::basic_string<char>, bool, long long, unsigned long long, double, std::allocator, adl_serializer>::operator MyType<MyType, 0>' requested here
MyType foo = f();
             ^
../single_include/nlohmann/json.hpp:13567:16: note: candidate template ignored: requirement 'std::is_same<typename std::remove_const<MyType>::type, basic_json_t>::value' was not satisfied [with BasicJsonType = MyType]
    basic_json get() const
               ^
../single_include/nlohmann/json.hpp:13590:19: note: candidate template ignored: requirement 'detail::is_basic_json<MyType>::value' was not satisfied [with BasicJsonType = MyType]
    BasicJsonType get() const
                  ^
../single_include/nlohmann/json.hpp:13640:15: note: candidate template ignored: requirement 'detail::has_from_json<basic_json_t, MyType>::value' was not satisfied [with ValueTypeCV = MyType, ValueType = MyType]
    ValueType get() const noexcept(noexcept(
              ^
../single_include/nlohmann/json.hpp:13691:15: note: candidate template ignored: requirement 'detail::has_non_default_from_json<basic_json_t, MyType>::value' was not satisfied [with ValueTypeCV = MyType, ValueType = MyType]
    ValueType get() const noexcept(noexcept(
              ^
../single_include/nlohmann/json.hpp:13728:17: note: candidate template ignored: requirement 'std::is_pointer<MyType>::value' was not satisfied [with PointerType = MyType]
    PointerType get() noexcept
                ^
../single_include/nlohmann/json.hpp:13740:33: note: candidate template ignored: requirement 'std::is_pointer<MyType>::value' was not satisfied [with PointerType = MyType]
    constexpr const PointerType get() const noexcept
                                ^

• What is the expected behavior?

The static_assert should fail instead.

• And what is the actual behavior instead?

The static_assert passes, lying about nlohmann::json being convertible to MyType.

• Which compiler and operating system are you using? Is it a supported compiler?

All Clangs fail.

• Did you use a released version of the library or the version from the develop branch?

Released version.

• If you experience a compilation error: can you compile and run the unit tests?

Yes.

[theodelrieu]

Hello Louis,

I recently merged #1228 that improved the library's SFINAE-correctness, but I forgot about operator T().

There are lots of issues with operator T() described in detail in #958, but it should do the right thing in an unevaluated context anyway, I'll fix it.

[nlohmann]

@ldionne Could you check whether #1238 fixes the issue?

[theodelrieu]

@nlohmann I've added a test in unit-udt.cpp with the same use-case, I believe it is safe to merge #1238.