ordered_json doesn't support construction from C array of custom type

[lethal-guitar]

Description

I have a C-style array of structs, plus an appropriate to_json overload. I can succesfully convert the array to JSON using a regular nlohmann::json object. However, trying to do the same using nlohmann::ordered_json fails to compile - see details below.

Reproduction steps

See the example code below.

Expected vs. actual results

I'd expect the code to compile, but I get a compiler error.

Minimal code example

Build with -std=c++17 -O3.

#include <nlohmann/json.hpp>

struct Example
{
    int bla;
};

NLOHMANN_DEFINE_TYPE_NON_INTRUSIVE(Example, bla);

int main()
{
    Example states[45];

    // This works:
    nlohmann::json j;
    j["test"] = states;

    // This doesn't compile:
    nlohmann::ordered_json oj;
    oj["test"] = states;
}

Error messages

gcc 9.4.0 reports:

In file included from /opt/compiler-explorer/gcc-9.4.0/include/c++/9.4.0/vector:66,
                 from /opt/compiler-explorer/gcc-9.4.0/include/c++/9.4.0/functional:62,
                 from /opt/compiler-explorer/gcc-9.4.0/include/c++/9.4.0/pstl/glue_algorithm_defs.h:13,
                 from /opt/compiler-explorer/gcc-9.4.0/include/c++/9.4.0/algorithm:71,
                 from /opt/compiler-explorer/libs/nlohmann_json/v3.11.1/single_include/nlohmann/json.hpp:21,
                 from <source>:1:
/opt/compiler-explorer/gcc-9.4.0/include/c++/9.4.0/bits/stl_uninitialized.h: In instantiation of '_ForwardIterator std::uninitialized_copy(_InputIterator, _InputIterator, _ForwardIterator) [with _InputIterator = const Example*; _ForwardIterator = nlohmann::json_v3_11_1::basic_json<nlohmann::json_v3_11_1::ordered_map>*]':
/opt/compiler-explorer/gcc-9.4.0/include/c++/9.4.0/bits/stl_uninitialized.h:307:37:   required from '_ForwardIterator std::__uninitialized_copy_a(_InputIterator, _InputIterator, _ForwardIterator, std::allocator<_Tp>&) [with _InputIterator = const Example*; _ForwardIterator = nlohmann::json_v3_11_1::basic_json<nlohmann::json_v3_11_1::ordered_map>*; _Tp = nlohmann::json_v3_11_1::basic_json<nlohmann::json_v3_11_1::ordered_map>]'
/opt/compiler-explorer/gcc-9.4.0/include/c++/9.4.0/bits/stl_vector.h:1582:33:   required from 'void std::vector<_Tp, _Alloc>::_M_range_initialize(_ForwardIterator, _ForwardIterator, std::forward_iterator_tag) [with _ForwardIterator = const Example*; _Tp = nlohmann::json_v3_11_1::basic_json<nlohmann::json_v3_11_1::ordered_map>; _Alloc = std::allocator<nlohmann::json_v3_11_1::basic_json<nlohmann::json_v3_11_1::ordered_map> >]'
/opt/compiler-explorer/gcc-9.4.0/include/c++/9.4.0/bits/stl_vector.h:654:4:   required from 'std::vector<_Tp, _Alloc>::vector(_InputIterator, _InputIterator, const allocator_type&) [with _InputIterator = const Example*; <template-parameter-2-2> = void; _Tp = nlohmann::json_v3_11_1::basic_json<nlohmann::json_v3_11_1::ordered_map>; _Alloc = std::allocator<nlohmann::json_v3_11_1::basic_json<nlohmann::json_v3_11_1::ordered_map> >; std::vector<_Tp, _Alloc>::allocator_type = std::allocator<nlohmann::json_v3_11_1::basic_json<nlohmann::json_v3_11_1::ordered_map> >]'
/opt/compiler-explorer/gcc-9.4.0/include/c++/9.4.0/ext/new_allocator.h:146:4:   required from 'void __gnu_cxx::new_allocator<_Tp>::construct(_Up*, _Args&& ...) [with _Up = std::vector<nlohmann::json_v3_11_1::basic_json<nlohmann::json_v3_11_1::ordered_map>, std::allocator<nlohmann::json_v3_11_1::basic_json<nlohmann::json_v3_11_1::ordered_map> > >; _Args = {const Example*, const Example*}; _Tp = std::vector<nlohmann::json_v3_11_1::basic_json<nlohmann::json_v3_11_1::ordered_map>, std::allocator<nlohmann::json_v3_11_1::basic_json<nlohmann::json_v3_11_1::ordered_map> > >]'
/opt/compiler-explorer/gcc-9.4.0/include/c++/9.4.0/bits/alloc_traits.h:483:4:   [ skipping 2 instantiation contexts, use -ftemplate-backtrace-limit=0 to disable ]
/opt/compiler-explorer/libs/nlohmann_json/v3.11.1/single_include/nlohmann/json.hpp:5412:25:   required from 'static void nlohmann::json_v3_11_1::detail::external_constructor<nlohmann::json_v3_11_1::detail::value_t::array>::construct(BasicJsonType&, const CompatibleArrayType&) [with BasicJsonType = nlohmann::json_v3_11_1::basic_json<nlohmann::json_v3_11_1::ordered_map>; CompatibleArrayType = Example [45]; typename std::enable_if<(! std::is_same<CompatibleArrayType, typename BasicJsonType::array_t>::value), int>::type <anonymous> = 0]'
/opt/compiler-explorer/libs/nlohmann_json/v3.11.1/single_include/nlohmann/json.hpp:5607:52:   required from 'void nlohmann::json_v3_11_1::detail::to_json(BasicJsonType&, const T (&)[N]) [with BasicJsonType = nlohmann::json_v3_11_1::basic_json<nlohmann::json_v3_11_1::ordered_map>; T = Example; long unsigned int N = 45; typename std::enable_if<(! std::is_constructible<typename BasicJsonType::string_t, const T (&)[N]>::value), int>::type <anonymous> = 0]'
/opt/compiler-explorer/libs/nlohmann_json/v3.11.1/single_include/nlohmann/json.hpp:5650:23:   required from 'decltype ((nlohmann::json_v3_11_1::detail::to_json(j, forward<T>(val)), void())) nlohmann::json_v3_11_1::detail::to_json_fn::operator()(BasicJsonType&, T&&) const [with BasicJsonType = nlohmann::json_v3_11_1::basic_json<nlohmann::json_v3_11_1::ordered_map>; T = Example (&)[45]; decltype ((nlohmann::json_v3_11_1::detail::to_json(j, forward<T>(val)), void())) = void]'
/opt/compiler-explorer/libs/nlohmann_json/v3.11.1/single_include/nlohmann/json.hpp:5706:28:   required from 'static decltype ((nlohmann::json_v3_11_1::to_json(j, forward<TargetType>(val)), void())) nlohmann::json_v3_11_1::adl_serializer<T, SFINAE>::to_json(BasicJsonType&, TargetType&&) [with BasicJsonType = nlohmann::json_v3_11_1::basic_json<nlohmann::json_v3_11_1::ordered_map>; TargetType = Example (&)[45]; ValueType = Example [45]; <template-parameter-1-2> = void; decltype ((nlohmann::json_v3_11_1::to_json(j, forward<TargetType>(val)), void())) = void]'
/opt/compiler-explorer/libs/nlohmann_json/v3.11.1/single_include/nlohmann/json.hpp:19798:35:   required from 'nlohmann::json_v3_11_1::basic_json<ObjectType, ArrayType, StringType, BooleanType, NumberIntegerType, NumberUnsignedType, NumberFloatType, AllocatorType, JSONSerializer, BinaryType>::basic_json(CompatibleType&&) [with CompatibleType = Example (&)[45]; U = Example [45]; typename std::enable_if<((! nlohmann::json_v3_11_1::detail::is_basic_json<U>::value) && nlohmann::json_v3_11_1::detail::is_compatible_type<nlohmann::json_v3_11_1::basic_json<ObjectType, ArrayType, StringType, BooleanType, NumberIntegerType, NumberUnsignedType, NumberFloatType, AllocatorType, JSONSerializer, BinaryType>, U>::value), int>::type <anonymous> = 0; ObjectType = nlohmann::json_v3_11_1::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::json_v3_11_1::adl_serializer; BinaryType = std::vector<unsigned char>]'
<source>:20:18:   required from here
/opt/compiler-explorer/gcc-9.4.0/include/c++/9.4.0/bits/stl_uninitialized.h:127:72: error: static assertion failed: result type must be constructible from value type of input range
  127 |       static_assert(is_constructible<_ValueType2, decltype(*__first)>::value,
      |

Clang outputs a significantly longer error message, which boils down to "No matching constructor found".

Compiler and operating system

GCC 9.4 on Linux (godbolt), Apple Clang 10 on Mac OS

Library version

3.11.1

Validation

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

[falbrechtskirchinger]

This will be fixed by #2843. In the meantime, this can be solved by writing from_json/to_json functions that accept arbitrary basic_json types (or just ordered_json in your case).

[lethal-guitar]

I see, thanks!

[nlohmann]

Fixed by merging #4597.