GCC 10 tests build failure

[xvitaly]

• What is the issue you have?

Unit tests build failure.

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

Build using GCC 10.0.

• What is the expected behavior?

Successful build.

• And what is the actual behavior instead?

Build failure:

[40/83] /usr/bin/c++  -DDOCTEST_CONFIG_SUPER_FAST_ASSERTS -I../test/thirdparty/doctest -I../test/thirdparty/fifo_map -I../single_include -O2 -g -pipe -Wall -Werror=format-security -Wp,-D_FORTIFY_SOURCE=2 -Wp,-D_GLIBCXX_ASSERTIONS -fexceptions -fstack-protector-strong -grecord-gcc-switches -specs=/usr/lib/rpm/redhat/redhat-hardened-cc1 -specs=/usr/lib/rpm/redhat/redhat-annobin-cc1 -m64 -mtune=generic -fasynchronous-unwind-tables -fstack-clash-protection -fcf-protection   -Wno-deprecated -Wno-float-equal -Wno-deprecated-declarations -MD -MT test/CMakeFiles/test-concepts.dir/src/unit-concepts.cpp.o -MF test/CMakeFiles/test-concepts.dir/src/unit-concepts.cpp.o.d -o test/CMakeFiles/test-concepts.dir/src/unit-concepts.cpp.o -c ../test/src/unit-concepts.cpp
FAILED: test/CMakeFiles/test-concepts.dir/src/unit-concepts.cpp.o 
/usr/bin/c++  -DDOCTEST_CONFIG_SUPER_FAST_ASSERTS -I../test/thirdparty/doctest -I../test/thirdparty/fifo_map -I../single_include -O2 -g -pipe -Wall -Werror=format-security -Wp,-D_FORTIFY_SOURCE=2 -Wp,-D_GLIBCXX_ASSERTIONS -fexceptions -fstack-protector-strong -grecord-gcc-switches -specs=/usr/lib/rpm/redhat/redhat-hardened-cc1 -specs=/usr/lib/rpm/redhat/redhat-annobin-cc1 -m64 -mtune=generic -fasynchronous-unwind-tables -fstack-clash-protection -fcf-protection   -Wno-deprecated -Wno-float-equal -Wno-deprecated-declarations -MD -MT test/CMakeFiles/test-concepts.dir/src/unit-concepts.cpp.o -MF test/CMakeFiles/test-concepts.dir/src/unit-concepts.cpp.o.d -o test/CMakeFiles/test-concepts.dir/src/unit-concepts.cpp.o -c ../test/src/unit-concepts.cpp
In file included from /usr/include/c++/10/bits/move.h:57,
                 from /usr/include/c++/10/bits/stl_pair.h:59,
                 from /usr/include/c++/10/utility:70,
                 from /usr/include/c++/10/algorithm:60,
                 from ../single_include/nlohmann/json.hpp:37,
                 from ../test/src/unit-concepts.cpp:32:
/usr/include/c++/10/type_traits: In instantiation of 'struct std::is_constructible<nlohmann::basic_json<>, const nlohmann::basic_json<std::map, std::vector, std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >, bool, long int, long unsigned int, double, std::allocator, nlohmann::adl_serializer>&>':
../single_include/nlohmann/json.hpp:11120:79:   required by substitution of 'template<class ... Args, typename std::enable_if<std::is_constructible<nlohmann::basic_json<>, Args ...>::value, int>::type <anonymous> > nlohmann::detail::json_ref<nlohmann::basic_json<> >::json_ref(Args&& ...) [with Args = {const nlohmann::basic_json<std::map, std::vector, std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >, bool, long int, long unsigned int, double, std::allocator, nlohmann::adl_serializer>&}; typename std::enable_if<std::is_constructible<nlohmann::basic_json<>, Args ...>::value, int>::type <anonymous> = <missing>]'
/usr/include/c++/10/type_traits:897:30:   required from 'struct std::__is_constructible_impl<nlohmann::basic_json<>, const nlohmann::basic_json<std::map, std::vector, std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >, bool, long int, long unsigned int, double, std::allocator, nlohmann::adl_serializer>&>'
/usr/include/c++/10/type_traits:926:12:   required from 'struct std::__is_copy_constructible_impl<nlohmann::basic_json<>, true>'
/usr/include/c++/10/type_traits:932:12:   required from 'struct std::is_copy_constructible<nlohmann::basic_json<> >'
../test/src/unit-concepts.cpp:104:13:   required from here
/usr/include/c++/10/type_traits:902:12: error: invalid use of incomplete type 'struct std::__is_constructible_impl<nlohmann::basic_json<>, const nlohmann::basic_json<std::map, std::vector, std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >, bool, long int, long unsigned int, double, std::allocator, nlohmann::adl_serializer>&>'
  902 |     struct is_constructible
      |            ^~~~~~~~~~~~~~~~
/usr/include/c++/10/type_traits:896:12: note: declaration of 'struct std::__is_constructible_impl<nlohmann::basic_json<>, const nlohmann::basic_json<std::map, std::vector, std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >, bool, long int, long unsigned int, double, std::allocator, nlohmann::adl_serializer>&>'
  896 |     struct __is_constructible_impl
      |            ^~~~~~~~~~~~~~~~~~~~~~~
In file included from /usr/include/c++/10/utility:70,
                 from /usr/include/c++/10/algorithm:60,
                 from ../single_include/nlohmann/json.hpp:37,
                 from ../test/src/unit-concepts.cpp:32:
/usr/include/c++/10/bits/stl_pair.h: In instantiation of 'static constexpr bool std::_PCC<<anonymous>, _T1, _T2>::_ConstructiblePair() [with _U1 = const std::__cxx11::basic_string<char>; _U2 = nlohmann::basic_json<>; bool <anonymous> = true; _T1 = const std::__cxx11::basic_string<char>; _T2 = nlohmann::basic_json<>]':
/usr/include/c++/10/bits/stl_pair.h:261:35:   required by substitution of 'template<class _U1, class _U2, typename std::enable_if<(_ConstructiblePair<_U1, _U2>() && _ImplicitlyConvertiblePair<_U1, _U2>()), bool>::type <anonymous> > constexpr std::pair<const std::__cxx11::basic_string<char>, nlohmann::basic_json<> >::pair(const std::__cxx11::basic_string<char>&, const nlohmann::basic_json<>&) [with _U1 = const std::__cxx11::basic_string<char>; _U2 = nlohmann::basic_json<>; typename std::enable_if<(_ConstructiblePair<_U1, _U2>() && _ImplicitlyConvertiblePair<_U1, _U2>()), bool>::type <anonymous> = <missing>]'
/usr/include/c++/10/bits/alloc_traits.h:504:20:   required from 'static void std::allocator_traits<std::allocator<_CharT> >::construct(std::allocator_traits<std::allocator<_CharT> >::allocator_type&, _Up*, _Args&& ...) [with _Up = std::pair<const std::__cxx11::basic_string<char>, nlohmann::basic_json<> >; _Args = {std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >, nlohmann::basic_json<std::map, std::vector, std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >, bool, long int, long unsigned int, double, std::allocator, nlohmann::adl_serializer>}; _Tp = std::_Rb_tree_node<std::pair<const std::__cxx11::basic_string<char>, nlohmann::basic_json<> > >; std::allocator_traits<std::allocator<_CharT> >::allocator_type = std::allocator<std::_Rb_tree_node<std::pair<const std::__cxx11::basic_string<char>, nlohmann::basic_json<> > > >]'
/usr/include/c++/10/bits/stl_tree.h:614:32:   required from 'void std::_Rb_tree<_Key, _Val, _KeyOfValue, _Compare, _Alloc>::_M_construct_node(std::_Rb_tree<_Key, _Val, _KeyOfValue, _Compare, _Alloc>::_Link_type, _Args&& ...) [with _Args = {std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >, nlohmann::basic_json<std::map, std::vector, std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >, bool, long int, long unsigned int, double, std::allocator, nlohmann::adl_serializer>}; _Key = std::__cxx11::basic_string<char>; _Val = std::pair<const std::__cxx11::basic_string<char>, nlohmann::basic_json<> >; _KeyOfValue = std::_Select1st<std::pair<const std::__cxx11::basic_string<char>, nlohmann::basic_json<> > >; _Compare = std::less<void>; _Alloc = std::allocator<std::pair<const std::__cxx11::basic_string<char>, nlohmann::basic_json<> > >; std::_Rb_tree<_Key, _Val, _KeyOfValue, _Compare, _Alloc>::_Link_type = std::_Rb_tree_node<std::pair<const std::__cxx11::basic_string<char>, nlohmann::basic_json<> > >*]'
/usr/include/c++/10/bits/stl_tree.h:631:21:   required from 'std::_Rb_tree_node<_Val>* std::_Rb_tree<_Key, _Val, _KeyOfValue, _Compare, _Alloc>::_M_create_node(_Args&& ...) [with _Args = {std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >, nlohmann::basic_json<std::map, std::vector, std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >, bool, long int, long unsigned int, double, std::allocator, nlohmann::adl_serializer>}; _Key = std::__cxx11::basic_string<char>; _Val = std::pair<const std::__cxx11::basic_string<char>, nlohmann::basic_json<> >; _KeyOfValue = std::_Select1st<std::pair<const std::__cxx11::basic_string<char>, nlohmann::basic_json<> > >; _Compare = std::less<void>; _Alloc = std::allocator<std::pair<const std::__cxx11::basic_string<char>, nlohmann::basic_json<> > >; std::_Rb_tree<_Key, _Val, _KeyOfValue, _Compare, _Alloc>::_Link_type = std::_Rb_tree_node<std::pair<const std::__cxx11::basic_string<char>, nlohmann::basic_json<> > >*]'
/usr/include/c++/10/bits/stl_tree.h:2399:33:   required from 'std::pair<std::_Rb_tree_iterator<_Val>, bool> std::_Rb_tree<_Key, _Val, _KeyOfValue, _Compare, _Alloc>::_M_emplace_unique(_Args&& ...) [with _Args = {std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >, nlohmann::basic_json<std::map, std::vector, std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >, bool, long int, long unsigned int, double, std::allocator, nlohmann::adl_serializer>}; _Key = std::__cxx11::basic_string<char>; _Val = std::pair<const std::__cxx11::basic_string<char>, nlohmann::basic_json<> >; _KeyOfValue = std::_Select1st<std::pair<const std::__cxx11::basic_string<char>, nlohmann::basic_json<> > >; _Compare = std::less<void>; _Alloc = std::allocator<std::pair<const std::__cxx11::basic_string<char>, nlohmann::basic_json<> > >]'
/usr/include/c++/10/bits/stl_map.h:577:33:   required from 'std::pair<typename std::_Rb_tree<_Key, std::pair<const _Key, _Tp>, std::_Select1st<std::pair<const _Key, _Tp> >, _Compare, typename __gnu_cxx::__alloc_traits<_Allocator>::rebind<std::pair<const _Key, _Tp> >::other>::iterator, bool> std::map<_Key, _Tp, _Compare, _Alloc>::emplace(_Args&& ...) [with _Args = {std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >, nlohmann::basic_json<std::map, std::vector, std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >, bool, long int, long unsigned int, double, std::allocator, nlohmann::adl_serializer>}; _Key = std::__cxx11::basic_string<char>; _Tp = nlohmann::basic_json<>; _Compare = std::less<void>; _Alloc = std::allocator<std::pair<const std::__cxx11::basic_string<char>, nlohmann::basic_json<> > >; typename std::_Rb_tree<_Key, std::pair<const _Key, _Tp>, std::_Select1st<std::pair<const _Key, _Tp> >, _Compare, typename __gnu_cxx::__alloc_traits<_Allocator>::rebind<std::pair<const _Key, _Tp> >::other>::iterator = std::_Rb_tree<std::__cxx11::basic_string<char>, std::pair<const std::__cxx11::basic_string<char>, nlohmann::basic_json<> >, std::_Select1st<std::pair<const std::__cxx11::basic_string<char>, nlohmann::basic_json<> > >, std::less<void>, std::allocator<std::pair<const std::__cxx11::basic_string<char>, nlohmann::basic_json<> > > >::iterator]'
../single_include/nlohmann/json.hpp:16036:40:   required from 'nlohmann::basic_json<ObjectType, ArrayType, StringType, BooleanType, NumberIntegerType, NumberUnsignedType, NumberFloatType, AllocatorType, JSONSerializer>::basic_json(nlohmann::basic_json<ObjectType, ArrayType, StringType, BooleanType, NumberIntegerType, NumberUnsignedType, NumberFloatType, AllocatorType, JSONSerializer>::initializer_list_t, bool, nlohmann::basic_json<ObjectType, ArrayType, StringType, BooleanType, NumberIntegerType, NumberUnsignedType, NumberFloatType, AllocatorType, JSONSerializer>::value_t) [with ObjectType = std::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; nlohmann::basic_json<ObjectType, ArrayType, StringType, BooleanType, NumberIntegerType, NumberUnsignedType, NumberFloatType, AllocatorType, JSONSerializer>::initializer_list_t = std::initializer_list<nlohmann::detail::json_ref<nlohmann::basic_json<> > >; nlohmann::basic_json<ObjectType, ArrayType, StringType, BooleanType, NumberIntegerType, NumberUnsignedType, NumberFloatType, AllocatorType, JSONSerializer>::value_t = nlohmann::detail::value_t]'
../test/src/unit-concepts.cpp:154:32:   required from here
/usr/include/c++/10/bits/stl_pair.h:106:45: error: 'value' is not a member of 'std::__and_<std::is_constructible<const std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >, const std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >&>, std::is_constructible<nlohmann::basic_json<std::map, std::vector, std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >, bool, long int, long unsigned int, double, std::allocator, nlohmann::adl_serializer>, const nlohmann::basic_json<std::map, std::vector, std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >, bool, long int, long unsigned int, double, std::allocator, nlohmann::adl_serializer>&> >'
  106 |         is_constructible<_T2, const _U2&>>::value;
      |                                             ^~~~~
In file included from /usr/include/c++/10/memory:66,
                 from ../single_include/nlohmann/json.hpp:45,
                 from ../test/src/unit-concepts.cpp:32:
/usr/include/c++/10/bits/stl_uninitialized.h: In instantiation of '_ForwardIterator std::uninitialized_copy(_InputIterator, _InputIterator, _ForwardIterator) [with _InputIterator = __gnu_cxx::__normal_iterator<const nlohmann::basic_json<>*, std::vector<nlohmann::basic_json<>, std::allocator<nlohmann::basic_json<> > > >; _ForwardIterator = nlohmann::basic_json<>*]':
/usr/include/c++/10/bits/stl_uninitialized.h:325:37:   required from '_ForwardIterator std::__uninitialized_copy_a(_InputIterator, _InputIterator, _ForwardIterator, std::allocator<_Tp>&) [with _InputIterator = __gnu_cxx::__normal_iterator<const nlohmann::basic_json<>*, std::vector<nlohmann::basic_json<>, std::allocator<nlohmann::basic_json<> > > >; _ForwardIterator = nlohmann::basic_json<>*; _Tp = nlohmann::basic_json<>]'
/usr/include/c++/10/bits/stl_vector.h:555:31:   required from 'std::vector<_Tp, _Alloc>::vector(const std::vector<_Tp, _Alloc>&) [with _Tp = nlohmann::basic_json<>; _Alloc = std::allocator<nlohmann::basic_json<> >]'
/usr/include/c++/10/ext/new_allocator.h:151:4:   required from 'void __gnu_cxx::new_allocator<_Tp>::construct(_Up*, _Args&& ...) [with _Up = std::vector<nlohmann::basic_json<>, std::allocator<nlohmann::basic_json<> > >; _Args = {const std::vector<nlohmann::basic_json<std::map, std::vector, std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >, bool, long int, long unsigned int, double, std::allocator, nlohmann::adl_serializer>, std::allocator<nlohmann::basic_json<std::map, std::vector, std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >, bool, long int, long unsigned int, double, std::allocator, nlohmann::adl_serializer> > >&}; _Tp = std::vector<nlohmann::basic_json<>, std::allocator<nlohmann::basic_json<> > >]'
/usr/include/c++/10/bits/alloc_traits.h:507:17:   required from 'static void std::allocator_traits<std::allocator<_CharT> >::construct(std::allocator_traits<std::allocator<_CharT> >::allocator_type&, _Up*, _Args&& ...) [with _Up = std::vector<nlohmann::basic_json<>, std::allocator<nlohmann::basic_json<> > >; _Args = {const std::vector<nlohmann::basic_json<std::map, std::vector, std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >, bool, long int, long unsigned int, double, std::allocator, nlohmann::adl_serializer>, std::allocator<nlohmann::basic_json<std::map, std::vector, std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >, bool, long int, long unsigned int, double, std::allocator, nlohmann::adl_serializer> > >&}; _Tp = std::vector<nlohmann::basic_json<>, std::allocator<nlohmann::basic_json<> > >; std::allocator_traits<std::allocator<_CharT> >::allocator_type = std::allocator<std::vector<nlohmann::basic_json<>, std::allocator<nlohmann::basic_json<> > > >]'
../single_include/nlohmann/json.hpp:15381:35:   required from 'static T* nlohmann::basic_json<ObjectType, ArrayType, StringType, BooleanType, NumberIntegerType, NumberUnsignedType, NumberFloatType, AllocatorType, JSONSerializer>::create(Args&& ...) [with T = std::vector<nlohmann::basic_json<>, std::allocator<nlohmann::basic_json<> > >; Args = {const std::vector<nlohmann::basic_json<std::map, std::vector, std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >, bool, long int, long unsigned int, double, std::allocator, nlohmann::adl_serializer>, std::allocator<nlohmann::basic_json<std::map, std::vector, std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >, bool, long int, long unsigned int, double, std::allocator, nlohmann::adl_serializer> > >&}; ObjectType = std::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]'
../single_include/nlohmann/json.hpp:15533:36:   required from 'nlohmann::basic_json<ObjectType, ArrayType, StringType, BooleanType, NumberIntegerType, NumberUnsignedType, NumberFloatType, AllocatorType, JSONSerializer>::json_value::json_value(const array_t&) [with ObjectType = std::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; nlohmann::basic_json<ObjectType, ArrayType, StringType, BooleanType, NumberIntegerType, NumberUnsignedType, NumberFloatType, AllocatorType, JSONSerializer>::array_t = std::vector<nlohmann::basic_json<>, std::allocator<nlohmann::basic_json<> > >]'
../single_include/nlohmann/json.hpp:16365:25:   required from 'nlohmann::basic_json<ObjectType, ArrayType, StringType, BooleanType, NumberIntegerType, NumberUnsignedType, NumberFloatType, AllocatorType, JSONSerializer>::basic_json(const nlohmann::basic_json<ObjectType, ArrayType, StringType, BooleanType, NumberIntegerType, NumberUnsignedType, NumberFloatType, AllocatorType, JSONSerializer>&) [with ObjectType = std::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]'
../single_include/nlohmann/json.hpp:11138:17:   required from 'nlohmann::detail::json_ref<BasicJsonType>::value_type nlohmann::detail::json_ref<BasicJsonType>::moved_or_copied() const [with BasicJsonType = nlohmann::basic_json<>; nlohmann::detail::json_ref<BasicJsonType>::value_type = nlohmann::basic_json<>]'
../single_include/nlohmann/json.hpp:16035:59:   required from 'nlohmann::basic_json<ObjectType, ArrayType, StringType, BooleanType, NumberIntegerType, NumberUnsignedType, NumberFloatType, AllocatorType, JSONSerializer>::basic_json(nlohmann::basic_json<ObjectType, ArrayType, StringType, BooleanType, NumberIntegerType, NumberUnsignedType, NumberFloatType, AllocatorType, JSONSerializer>::initializer_list_t, bool, nlohmann::basic_json<ObjectType, ArrayType, StringType, BooleanType, NumberIntegerType, NumberUnsignedType, NumberFloatType, AllocatorType, JSONSerializer>::value_t) [with ObjectType = std::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; nlohmann::basic_json<ObjectType, ArrayType, StringType, BooleanType, NumberIntegerType, NumberUnsignedType, NumberFloatType, AllocatorType, JSONSerializer>::initializer_list_t = std::initializer_list<nlohmann::detail::json_ref<nlohmann::basic_json<> > >; nlohmann::basic_json<ObjectType, ArrayType, StringType, BooleanType, NumberIntegerType, NumberUnsignedType, NumberFloatType, AllocatorType, JSONSerializer>::value_t = nlohmann::detail::value_t]'
../test/src/unit-concepts.cpp:154:32:   required from here
/usr/include/c++/10/bits/stl_uninitialized.h:137:72: error: 'value' is not a member of 'std::is_constructible<nlohmann::basic_json<>, const nlohmann::basic_json<std::map, std::vector, std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >, bool, long int, long unsigned int, double, std::allocator, nlohmann::adl_serializer>&>'
  137 |       static_assert(is_constructible<_ValueType2, decltype(*__first)>::value,
      |                                                                        ^~~~~

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

GCC 10.0.1 from Fedora 32.

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

Latest release.

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

[stac47]

The problem here is that std::is_copy_constructible<json>::value (which is approximatively the same as std::is_constructible<json, const json&>::value will try to figure out if the constructor basic_json(const detail::json_ref<basic_json>& ref) is candidate for the lookup of copy constructor. This boils down to evaluating

    template <
        class... Args,
        enable_if_t<std::is_constructible<value_type, Args...>::value, int> = 0 >
    json_ref(Args && ... args)
        : owned_value(std::forward<Args>(args)...), value_ref(&owned_value),
          is_rvalue(true) {}

With Args={const json&}. The enable_it_t<std::is_constructible<json, const json&>::value, int> = 0> creates an unresolvable cycle. Hence, json type is still considered as incomplete so cannot be used in the std::is_copy_constructible trait.

GCC 10 improved the detection of incomplete types usage in this commit: https://gcc.gnu.org/viewcvs/gcc?view=revision&revision=271806

I think this ticket deserves the maintainer attention because it will lead to compilation issues on user side of the library when they compile their code with GCC10.

[nlohmann]

Any idea how to fix this?

[ArtemSarmini]

Thing is, this variadic perfect-forwarding ctor is matching copy/move ctor.
We could first check if (pseudocode) !((sizeof...(Args) == 1) && is_same_v<decay_t<Args_0>, json_ref>) to sfinae that ctor out in favor of copy/move. Ping me if PR is wanted.

[ArtemSarmini]

I was wrong, that's mutual recursion cycle. Will try to figure something out.