GCC 10 tests build failure

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?

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.

Any idea how to fix this?

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.

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

Merge pull request #2034 from ArtemSarmini/gh-1920-gcc-10-incomplete-…

nlohmann-json: backport gcc10 fix

json: use latest to get gcc10 fixes

nlohmann-json: backport gcc10 fix

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GCC 10 tests build failure #1920

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stac47 commented on Feb 26, 2020

nlohmann commented on Feb 28, 2020

ArtemSarmini commented on Mar 16, 2020

ArtemSarmini commented on Apr 12, 2020

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