[Bug libstdc++/93983] New: std::filesystem::path is not concept-friendly

[lyberta at lyberta dot net]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=93983

            Bug ID: 93983
           Summary: std::filesystem::path is not concept-friendly
           Product: gcc
           Version: 10.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: libstdc++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: lyberta at lyberta dot net
  Target Milestone: ---

#include <concepts>
#include <filesystem>

struct Foo
{
    Foo(const std::filesystem::path& p);
};

static_assert(std::copyable<Foo>);

Error:
In file included from <source>:1:

/opt/compiler-explorer/gcc-trunk-20200229/include/c++/10.0.1/bits/stl_iterator_base_types.h:
In instantiation of 'struct std::iterator_traits<Foo>':

/opt/compiler-explorer/gcc-trunk-20200229/include/c++/10.0.1/bits/fs_path.h:84:11:
  required by substitution of 'template<class _Iter, class _Iter_traits> using
__is_path_iter_src = std::__and_<std::__or_<std::is_same<typename
std::remove_const<typename _Iter_traits::value_type>::type, char>,
std::is_same<typename std::remove_const<typename
_Iter_traits::value_type>::type, char8_t>, std::is_same<typename
std::remove_const<typename _Iter_traits::value_type>::type, wchar_t>,
std::is_same<typename std::remove_const<typename
_Iter_traits::value_type>::type, char16_t>, std::is_same<typename
std::remove_const<typename _Iter_traits::value_type>::type, char32_t> >,
std::is_base_of<std::input_iterator_tag, typename
_Iter_traits::iterator_category> > [with _Iter = Foo; _Iter_traits =
std::iterator_traits<Foo>]'

/opt/compiler-explorer/gcc-trunk-20200229/include/c++/10.0.1/bits/fs_path.h:91:5:
  required by substitution of 'template<class _Iter>
std::filesystem::__cxx11::__detail::__is_path_iter_src<_Iter>
std::filesystem::__cxx11::__detail::__is_path_src(_Iter, int) [with _Iter =
Foo]'

/opt/compiler-explorer/gcc-trunk-20200229/include/c++/10.0.1/bits/fs_path.h:115:29:
  required from 'struct
std::filesystem::__cxx11::__detail::__constructible_from<Foo, void>'

/opt/compiler-explorer/gcc-trunk-20200229/include/c++/10.0.1/type_traits:138:12:
  required from 'struct std::__and_<std::__not_<std::is_void<Foo> >,
std::filesystem::__cxx11::__detail::__constructible_from<Foo, void> >'

/opt/compiler-explorer/gcc-trunk-20200229/include/c++/10.0.1/type_traits:143:12:
  required from 'struct std::__and_<std::__not_<std::is_same<Foo,
std::filesystem::__cxx11::path> >, std::__not_<std::is_void<Foo> >,
std::filesystem::__cxx11::__detail::__constructible_from<Foo, void> >'

/opt/compiler-explorer/gcc-trunk-20200229/include/c++/10.0.1/bits/fs_path.h:119:11:
  required by substitution of 'template<class _Tp1, class _Tp2> using _Path =
typename std::enable_if<std::__and_<std::__not_<std::is_same<typename
std::remove_cv< <template-parameter-1-1> >::type,
std::filesystem::__cxx11::path> >, std::__not_<std::is_void<typename
std::remove_pointer<_Tp>::type> >,
std::filesystem::__cxx11::__detail::__constructible_from<_Tp1, _Tp2> >::value,
std::filesystem::__cxx11::path>::type [with _Tp1 = Foo; _Tp2 = void]'

/opt/compiler-explorer/gcc-trunk-20200229/include/c++/10.0.1/bits/fs_path.h:219:7:
  required by substitution of 'template<class _Source, class _Require>
std::filesystem::__cxx11::path::path(const _Source&,
std::filesystem::__cxx11::path::format) [with _Source = Foo; _Require =
<missing>]'

/opt/compiler-explorer/gcc-trunk-20200229/include/c++/10.0.1/type_traits:901:30:
  required from 'struct std::__is_constructible_impl<Foo, Foo>'

/opt/compiler-explorer/gcc-trunk-20200229/include/c++/10.0.1/type_traits:906:12:
  required from 'struct std::is_constructible<Foo, Foo>'

/opt/compiler-explorer/gcc-trunk-20200229/include/c++/10.0.1/type_traits:3107:25:
  required from 'constexpr const bool std::is_constructible_v<Foo, Foo>'

/opt/compiler-explorer/gcc-trunk-20200229/include/c++/10.0.1/concepts:139:30:  
required from here

/opt/compiler-explorer/gcc-trunk-20200229/include/c++/10.0.1/concepts:139:30:
error: the value of 'std::is_constructible_v<Foo, Foo>' is not usable in a
constant expression

  139 |       = destructible<_Tp> && is_constructible_v<_Tp, _Args...>;

      |                              ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

In file included from
/opt/compiler-explorer/gcc-trunk-20200229/include/c++/10.0.1/concepts:44,

                 from <source>:1:

/opt/compiler-explorer/gcc-trunk-20200229/include/c++/10.0.1/type_traits:3107:25:
note: 'std::is_constructible_v<Foo, Foo>' used in its own initializer

 3107 |   inline constexpr bool is_constructible_v =

      |                         ^~~~~~~~~~~~~~~~~~