[Bug libstdc++/121061] New: extents/mdspan constructor is ILL-FORMED for IntegrLike with mutable conversions

[tkaminsk at gcc dot gnu.org via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=121061

            Bug ID: 121061
           Summary: extents/mdspan constructor is ILL-FORMED for
                    IntegrLike with mutable conversions
           Product: gcc
           Version: 16.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: libstdc++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: tkaminsk at gcc dot gnu.org
  Target Milestone: ---

For the following code:
```
template<bool Const>
class IntLike
{
public:
  explicit
  IntLike(int i)
  : _M_i(i)
  { }

  IntLike() = delete;
  IntLike(const IntLike&) = delete;
  IntLike(IntLike&&) = delete;

  const IntLike&
  operator=(const IntLike&) = delete;

  const IntLike&
  operator=(IntLike&&) = delete;

  constexpr
  operator int() const noexcept
    requires (Const)
  { return _M_i; }

  constexpr
  operator int() noexcept
    requires (!Const)
  { return _M_i; }

private:
  int _M_i;
};
```
The static assertion fails incorrectly: 
static_assert(!std::is_constructible_v<std::extents<int, std::dynamic_extent>,
std::array<IntLike<false>, 1>>);.
It should be eliminated by https://eel.is/c++draft/mdspan.extents#cons-9.1.

We emit the ill-formed error from corresponding cosntructor:
std::array<IntLike<false>, 1> a1{IntLike<false>(1)};
std::extents<int, std::dynamic_extent> e1(a1);

See: https://godbolt.org/z/6GnqnascY