[Bug libstdc++/103013] New: Underconstrained constructor span(_Range&&)

[andrey.davydov at jetbrains dot com via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=103013

            Bug ID: 103013
           Summary: Underconstrained constructor span(_Range&&)
           Product: gcc
           Version: 11.2.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: libstdc++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: andrey.davydov at jetbrains dot com
  Target Milestone: ---

During calculation of type trait `is_move_constructible<span>` span constructor
from `_Range &&` is checked and concept `contiguous_range<span<T>>`
instantiated. It requires T to be complete type
(https://gcc.godbolt.org/z/fYWa3KdTs). It means that
`optional<span<Incomplete>>` couldn't be instantiated
(https://gcc.godbolt.org/z/1PWdE4YK3).

This issue could be solved if make the constructor span(_Range&&) more
constrained, something like this:
template<typename _Range>
  requires (!__is_same(_Range, span)) && ...
span(_Range&& __range);