[Bug libstdc++/122708] New: views::concat is underconstrained

[hewillk at gmail dot com via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=122708

            Bug ID: 122708
           Summary: views::concat is underconstrained
           Product: gcc
           Version: 15.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: libstdc++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: hewillk at gmail dot com
  Target Milestone: ---

Seems like we also need viewable_range:

#include <vector>
#include <ranges>

template<class R>
concept can_concat = requires (R& r) { 
  std::views::concat(r);
};

auto r = std::views::all(std::vector{42});
static_assert(can_concat<decltype(r)>);  // hard error

https://godbolt.org/z/TvKcavc6W