[Bug libstdc++/111535] New: _RangeAdaptorClosure's (range | adaptor) operator is underconstrained

[Bug libstdc++/111535] New: _RangeAdaptorClosure's (range | adaptor) operator is underconstrained

[hewillk at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=111535

            Bug ID: 111535
           Summary: _RangeAdaptorClosure's (range | adaptor) operator is
                    underconstrained
           Product: gcc
           Version: 14.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: libstdc++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: hewillk at gmail dot com
  Target Milestone: ---

// range | adaptor is equivalent to adaptor(range).
    template<typename _Self, typename _Range>
    requires __is_range_adaptor_closure<_Self>
      && __adaptor_invocable<_Self, _Range>
    constexpr auto
    operator|(_Range&& __r, _Self&& __self)
    { return std::forward<_Self>(__self)(std::forward<_Range>(__r)); }

The pipe operator here does not constrain the Range template parameter, but the
range adaptor closure object requires arguments to model the range.

This makes libstdc++ accept the following

#include <ranges>

struct closure : std::ranges::range_adaptor_closure<closure> {
  int operator()(int);
};

int main() {
  auto r = 42 | closure{};
}

https://godbolt.org/z/hj9a68ssT