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

[eddiejnolan at gmail dot com via Gcc-bugs]

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

--- Comment #4 from Eddie Nolan <eddiejnolan at gmail dot com> ---
To clarify, I agree that the example in this bug, 111535, is not a proper range
adaptor closure object:

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

This is the example, from bug 116217, that I think is a proper range adaptor
closure object, but which is restricted by the wording from using the pipe
operator with non-range parameters:

struct foo : std::ranges::range_adaptor_closure<foo> {
  auto operator()(auto const&) const {
    return std::ranges::empty_view<int>{};
  }
};