[Bug libstdc++/93628] New: ranges::equal_to doesn't work for types convertible to function pointers

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=93628

            Bug ID: 93628
           Summary: ranges::equal_to doesn't work for types convertible to
                    function pointers
           Product: gcc
           Version: 10.0
            Status: UNCONFIRMED
          Keywords: rejects-valid
          Severity: normal
          Priority: P3
         Component: libstdc++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: redi at gcc dot gnu.org
  Target Milestone: ---

#include <functional>

using F = void();
static_assert(std::equality_comparable_with<F*, F*>);

struct S {
  constexpr operator F*() const { return nullptr; }
};

void operator!=(S const&, S const&) {}

// concept not satisfied because operator!= returns void
static_assert(!std::equality_comparable_with<S,S>);

// but S can be compared using == via conversion to F*
static_assert(S{} == S{});

// so these should be valid:
static_assert(std::ranges::equal_to{}(S{}, S{}));
static_assert(!std::ranges::not_equal_to{}(S{}, S{}));


The problem is that our implementation of BUILTIN-PTR-CMP(T, ==, U) checks for
conversion to void*, which isn't possible for function pointers. There's no
type that all function pointers are implicitly convertible.

This might not be implementable without a new built-in.