[Bug libstdc++/93479] compare_three_way allows comparing arrays

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=93479

--- Comment #2 from Jonathan Wakely <redi at gcc dot gnu.org> ---
i.e. 

--- a/libstdc++-v3/libsupc++/compare
+++ b/libstdc++-v3/libsupc++/compare
@@ -525,7 +525,9 @@ namespace std
     // BUILTIN-PTR-THREE-WAY(T, U)
     template<typename _Tp, typename _Up>
       concept __3way_builtin_ptr_cmp
-       = convertible_to<_Tp, const volatile void*>
+       = requires(_Tp&& __t, _Up&& __u)
+           { static_cast<_Tp&&>(__t) <=> static_cast<_Up&&>(__u); }
+         && convertible_to<_Tp, const volatile void*>
          && convertible_to<_Up, const volatile void*>
          && ! requires(_Tp&& __t, _Up&& __u)
             { operator<=>(static_cast<_Tp&&>(__t), static_cast<_Up&&>(__u)); }