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

[barry.revzin at gmail dot com]

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

            Bug ID: 93479
           Summary: compare_three_way allows comparing arrays
           Product: gcc
           Version: 10.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: libstdc++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: barry.revzin at gmail dot com
  Target Milestone: ---

This program:

#include <compare>

bool check(int(&x)[4], int(&y)[4]) {
    return std::compare_three_way{}(x, y) == 0;
}

Successfully compiles, with check() doing an address comparison on x and y. But
compare_three_way is just the function object for <=> and x <=> y is ill-formed
(gcc correctly rejects that).  

The check for __3way_builtin_ptr_cmp needs to reject arrays, since the
expression declval<T>() <=> declval<U>() does not resolve to a built-in
operator comparing pointers (it doesn't resolve to any operator).