[Bug libstdc++/84580] New: Equality and relational ops for containers behave differently in Debug Mode

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=84580

            Bug ID: 84580
           Summary: Equality and relational ops for containers behave
                    differently in Debug Mode
           Product: gcc
           Version: 8.0
            Status: UNCONFIRMED
          Keywords: wrong-code
          Severity: normal
          Priority: P3
         Component: libstdc++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: redi at gcc dot gnu.org
  Target Milestone: ---

This passes in normal mode but fails the second assertion when compiled with
-D_GLIBCXX_DEBUG

#include <cassert>
#include <vector>

struct foo { };

typedef std::vector<foo*> vect;

bool operator==(const vect&, const vect&) { return true; }

int main() {
  vect v1;
  vect v2;
  v1.push_back(nullptr);

  assert(v1 == v2);
  assert(!(v1 != v2));
}

The problem is that in Debug Mode (v1 != v2) doesn't call (v1 == v2) which
would use the custom ::operator== overload.

Instead it calls (v1.base() != v2.base()) which then calls operator==, but with
arguments of type std::__cxx1998::vector, and so won't use the ::operator==
overload.

I think we need to make std::__debug::operator!= use == so that ADL can find
the same overload as would be found in normal mode, and similarly for >, <= and
>= (from Table 77, Optional container operations).