[Bug libstdc++/87020] New: comparison operator isn't called for stateless allocator without is_always_equal for C++11/14

[roland at rschulz dot eu]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=87020

            Bug ID: 87020
           Summary: comparison operator isn't called for stateless
                    allocator without is_always_equal for C++11/14
           Product: gcc
           Version: 8.2.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: libstdc++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: roland at rschulz dot eu
  Target Milestone: ---

For an empty allocator the comparison operators aren't used, unless
is_always_equal=std::false_type is present. This is the correct behavior with
C++17 but for C++11/14 is_always_equal shouldn't have any effect.

This is low impact because any well defined and state-less allocator should
always be equal. But it isn't strictly compliant. This is present since GCC 6.

Reproducer:
https://godbolt.org/z/-5IHwC