[Bug libstdc++/71960] New: __glibcxx_assert and Debug Mode checks can't be used in constexpr functions

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=71960

            Bug ID: 71960
           Summary: __glibcxx_assert and Debug Mode checks can't be used
                    in constexpr functions
           Product: gcc
           Version: 7.0
            Status: UNCONFIRMED
          Keywords: rejects-valid
          Severity: minor
          Priority: P3
         Component: libstdc++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: redi at gcc dot gnu.org
  Target Milestone: ---

This affects clamp, min_element, max_element and minmax_element, at least.

For the simple __glibcxx_assert uses it would be nice to have a constexpr
replacement for __replacement_assert which could be used in those places.

In more complex cases we could drop the 'constexpr' specifier but that would
mean some valid code wouldn't compile in Debug Mode.