[Bug libstdc++/77801] New: std::experimental::gcd std::experimental::lcd use non-constexpr std::abs

["ma30002000 at yahoo dot de" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=77801

            Bug ID: 77801
           Summary: std::experimental::gcd std::experimental::lcd use
                    non-constexpr std::abs
           Product: gcc
           Version: 6.2.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: libstdc++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: ma30002000 at yahoo dot de
  Target Milestone: ---

std::experimental::gcd std::experimental::lcd use non-constexpr std::abs, i.e.
they cannot be used in constexprs. 

As I understand, according to all currently existing c++ standards, std::abs is
not defined as constexr (although it probably should be).

Anyway, at least lcd and gcd should be usable as constexprs.