[Bug libstdc++/80493] New: std::experimental::optional::swap is ill formed

[a...@cloudius-systems.com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=80493

            Bug ID: 80493
           Summary: std::experimental::optional::swap is ill formed
           Product: gcc
           Version: 6.3.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: libstdc++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: a...@cloudius-systems.com
  Target Milestone: ---

From std::experimental::optional:


      void
      swap(optional& __other)
      noexcept(is_nothrow_move_constructible<_Tp>()
               && noexcept(swap(declval<_Tp&>(), declval<_Tp&>())))
      {


The operand to the noexcept operator calls swap(T&, T&); but that is hidden by
optional::swap(optional&).  Clang correctly rejects this code, but g++ accepts
it.