[Bug libstdc++/71187] New: declval() can be implemented without requiring a template instantiation

["eric.niebler at gmail dot com" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=71187

            Bug ID: 71187
           Summary: declval() can be implemented without requiring a
                    template instantiation
           Product: gcc
           Version: 5.3.0
            Status: UNCONFIRMED
          Severity: enhancement
          Priority: P3
         Component: libstdc++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: eric.niebler at gmail dot com
  Target Milestone: ---

declval gets used *everywhere*. It doesn't need to instantiate a template.
Rather than:

  template<typename _Tp>
    inline typename add_rvalue_reference<_Tp>::type
    declval() noexcept
    {
      static_assert(__declval_protector<_Tp>::__stop,
                    "declval() must not be used!");
      return __declval_protector<_Tp>::__delegate();
    }

declval can be defined as:

  template<typename _Tp, typename _Up = _Tp&&>
    _Up
    __declval(int);

  template<typename _Tp>
    _Tp
    __declval(long);

  template<typename _Tp>
    auto declval() noexcept -> decltype(__declval<_Tp>(0))
    {
      static_assert(__declval_protector<_Tp>::__stop,
                    "declval() must not be used!");
      return __declval_protector<_Tp>::__delegate();
    }


In a large code base that makes heavy use of templates, this small change is a
measured 4% compile-time win (g++ (GCC) 5.x 20160302).

I think the as-if rule gives implementers enough latitude to make this change.