[PATCH] Do not use tuple-like interface for pair in unordered containers

[Jonathan Wakely]

On 23/07/21 19:21 +0100, Jonathan Wakely wrote:
>I've been experimenting with this patch, which removes the need to use
>std::tuple_element and std::get to access the members of a std::pair
>in unordered_{map,multimap}.
>
>I'm in the process of refactoring the <utility> header to reduce
>header dependencies throughout the library, and this is the only use
>of the tuple-like interface for std::pair in the library.
>
>Using tuple_element and std::get resolved PR 53339 by allowing the
>std::pair type to be incomplete, however that is no longer supported
>anyway (the 23_containers/unordered_map/requirements/53339.cc test
>case is XFAILed). That means we could just define _Select1st as:
>
>  struct _Select1st
>  {
>    template<typename _Tp>
>      auto
>      operator()(_Tp&& __x) const noexcept
>      -> decltype(std::forward<_Tp>(__x).first)
>      { return std::forward<_Tp>(__x).first; }
>  };
>
>But the approach in the patch seems OK too.

Actually I have a fix for PR 53339 so that we can support incomplete
types again. So we don't want to access the .first member in the
return type, as that requires a complete type.