Use if-constexpr instead of overloading for customization point

This combines two of the std::ranges::swap.operator() overloads into a
single function template. Using if-constexpr to choose between
implementations should give the compiler less work to do than using
overloading.

	* include/std/concepts (std::ranges::swap): Use a single overload for
	the non-array cases, and switch using if-constexpr.

From-SVN: r277635

libstdc++-v3/ChangeLog

 2019-10-30  Jonathan Wakely  <jwakely@redhat.com>
 
+	* include/std/concepts (std::ranges::swap): Use a single overload for
+	the non-array cases, and switch using if-constexpr.
+
 	* include/bits/stl_iterator.h (__normal_iterator::iterator_concept):
 	Guard with __cpp_lib_concepts macro.
 

libstdc++-v3/include/std/concepts

@@ -173,12 +173,37 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 
       struct _Swap
       {
+      private:
+	template<typename _Tp, typename _Up>
+	  static constexpr bool
+	  _S_noexcept()
+	  {
+	    if constexpr (__adl_swap<_Tp, _Up>)
+	      return noexcept(swap(std::declval<_Tp>(), std::declval<_Up>()));
+	    else
+	      return is_nothrow_move_constructible_v<remove_reference_t<_Tp>>
+		   && is_nothrow_move_assignable_v<remove_reference_t<_Tp>>;
+	  }
+
+      public:
 	template<typename _Tp, typename _Up>
 	  requires __adl_swap<_Tp, _Up>
+	  || (same_as<_Tp, _Up> && is_lvalue_reference_v<_Tp>
+	      && move_constructible<remove_reference_t<_Tp>>
+	      && assignable_from<_Tp, remove_reference_t<_Tp>>)
 	  constexpr void
 	  operator()(_Tp&& __t, _Up&& __u) const
-	  noexcept(noexcept(swap(std::declval<_Tp>(), std::declval<_Up>())))
-	  { swap(static_cast<_Tp&&>(__t), static_cast<_Up&&>(__u)); }
+	  noexcept(_S_noexcept<_Tp, _Up>())
+	  {
+	    if constexpr (__adl_swap<_Tp, _Up>)
+	      swap(static_cast<_Tp&&>(__t), static_cast<_Up&&>(__u));
+	    else
+	      {
+		auto __tmp = static_cast<remove_reference_t<_Tp>&&>(__t);
+		__t = static_cast<remove_reference_t<_Tp>&&>(__u);
+		__u = static_cast<remove_reference_t<_Tp>&&>(__tmp);
+	      }
+	  }
 
 	template<typename _Tp, typename _Up, size_t _Num>
 	  requires requires(const _Swap& __swap, _Tp& __e1, _Up& __e2) {
@@ -191,19 +216,6 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 	    for (size_t __n = 0; __n < _Num; ++__n)
 	      (*this)(__e1[__n], __e2[__n]);
 	  }
-
-	template<typename _Tp>
-	  requires (!__adl_swap<_Tp&, _Tp&>
-	    && move_constructible<_Tp> && assignable_from<_Tp&, _Tp>)
-	  constexpr void
-	  operator()(_Tp& __e1, _Tp& __e2) const
-	  noexcept(is_nothrow_move_constructible_v<_Tp>
-		   && is_nothrow_move_assignable_v<_Tp>)
-	  {
-	    _Tp __tmp = static_cast<_Tp&&>(__e1);
-	    __e1 = static_cast<_Tp&&>(__e2);
-	    __e2 = static_cast<_Tp&&>(__tmp);
-	  }
       };
     } // namespace __cust_swap