libstdc++: Add missing P0896 changes to <iterator>

I noticed that the following changes from this paper were not yet
implemented.

libstdc++-v3/ChangeLog:

	* include/bits/stl_iterator.h (reverse_iterator::iter_move):
	Define for C++20 as per P0896.
	(reverse_iterator::iter_swap): Likewise.
	(move_iterator::operator*): Apply P0896 changes for C++20.
	(move_iterator::operator[]): Likewise.
	* testsuite/24_iterators/reverse_iterator/cust.cc: New test.

libstdc++-v3/include/bits/stl_iterator.h

@@ -362,6 +362,31 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
       operator[](difference_type __n) const
       { return *(*this + __n); }
 
+#if __cplusplus > 201703L && __cpp_lib_concepts
+      friend constexpr iter_rvalue_reference_t<_Iterator>
+      iter_move(const reverse_iterator& __i)
+      noexcept(is_nothrow_copy_constructible_v<_Iterator>
+	       && noexcept(ranges::iter_move(--std::declval<_Iterator&>())))
+      {
+	auto __tmp = __i.base();
+	return ranges::iter_move(--__tmp);
+      }
+
+      template<indirectly_swappable<_Iterator> _Iter2>
+	friend constexpr void
+	iter_swap(const reverse_iterator& __x,
+		  const reverse_iterator<_Iter2>& __y)
+	noexcept(is_nothrow_copy_constructible_v<_Iterator>
+		 && is_nothrow_copy_constructible_v<_Iter2>
+		 && noexcept(ranges::iter_swap(--std::declval<_Iterator&>(),
+					       --std::declval<_Iter2&>())))
+	{
+	  auto __xtmp = __x.base();
+	  auto __ytmp = __y.base();
+	  ranges::iter_swap(--__xtmp, --__ytmp);
+	}
+#endif
+
     private:
       template<typename _Tp>
 	static _GLIBCXX17_CONSTEXPR _Tp*
@@ -1379,7 +1404,11 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 
       _GLIBCXX17_CONSTEXPR reference
       operator*() const
+#if __cplusplus > 201703L && __cpp_lib_concepts
+      { return ranges::iter_move(_M_current); }
+#else
       { return static_cast<reference>(*_M_current); }
+#endif
 
       _GLIBCXX17_CONSTEXPR pointer
       operator->() const
@@ -1445,7 +1474,11 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 
       _GLIBCXX17_CONSTEXPR reference
       operator[](difference_type __n) const
+#if __cplusplus > 201703L && __cpp_lib_concepts
+      { return ranges::iter_move(_M_current + __n); }
+#else
       { return std::move(_M_current[__n]); }
+#endif
 
 #if __cplusplus > 201703L && __cpp_lib_concepts
       template<sentinel_for<_Iterator> _Sent>

libstdc++-v3/testsuite/24_iterators/reverse_iterator/cust.cc

@@ -0,0 +1,52 @@
+// Copyright (C) 2019-2020 Free Software Foundation, Inc.
+//
+// This file is part of the GNU ISO C++ Library.  This library is free
+// software; you can redistribute it and/or modify it under the
+// terms of the GNU General Public License as published by the
+// Free Software Foundation; either version 3, or (at your option)
+// any later version.
+
+// This library is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU General Public License for more details.
+
+// You should have received a copy of the GNU General Public License along
+// with this library; see the file COPYING3.  If not see
+// <http://www.gnu.org/licenses/>.
+
+// { dg-options "-std=gnu++2a" }
+// { dg-do compile { target c++2a } }
+
+#include <iterator>
+#include <testsuite_hooks.h>
+
+// This test is an adaptation of 24_iterators/move_iterator/cust.cc.
+
+constexpr bool
+test01()
+{
+  struct X
+  {
+    constexpr X(int i) noexcept : i(i) { }
+    constexpr X(X&& x) noexcept : i(x.i) { x.i = -1; }
+    constexpr X& operator=(X&& x) noexcept { i = x.i; x.i = 0; return *this; }
+    int i;
+  };
+
+  X arr[] = { 1, 2 };
+  std::reverse_iterator<X*> i(arr + 1), j(arr + 2);
+  static_assert(noexcept(std::ranges::iter_swap(i, j)));
+  std::ranges::iter_swap(i, j);
+  VERIFY( arr[0].i == 2 );
+  VERIFY( arr[1].i == 1 );
+
+  static_assert(noexcept(std::ranges::iter_move(i)));
+  X x = std::ranges::iter_move(i);
+  VERIFY( arr[0].i == -1 );
+  VERIFY( x.i == 2 );
+
+  return true;
+}
+
+static_assert(test01());