libstdc++: Implement missing P0896R4 changes to reverse_iterator [PR100639]

This implements the P0896R4 changes to reverse_iterator's member types
value_type, difference_type and reference in C++20 mode, which fixes
taking the reverse_iterator of an iterator with a non-integral
difference_type (such as iota_view<long long>).

libstdc++-v3/ChangeLog:

	PR libstdc++/100639
	* include/bits/stl_iterator.h (reverse_iterator::difference_type):
	In C++20 mode, define in terms of iter_difference_t as per P0896R4.
	(reverse_iterator::reference): Likewise, but with iter_reference_t.
	(reverse_iterator::value_type): Likewise, but with iter_value_t.
	* testsuite/std/ranges/adaptors/reverse.cc (test08): New test.
	* testsuite/24_iterators/reverse_iterator/100639.cc: New test.

3 files changed, 53 insertions, 3 deletions

diff --git a/libstdc++-v3/include/bits/stl_iterator.h b/libstdc++-v3/include/bits/stl_iterator.h
index 2409cd7..8768624 100644
--- a/libstdc++-v3/include/bits/stl_iterator.h
+++ b/libstdc++-v3/include/bits/stl_iterator.h
@@ -149,11 +149,11 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 
     public:
       typedef _Iterator					iterator_type;
-      typedef typename __traits_type::difference_type	difference_type;
       typedef typename __traits_type::pointer		pointer;
+#if __cplusplus <= 201703L
+      typedef typename __traits_type::difference_type	difference_type;
       typedef typename __traits_type::reference		reference;
-
-#if __cplusplus > 201703L && __cpp_lib_concepts
+#else
       using iterator_concept
 	= conditional_t<random_access_iterator<_Iterator>,
 			random_access_iterator_tag,
@@ -161,6 +161,9 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
       using iterator_category
 	= __detail::__clamp_iter_cat<typename __traits_type::iterator_category,
 				     random_access_iterator_tag>;
+      using value_type = iter_value_t<_Iterator>;
+      using difference_type = iter_difference_t<_Iterator>;
+      using reference = iter_reference_t<_Iterator>;
 #endif
 
       /**
diff --git a/libstdc++-v3/testsuite/24_iterators/reverse_iterator/100639.cc b/libstdc++-v3/testsuite/24_iterators/reverse_iterator/100639.cc
new file mode 100644
index 0000000..358d91d
--- /dev/null
+++ b/libstdc++-v3/testsuite/24_iterators/reverse_iterator/100639.cc
@@ -0,0 +1,37 @@
+// Copyright (C) 2021 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=c++2a" }
+// { dg-do compile { target c++2a } }
+
+// PR libstdc++/100639
+
+#include <iterator>
+#include <ranges>
+
+void
+test01()
+{
+  using iter = std::ranges::iterator_t<std::ranges::iota_view<long long>>;
+  using riter = std::reverse_iterator<iter>;
+  static_assert(std::same_as<std::iter_reference_t<iter>,
+			     std::iter_reference_t<riter>>);
+  static_assert(std::same_as<std::iter_value_t<iter>,
+			     std::iter_value_t<riter>>);
+  static_assert(std::same_as<std::iter_difference_t<iter>,
+			     std::iter_difference_t<riter>>);
+}
diff --git a/libstdc++-v3/testsuite/std/ranges/adaptors/reverse.cc b/libstdc++-v3/testsuite/std/ranges/adaptors/reverse.cc
index 47e34eb..d9094a9 100644
--- a/libstdc++-v3/testsuite/std/ranges/adaptors/reverse.cc
+++ b/libstdc++-v3/testsuite/std/ranges/adaptors/reverse.cc
@@ -161,6 +161,15 @@ test07()
   static_assert(!requires { 0 | reverse; });
 }
 
+void
+test08()
+{
+  // PR libstdc++/100639
+  auto v = views::iota(1701ll, 3000ll) | views::reverse | views::take(5);
+  for (auto x : v)
+    ;
+}
+
 int
 main()
 {
@@ -171,4 +180,5 @@ main()
   test05();
   test06();
   test07();
+  test08();
 }