libstdc++: Implement P2520R0 changes to move_iterator's iterator_concept

libstdc++-v3/ChangeLog:

	* include/bits/stl_iterator.h (move_iterator::_S_iter_concept):
	Define.
	(__cpp_lib_move_iterator_concept): Define for C++20.
	(move_iterator::iterator_concept): Strengthen as per P2520R0.
	* include/std/version (__cpp_lib_move_iterator_concept): Define
	for C++20.
	* testsuite/24_iterators/move_iterator/p2520r0.cc: New test.

(cherry picked from commit 2b204accd07a3185b58b1edc6e9b019472857a5d)

3 files changed, 57 insertions, 1 deletions

diff --git a/libstdc++-v3/include/bits/stl_iterator.h b/libstdc++-v3/include/bits/stl_iterator.h
index 15b9affc4b9..2d4fc3435e1 100644
--- a/libstdc++-v3/include/bits/stl_iterator.h
+++ b/libstdc++-v3/include/bits/stl_iterator.h
@@ -1465,11 +1465,29 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 	    && convertible_to<const _Iter2&, _Iterator>;
 #endif
 
+#if __cplusplus > 201703L && __cpp_lib_concepts
+      static auto
+      _S_iter_concept()
+      {
+	if constexpr (random_access_iterator<_Iterator>)
+	  return random_access_iterator_tag{};
+	else if constexpr (bidirectional_iterator<_Iterator>)
+	  return bidirectional_iterator_tag{};
+	else if constexpr (forward_iterator<_Iterator>)
+	  return forward_iterator_tag{};
+	else
+	  return input_iterator_tag{};
+      }
+#endif
+
     public:
       using iterator_type = _Iterator;
 
 #if __cplusplus > 201703L && __cpp_lib_concepts
-      using iterator_concept = input_iterator_tag;
+      // This is P2520R0, a C++23 change, but we treat it as a DR against C++20.
+# define __cpp_lib_move_iterator_concept 202207L
+      using iterator_concept = decltype(_S_iter_concept());
+
       // iterator_category defined in __move_iter_cat
       using value_type = iter_value_t<_Iterator>;
       using difference_type = iter_difference_t<_Iterator>;
diff --git a/libstdc++-v3/include/std/version b/libstdc++-v3/include/std/version
index dcc5008d8a5..ab6a3241b90 100644
--- a/libstdc++-v3/include/std/version
+++ b/libstdc++-v3/include/std/version
@@ -276,6 +276,7 @@
 #define __cpp_lib_polymorphic_allocator 201902L
 #if __cpp_lib_concepts
 # define __cpp_lib_ranges 202110L
+# define __cpp_lib_move_iterator_concept 202207L
 #endif
 #if __cpp_lib_atomic_wait || _GLIBCXX_HAVE_POSIX_SEMAPHORE
 # define __cpp_lib_semaphore 201907L
diff --git a/libstdc++-v3/testsuite/24_iterators/move_iterator/p2520r0.cc b/libstdc++-v3/testsuite/24_iterators/move_iterator/p2520r0.cc
new file mode 100644
index 00000000000..883d6cc09e0
--- /dev/null
+++ b/libstdc++-v3/testsuite/24_iterators/move_iterator/p2520r0.cc
@@ -0,0 +1,37 @@
+// { dg-options "-std=gnu++20" }
+// { dg-do compile { target c++20 } }
+
+// Verify P2520R0 changes to move_iterator's iterator_concept, which we treat
+// as a DR against C++20.
+
+#include <iterator>
+#if __cpp_lib_move_iterator_concept != 202207L
+# error "Feature-test macro __cpp_lib_move_iterator_concept has wrong value in <iterator>"
+#endif
+
+#undef __cpp_lib_move_iterator_concept
+#include <version>
+#if __cpp_lib_move_iterator_concept != 202207L
+# error "Feature-test macro __cpp_lib_move_iterator_concept has wrong value in <version>"
+#endif
+
+#include <testsuite_iterators.h>
+
+using __gnu_test::test_input_range;
+using __gnu_test::test_forward_range;
+using __gnu_test::test_bidirectional_range;
+using __gnu_test::test_random_access_range;
+
+using ty1 = std::move_iterator<decltype(std::declval<test_input_range<int>&>().begin())>;
+static_assert(std::same_as<ty1::iterator_concept, std::input_iterator_tag>);
+
+using ty2 = std::move_iterator<decltype(std::declval<test_forward_range<int>&>().begin())>;
+static_assert(std::same_as<ty2::iterator_concept, std::forward_iterator_tag>);
+
+using ty3 = std::move_iterator<decltype(std::declval<test_bidirectional_range<int>&>().begin())>;
+static_assert(std::same_as<ty3::iterator_concept, std::bidirectional_iterator_tag>);
+
+using ty4 = std::move_iterator<decltype(std::declval<test_random_access_range<int>&>().begin())>;
+static_assert(std::same_as<ty4::iterator_concept, std::random_access_iterator_tag>);
+
+static_assert(std::random_access_iterator<std::move_iterator<int*>>);