libstdc++: Add constexpr to std::counted_iterator post-increment (LWG 3643)

libstdc++-v3/ChangeLog:

	* include/bits/stl_iterator.h (counted_iterator::operator++(int)):
	Add 'constexpr' as per LWG 3643.
	* testsuite/24_iterators/counted_iterator/lwg3643.cc: New test.

1 files changed, 27 insertions, 0 deletions

diff --git a/libstdc++-v3/testsuite/24_iterators/counted_iterator/lwg3643.cc b/libstdc++-v3/testsuite/24_iterators/counted_iterator/lwg3643.cc
new file mode 100644
index 00000000000..e6f12b46c12
--- /dev/null
+++ b/libstdc++-v3/testsuite/24_iterators/counted_iterator/lwg3643.cc
@@ -0,0 +1,27 @@
+// { dg-options "-std=gnu++20" }
+// { dg-do compile { target c++20 } }
+
+#include <iterator>
+
+struct InputIterator
+{
+  using difference_type = int;
+  using value_type = int;
+
+  constexpr int operator*() const noexcept { return 0; }
+  InputIterator& operator++() { return *this; }
+  constexpr void operator++(int) { }
+};
+
+static_assert( std::input_iterator<InputIterator> );
+static_assert( !std::forward_iterator<InputIterator> );
+
+constexpr bool
+test_lwg3643()
+{
+  std::counted_iterator<InputIterator> iter({}, 1);
+  iter++;
+  return iter == std::default_sentinel;
+}
+
+static_assert( test_lwg3643() );