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.

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

@@ -2341,7 +2341,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 	return *this;
       }
 
-      decltype(auto)
+      constexpr decltype(auto)
       operator++(int)
       {
 	__glibcxx_assert(_M_length > 0);
@@ -2353,7 +2353,6 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 	    ++_M_length;
 	    __throw_exception_again;
 	  }
-
       }
 
       constexpr counted_iterator

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() );