[gcc r11-10136] libstdc++: Add constexpr to std::counted_iterator post-increment (LWG 3643)

[Jonathan Wakely <redi@gcc.gnu.org>]

https://gcc.gnu.org/g:debbc74f170c9e3ccbb7eaea13fb483a619a6dbf

commit r11-10136-gdebbc74f170c9e3ccbb7eaea13fb483a619a6dbf
Author: Jonathan Wakely <jwakely@redhat.com>
Date:   Thu May 26 12:41:03 2022 +0100

    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.
    
    (cherry picked from commit 47b20d027ade5bfbd932724d0ea2aedee7421243)

Diff:
---
 libstdc++-v3/include/bits/stl_iterator.h           |  3 +--
 .../24_iterators/counted_iterator/lwg3643.cc       | 27 ++++++++++++++++++++++
 2 files changed, 28 insertions(+), 2 deletions(-)

diff --git a/libstdc++-v3/include/bits/stl_iterator.h b/libstdc++-v3/include/bits/stl_iterator.h
index 4247bb116c3..38a828e633e 100644
--- a/libstdc++-v3/include/bits/stl_iterator.h
+++ b/libstdc++-v3/include/bits/stl_iterator.h
@@ -2223,7 +2223,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 	return *this;
       }
 
-      decltype(auto)
+      constexpr decltype(auto)
       operator++(int)
       {
 	__glibcxx_assert(_M_length > 0);
@@ -2235,7 +2235,6 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 	    ++_M_length;
 	    __throw_exception_again;
 	  }
-
       }
 
       constexpr counted_iterator
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() );