[gcc r16-5806] libstdc++: Inconsistent const in flat_map's value_type [PR122921]

[Patrick Palka]

https://gcc.gnu.org/g:3e02f86cd13c787331281ab098901f8a67381945

commit r16-5806-g3e02f86cd13c787331281ab098901f8a67381945
Author: Patrick Palka <ppalka@redhat.com>
Date:   Mon Dec 1 17:08:01 2025 -0500

    libstdc++: Inconsistent const in flat_map's value_type [PR122921]
    
    flat_map's value_type is pair<key_type, mapped_type>, which we correctly
    define within the container but incorrectly within the iterator.
    
            PR libstdc++/122921
    
    libstdc++-v3/ChangeLog:
    
            * include/std/flat_map (_Flat_map_impl::_Iterator::value_type):
            Remove const from key_type to make consistent with the
            container's value_type.
            * testsuite/23_containers/flat_map/1.cc (test09): New test.
            * testsuite/23_containers/flat_multimap/1.cc (test09): New test.
    
    Reported-by: Vincent X
    Reviewed-by: Jonathan Wakely <jwakely@redhat.com>

Diff:
---
 libstdc++-v3/include/std/flat_map                       |  2 +-
 libstdc++-v3/testsuite/23_containers/flat_map/1.cc      | 12 ++++++++++++
 libstdc++-v3/testsuite/23_containers/flat_multimap/1.cc | 12 ++++++++++++
 3 files changed, 25 insertions(+), 1 deletion(-)

diff --git a/libstdc++-v3/include/std/flat_map b/libstdc++-v3/include/std/flat_map
index de006ad1c533..e48c3eae07fd 100644
--- a/libstdc++-v3/include/std/flat_map
+++ b/libstdc++-v3/include/std/flat_map
@@ -953,7 +953,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
     public:
       using iterator_category = input_iterator_tag;
       using iterator_concept  = random_access_iterator_tag;
-      using value_type        = pair<const key_type, mapped_type>;
+      using value_type        = pair<key_type, mapped_type>;
       using reference         = pair<const key_type&,
 				     ranges::__maybe_const_t<_Const, mapped_type>&>;
       using difference_type   = ptrdiff_t;
diff --git a/libstdc++-v3/testsuite/23_containers/flat_map/1.cc b/libstdc++-v3/testsuite/23_containers/flat_map/1.cc
index 01278d7dc33c..5bcda3464f1f 100644
--- a/libstdc++-v3/testsuite/23_containers/flat_map/1.cc
+++ b/libstdc++-v3/testsuite/23_containers/flat_map/1.cc
@@ -263,6 +263,17 @@ test08()
   m[k] = 0;
 }
 
+void
+test09()
+{
+  // PR libstdc++/122921 - The value_type of flat_map's iterator should be
+  // pair<Key, T> instead of pair<const Key, T>
+  using type = std::flat_map<int, int>;
+  using value_type = std::ranges::range_value_t<type>;
+  using value_type = type::value_type;
+  using value_type = std::pair<int, int>;
+}
+
 int
 main()
 {
@@ -277,4 +288,5 @@ main()
   test06();
   test07();
   test08();
+  test09();
 }
diff --git a/libstdc++-v3/testsuite/23_containers/flat_multimap/1.cc b/libstdc++-v3/testsuite/23_containers/flat_multimap/1.cc
index d746614401de..f143f3acd1b5 100644
--- a/libstdc++-v3/testsuite/23_containers/flat_multimap/1.cc
+++ b/libstdc++-v3/testsuite/23_containers/flat_multimap/1.cc
@@ -232,6 +232,17 @@ test07()
   VERIFY( std::ranges::equal(m, (std::pair<int,int>[]){{3,4},{3,3}}) );
 }
 
+void
+test09()
+{
+  // PR libstdc++/122921 - The value_type of flat_map's iterator should be
+  // pair<Key, T> instead of pair<const Key, T>
+  using type = std::flat_multimap<int, int>;
+  using value_type = std::ranges::range_value_t<type>;
+  using value_type = type::value_type;
+  using value_type = std::pair<int, int>;
+}
+
 int
 main()
 {
@@ -245,4 +256,5 @@ main()
   test05();
   test06();
   test07();
+  test09();
 }