From e9987c3fb4e778742e7539416df702cf869c05bc Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Vojt=C4=9Bch=20Michal?= <vojta.michall@email.cz>
Date: Sun, 4 Jan 2026 13:10:52 +0100
Subject: [PATCH 1/3] Add tests

---
 tests/std/tests/P0429R9_flat_map/test.cpp | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/tests/std/tests/P0429R9_flat_map/test.cpp b/tests/std/tests/P0429R9_flat_map/test.cpp
index 74e88ddc81d..38aad5ca5ec 100644
--- a/tests/std/tests/P0429R9_flat_map/test.cpp
+++ b/tests/std/tests/P0429R9_flat_map/test.cpp
@@ -16,6 +16,11 @@
 
 using namespace std;
 
+// See GH-5965: Speculative resolution of LWG-3963 "Different flat_(multi)map specializations
+// should be able to share same nested classes" is not likely to be accepted
+static_assert(!is_same_v<flat_map<int, int>::containers, flat_multimap<int, int>::containers>);
+static_assert(!is_same_v<flat_map<int, int>::value_compare, flat_multimap<int, int>::value_compare>);
+
 template <class T, template <class...> class Tmpl>
 constexpr bool is_specialization_v = false;
 template <template <class...> class Tmpl, class... Ts>

From 4ebfb6a8255433d2928cba9a78d5ad2b11095076 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Vojt=C4=9Bch=20Michal?= <vojta.michall@email.cz>
Date: Sun, 4 Jan 2026 13:19:01 +0100
Subject: [PATCH 2/3] Move definition of flat_(multi)map::value_compare and
 containers inline.

---
 stl/inc/flat_map | 42 ++++++++++++++++--------------------------
 1 file changed, 16 insertions(+), 26 deletions(-)

diff --git a/stl/inc/flat_map b/stl/inc/flat_map
index 1e6eac4961b..2fd94bba83b 100644
--- a/stl/inc/flat_map
+++ b/stl/inc/flat_map
@@ -33,30 +33,6 @@ _STL_DISABLE_CLANG_WARNINGS
 #undef msvc
 
 _STD_BEGIN
-template <class _Key, class _Mapped, class _KeyCompare>
-struct _Flat_map_value_compare_provider {
-    struct value_compare {
-    public:
-        _NODISCARD bool operator()(
-            pair<const _Key&, const _Mapped&> _Left, pair<const _Key&, const _Mapped&> _Right) const {
-            return _Key_comparator(_Left.first, _Right.first);
-        }
-
-        value_compare(_KeyCompare _Comp) : _Key_comparator(_Comp) {}
-
-    private:
-        _KeyCompare _Key_comparator;
-    };
-};
-
-template <class _KeyContainer, class _MappedContainer>
-struct _Flat_map_container_provider {
-    struct containers {
-        _KeyContainer keys;
-        _MappedContainer values;
-    };
-};
-
 // Implementation
 
 template <bool _IsUnique, class _Key, class _Mapped, class _Compare, class _KeyContainer, class _MappedContainer>
@@ -342,8 +318,22 @@ public:
         "mapped_container_type must support random-access iterators in order to be adapted. "
         "(N5014 [flat.map.overview]/7, [flat.multimap.overview]/7)");
 
-    using value_compare = _Flat_map_value_compare_provider<key_type, mapped_type, key_compare>::value_compare;
-    using containers    = _Flat_map_container_provider<key_container_type, mapped_container_type>::containers;
+    struct value_compare {
+    public:
+        _NODISCARD bool operator()(const_reference _Left, const_reference _Right) const {
+            return _Key_comparator(_Left.first, _Right.first);
+        }
+
+        value_compare(key_compare _Comp) : _Key_comparator(_Comp) {}
+
+    private:
+        key_compare _Key_comparator;
+    };
+
+    struct containers {
+        key_container_type keys;
+        mapped_container_type values;
+    };
 
 public:
     // [flat.map.cons] Constructors

From d2b0fa56b3e4f30818817d829cb97f2fe4a2670d Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Vojt=C4=9Bch=20Michal?= <vojta.michall@email.cz>
Date: Sun, 4 Jan 2026 21:03:29 +0100
Subject: [PATCH 3/3] Also update tests in P0429R9_flat_map_ms_specific

---
 .../test.compile.pass.cpp                     | 30 ++++++++-----------
 1 file changed, 13 insertions(+), 17 deletions(-)

diff --git a/tests/std/tests/P0429R9_flat_map_ms_specific/test.compile.pass.cpp b/tests/std/tests/P0429R9_flat_map_ms_specific/test.compile.pass.cpp
index cdba07f3cbe..335c8e14326 100644
--- a/tests/std/tests/P0429R9_flat_map_ms_specific/test.compile.pass.cpp
+++ b/tests/std/tests/P0429R9_flat_map_ms_specific/test.compile.pass.cpp
@@ -51,6 +51,10 @@ struct flat_map_unique_if_impl<false> {
 template <bool IsUnique, class Key, class Mapped, class Comp, class KeyCont, class MappedCont>
 using flat_map_unique_if = flat_map_unique_if_impl<IsUnique>::template type<Key, Mapped, Comp, KeyCont, MappedCont>;
 
+template <typename A, typename B>
+constexpr bool has_different_nested_types = !is_same_v<typename A::value_compare, typename B::value_compare>
+                                         && !is_same_v<typename A::containers, typename B::containers>;
+
 template <bool IsUnique, class Comparator, class Alloc1, class Alloc2>
 void test_scary_ness_one() {
     using Iter = flat_map<int, int>::iterator;
@@ -63,23 +67,15 @@ void test_scary_ness_one() {
         flat_map_unique_if<IsUnique, int, int, Comparator, vector<int, Alloc1>, vector<int, Alloc2>>::const_iterator;
     static_assert(is_same_v<ConstIter, OtherConstIter>);
 
-    using Cont = flat_map<int, int, less<int>, vector<int, Alloc1>, vector<int, Alloc2>>::containers;
-    using OtherCont =
-        flat_map_unique_if<IsUnique, int, int, Comparator, vector<int, Alloc1>, vector<int, Alloc2>>::containers;
-    static_assert(is_same_v<Cont, OtherCont>);
-
-    using ValueComp = flat_map<int, int, Comparator, vector<int, Alloc1>, vector<int, Alloc2>>::value_compare;
-    using OtherValueComp1 =
-        flat_map_unique_if<IsUnique, int, int, Comparator, vector<int, Alloc1>, vector<int, Alloc2>>::value_compare;
-    using OtherValueComp2 = flat_map_unique_if<IsUnique, int, int, Comparator, vector<int>, vector<int>>::value_compare;
-    using OtherValueComp3 =
-        flat_map_unique_if<IsUnique, int, int, Comparator, vector<int, Alloc1>, deque<int, Alloc2>>::value_compare;
-    using OtherValueComp4 =
-        flat_map_unique_if<IsUnique, int, int, Comparator, deque<int, Alloc1>, vector<int, Alloc2>>::value_compare;
-    static_assert(is_same_v<ValueComp, OtherValueComp1>);
-    static_assert(is_same_v<ValueComp, OtherValueComp2>);
-    static_assert(is_same_v<ValueComp, OtherValueComp3>);
-    static_assert(is_same_v<ValueComp, OtherValueComp4>);
+    using Cont       = flat_map_unique_if<IsUnique, int, int, Comparator, vector<int, Alloc1>, vector<int, Alloc2>>;
+    using OtherCont1 = flat_map_unique_if<!IsUnique, int, int, Comparator, vector<int, Alloc1>, vector<int, Alloc2>>;
+    using OtherCont2 = flat_map_unique_if<IsUnique, int, int, Comparator, deque<int>, deque<int>>;
+    using OtherCont3 = flat_map_unique_if<IsUnique, int, int, Comparator, vector<int, Alloc1>, deque<int, Alloc2>>;
+    using OtherCont4 = flat_map_unique_if<IsUnique, int, int, Comparator, deque<int, Alloc1>, vector<int, Alloc2>>;
+    static_assert(has_different_nested_types<Cont, OtherCont1>);
+    static_assert(has_different_nested_types<Cont, OtherCont2>);
+    static_assert(has_different_nested_types<Cont, OtherCont3>);
+    static_assert(has_different_nested_types<Cont, OtherCont4>);
 }
 
 void test_scary_ness() {