diff --git a/libcxx/docs/FeatureTestMacroTable.rst b/libcxx/docs/FeatureTestMacroTable.rst
index 358889d8dbc37..4b5cf57506bd3 100644
--- a/libcxx/docs/FeatureTestMacroTable.rst
+++ b/libcxx/docs/FeatureTestMacroTable.rst
@@ -424,6 +424,10 @@ Status
     ---------------------------------------------------------- -----------------
     ``__cpp_lib_constexpr_algorithms``                         ``202306L``
     ---------------------------------------------------------- -----------------
+    ``__cpp_lib_constexpr_flat_map``                           ``202502L``
+    ---------------------------------------------------------- -----------------
+    ``__cpp_lib_constexpr_flat_set``                           ``202502L``
+    ---------------------------------------------------------- -----------------
     ``__cpp_lib_constexpr_forward_list``                       ``202502L``
     ---------------------------------------------------------- -----------------
     ``__cpp_lib_constexpr_list``                               ``202502L``
diff --git a/libcxx/include/__flat_set/flat_multiset.h b/libcxx/include/__flat_set/flat_multiset.h
index 44d8af05a56af..5e04f3a954c56 100644
--- a/libcxx/include/__flat_set/flat_multiset.h
+++ b/libcxx/include/__flat_set/flat_multiset.h
@@ -108,16 +108,16 @@ class flat_multiset {
 
 public:
   // [flat.multiset.cons], constructors
-  _LIBCPP_HIDE_FROM_ABI flat_multiset() noexcept(is_nothrow_default_constructible_v<_KeyContainer> &&
-                                                 is_nothrow_default_constructible_v<_Compare>)
+  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX26 flat_multiset() noexcept(
+      is_nothrow_default_constructible_v<_KeyContainer> && is_nothrow_default_constructible_v<_Compare>)
       : __keys_(), __compare_() {}
 
-  _LIBCPP_HIDE_FROM_ABI flat_multiset(const flat_multiset&) = default;
+  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX26 flat_multiset(const flat_multiset&) = default;
 
   // The copy/move constructors are not specified in the spec, which means they should be defaulted.
   // However, the move constructor can potentially leave a moved-from object in an inconsistent
   // state if an exception is thrown.
-  _LIBCPP_HIDE_FROM_ABI flat_multiset(flat_multiset&& __other) noexcept(
+  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX26 flat_multiset(flat_multiset&& __other) noexcept(
       is_nothrow_move_constructible_v<_KeyContainer> && is_nothrow_move_constructible_v<_Compare>)
 #  if _LIBCPP_HAS_EXCEPTIONS
       try
@@ -134,14 +134,16 @@ class flat_multiset {
 #  endif // _LIBCPP_HAS_EXCEPTIONS
   }
 
-  _LIBCPP_HIDE_FROM_ABI explicit flat_multiset(const key_compare& __comp) : __keys_(), __compare_(__comp) {}
+  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX26 explicit flat_multiset(const key_compare& __comp)
+      : __keys_(), __compare_(__comp) {}
 
-  _LIBCPP_HIDE_FROM_ABI explicit flat_multiset(container_type __keys, const key_compare& __comp = key_compare())
+  _LIBCPP_HIDE_FROM_ABI
+  _LIBCPP_CONSTEXPR_SINCE_CXX26 explicit flat_multiset(container_type __keys, const key_compare& __comp = key_compare())
       : __keys_(std::move(__keys)), __compare_(__comp) {
     ranges::sort(__keys_, __compare_);
   }
 
-  _LIBCPP_HIDE_FROM_ABI
+  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX26
   flat_multiset(sorted_equivalent_t, container_type __keys, const key_compare& __comp = key_compare())
       : __keys_(std::move(__keys)), __compare_(__comp) {
     _LIBCPP_ASSERT_SEMANTIC_REQUIREMENT(ranges::is_sorted(__keys_, __compare_), "Key container is not sorted");
@@ -149,7 +151,7 @@ class flat_multiset {
 
   template <class _InputIterator>
     requires __has_input_iterator_category<_InputIterator>::value
-  _LIBCPP_HIDE_FROM_ABI
+  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX26
   flat_multiset(_InputIterator __first, _InputIterator __last, const key_compare& __comp = key_compare())
       : __keys_(), __compare_(__comp) {
     insert(__first, __last);
@@ -157,48 +159,53 @@ class flat_multiset {
 
   template <class _InputIterator>
     requires __has_input_iterator_category<_InputIterator>::value
-  _LIBCPP_HIDE_FROM_ABI flat_multiset(
+  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX26 flat_multiset(
       sorted_equivalent_t, _InputIterator __first, _InputIterator __last, const key_compare& __comp = key_compare())
       : __keys_(__first, __last), __compare_(__comp) {
     _LIBCPP_ASSERT_SEMANTIC_REQUIREMENT(ranges::is_sorted(__keys_, __compare_), "Key container is not sorted");
   }
 
   template <_ContainerCompatibleRange<value_type> _Range>
-  _LIBCPP_HIDE_FROM_ABI flat_multiset(from_range_t __fr, _Range&& __rg)
+  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX26 flat_multiset(from_range_t __fr, _Range&& __rg)
       : flat_multiset(__fr, std::forward<_Range>(__rg), key_compare()) {}
 
   template <_ContainerCompatibleRange<value_type> _Range>
-  _LIBCPP_HIDE_FROM_ABI flat_multiset(from_range_t, _Range&& __rg, const key_compare& __comp) : flat_multiset(__comp) {
+  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX26
+  flat_multiset(from_range_t, _Range&& __rg, const key_compare& __comp)
+      : flat_multiset(__comp) {
     insert_range(std::forward<_Range>(__rg));
   }
 
-  _LIBCPP_HIDE_FROM_ABI flat_multiset(initializer_list<value_type> __il, const key_compare& __comp = key_compare())
+  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX26
+  flat_multiset(initializer_list<value_type> __il, const key_compare& __comp = key_compare())
       : flat_multiset(__il.begin(), __il.end(), __comp) {}
 
-  _LIBCPP_HIDE_FROM_ABI
+  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX26
   flat_multiset(sorted_equivalent_t, initializer_list<value_type> __il, const key_compare& __comp = key_compare())
       : flat_multiset(sorted_equivalent, __il.begin(), __il.end(), __comp) {}
 
   template <class _Allocator>
     requires uses_allocator<container_type, _Allocator>::value
-  _LIBCPP_HIDE_FROM_ABI explicit flat_multiset(const _Allocator& __alloc)
+  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX26 explicit flat_multiset(const _Allocator& __alloc)
       : __keys_(std::make_obj_using_allocator<container_type>(__alloc)), __compare_() {}
 
   template <class _Allocator>
     requires uses_allocator<container_type, _Allocator>::value
-  _LIBCPP_HIDE_FROM_ABI flat_multiset(const key_compare& __comp, const _Allocator& __alloc)
+  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX26
+  flat_multiset(const key_compare& __comp, const _Allocator& __alloc)
       : __keys_(std::make_obj_using_allocator<container_type>(__alloc)), __compare_(__comp) {}
 
   template <class _Allocator>
     requires uses_allocator<container_type, _Allocator>::value
-  _LIBCPP_HIDE_FROM_ABI flat_multiset(const container_type& __keys, const _Allocator& __alloc)
+  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX26
+  flat_multiset(const container_type& __keys, const _Allocator& __alloc)
       : __keys_(std::make_obj_using_allocator<container_type>(__alloc, __keys)), __compare_() {
     ranges::sort(__keys_, __compare_);
   }
 
   template <class _Allocator>
     requires uses_allocator<container_type, _Allocator>::value
-  _LIBCPP_HIDE_FROM_ABI
+  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX26
   flat_multiset(const container_type& __keys, const key_compare& __comp, const _Allocator& __alloc)
       : __keys_(std::make_obj_using_allocator<container_type>(__alloc, __keys)), __compare_(__comp) {
     ranges::sort(__keys_, __compare_);
@@ -206,14 +213,15 @@ class flat_multiset {
 
   template <class _Allocator>
     requires uses_allocator<container_type, _Allocator>::value
-  _LIBCPP_HIDE_FROM_ABI flat_multiset(sorted_equivalent_t, const container_type& __keys, const _Allocator& __alloc)
+  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX26
+  flat_multiset(sorted_equivalent_t, const container_type& __keys, const _Allocator& __alloc)
       : __keys_(std::make_obj_using_allocator<container_type>(__alloc, __keys)), __compare_() {
     _LIBCPP_ASSERT_SEMANTIC_REQUIREMENT(ranges::is_sorted(__keys_, __compare_), "Key container is not sorted");
   }
 
   template <class _Allocator>
     requires uses_allocator<container_type, _Allocator>::value
-  _LIBCPP_HIDE_FROM_ABI
+  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX26
   flat_multiset(sorted_equivalent_t, const container_type& __keys, const key_compare& __comp, const _Allocator& __alloc)
       : __keys_(std::make_obj_using_allocator<container_type>(__alloc, __keys)), __compare_(__comp) {
     _LIBCPP_ASSERT_SEMANTIC_REQUIREMENT(ranges::is_sorted(__keys_, __compare_), "Key container is not sorted");
@@ -221,13 +229,14 @@ class flat_multiset {
 
   template <class _Allocator>
     requires uses_allocator<container_type, _Allocator>::value
-  _LIBCPP_HIDE_FROM_ABI flat_multiset(const flat_multiset& __other, const _Allocator& __alloc)
+  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX26
+  flat_multiset(const flat_multiset& __other, const _Allocator& __alloc)
       : __keys_(std::make_obj_using_allocator<container_type>(__alloc, __other.__keys_)),
         __compare_(__other.__compare_) {}
 
   template <class _Allocator>
     requires uses_allocator<container_type, _Allocator>::value
-  _LIBCPP_HIDE_FROM_ABI flat_multiset(flat_multiset&& __other, const _Allocator& __alloc)
+  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX26 flat_multiset(flat_multiset&& __other, const _Allocator& __alloc)
 #  if _LIBCPP_HAS_EXCEPTIONS
       try
 #  endif // _LIBCPP_HAS_EXCEPTIONS
@@ -243,14 +252,15 @@ class flat_multiset {
 
   template <class _InputIterator, class _Allocator>
     requires(__has_input_iterator_category<_InputIterator>::value && uses_allocator<container_type, _Allocator>::value)
-  _LIBCPP_HIDE_FROM_ABI flat_multiset(_InputIterator __first, _InputIterator __last, const _Allocator& __alloc)
+  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX26
+  flat_multiset(_InputIterator __first, _InputIterator __last, const _Allocator& __alloc)
       : __keys_(std::make_obj_using_allocator<container_type>(__alloc)), __compare_() {
     insert(__first, __last);
   }
 
   template <class _InputIterator, class _Allocator>
     requires(__has_input_iterator_category<_InputIterator>::value && uses_allocator<container_type, _Allocator>::value)
-  _LIBCPP_HIDE_FROM_ABI
+  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX26
   flat_multiset(_InputIterator __first, _InputIterator __last, const key_compare& __comp, const _Allocator& __alloc)
       : __keys_(std::make_obj_using_allocator<container_type>(__alloc)), __compare_(__comp) {
     insert(__first, __last);
@@ -258,7 +268,7 @@ class flat_multiset {
 
   template <class _InputIterator, class _Allocator>
     requires(__has_input_iterator_category<_InputIterator>::value && uses_allocator<container_type, _Allocator>::value)
-  _LIBCPP_HIDE_FROM_ABI
+  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX26
   flat_multiset(sorted_equivalent_t, _InputIterator __first, _InputIterator __last, const _Allocator& __alloc)
       : __keys_(std::make_obj_using_allocator<container_type>(__alloc, __first, __last)), __compare_() {
     _LIBCPP_ASSERT_SEMANTIC_REQUIREMENT(ranges::is_sorted(__keys_, __compare_), "Key container is not sorted");
@@ -266,53 +276,57 @@ class flat_multiset {
 
   template <class _InputIterator, class _Allocator>
     requires(__has_input_iterator_category<_InputIterator>::value && uses_allocator<container_type, _Allocator>::value)
-  _LIBCPP_HIDE_FROM_ABI
-  flat_multiset(sorted_equivalent_t,
-                _InputIterator __first,
-                _InputIterator __last,
-                const key_compare& __comp,
-                const _Allocator& __alloc)
+  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX26 flat_multiset(
+      sorted_equivalent_t,
+      _InputIterator __first,
+      _InputIterator __last,
+      const key_compare& __comp,
+      const _Allocator& __alloc)
       : __keys_(std::make_obj_using_allocator<container_type>(__alloc, __first, __last)), __compare_(__comp) {
     _LIBCPP_ASSERT_SEMANTIC_REQUIREMENT(ranges::is_sorted(__keys_, __compare_), "Key container is not sorted");
   }
 
   template <_ContainerCompatibleRange<value_type> _Range, class _Allocator>
     requires uses_allocator<container_type, _Allocator>::value
-  _LIBCPP_HIDE_FROM_ABI flat_multiset(from_range_t, _Range&& __rg, const _Allocator& __alloc)
+  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX26
+  flat_multiset(from_range_t, _Range&& __rg, const _Allocator& __alloc)
       : __keys_(std::make_obj_using_allocator<container_type>(__alloc)), __compare_() {
     insert_range(std::forward<_Range>(__rg));
   }
 
   template <_ContainerCompatibleRange<value_type> _Range, class _Allocator>
     requires uses_allocator<container_type, _Allocator>::value
-  _LIBCPP_HIDE_FROM_ABI flat_multiset(from_range_t, _Range&& __rg, const key_compare& __comp, const _Allocator& __alloc)
+  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX26
+  flat_multiset(from_range_t, _Range&& __rg, const key_compare& __comp, const _Allocator& __alloc)
       : __keys_(std::make_obj_using_allocator<container_type>(__alloc)), __compare_(__comp) {
     insert_range(std::forward<_Range>(__rg));
   }
 
   template <class _Allocator>
     requires uses_allocator<container_type, _Allocator>::value
-  _LIBCPP_HIDE_FROM_ABI flat_multiset(initializer_list<value_type> __il, const _Allocator& __alloc)
+  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX26
+  flat_multiset(initializer_list<value_type> __il, const _Allocator& __alloc)
       : flat_multiset(__il.begin(), __il.end(), __alloc) {}
 
   template <class _Allocator>
     requires uses_allocator<container_type, _Allocator>::value
-  _LIBCPP_HIDE_FROM_ABI
+  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX26
   flat_multiset(initializer_list<value_type> __il, const key_compare& __comp, const _Allocator& __alloc)
       : flat_multiset(__il.begin(), __il.end(), __comp, __alloc) {}
 
   template <class _Allocator>
     requires uses_allocator<container_type, _Allocator>::value
-  _LIBCPP_HIDE_FROM_ABI flat_multiset(sorted_equivalent_t, initializer_list<value_type> __il, const _Allocator& __alloc)
+  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX26
+  flat_multiset(sorted_equivalent_t, initializer_list<value_type> __il, const _Allocator& __alloc)
       : flat_multiset(sorted_equivalent, __il.begin(), __il.end(), __alloc) {}
 
   template <class _Allocator>
     requires uses_allocator<container_type, _Allocator>::value
-  _LIBCPP_HIDE_FROM_ABI flat_multiset(
+  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX26 flat_multiset(
       sorted_equivalent_t, initializer_list<value_type> __il, const key_compare& __comp, const _Allocator& __alloc)
       : flat_multiset(sorted_equivalent, __il.begin(), __il.end(), __comp, __alloc) {}
 
-  _LIBCPP_HIDE_FROM_ABI flat_multiset& operator=(initializer_list<value_type> __il) {
+  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX26 flat_multiset& operator=(initializer_list<value_type> __il) {
     clear();
     insert(__il);
     return *this;
@@ -321,9 +335,9 @@ class flat_multiset {
   // copy/move assignment are not specified in the spec (defaulted)
   // but move assignment can potentially leave moved from object in an inconsistent
   // state if an exception is thrown
-  _LIBCPP_HIDE_FROM_ABI flat_multiset& operator=(const flat_multiset&) = default;
+  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX26 flat_multiset& operator=(const flat_multiset&) = default;
 
-  _LIBCPP_HIDE_FROM_ABI flat_multiset& operator=(flat_multiset&& __other) noexcept(
+  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX26 flat_multiset& operator=(flat_multiset&& __other) noexcept(
       is_nothrow_move_assignable_v<_KeyContainer> && is_nothrow_move_assignable_v<_Compare>) {
     auto __clear_other_guard = std::__make_scope_guard([&]() noexcept { __other.clear() /* noexcept */; });
     auto __clear_self_guard  = std::__make_exception_guard([&]() noexcept { clear() /* noexcept */; });
@@ -334,30 +348,52 @@ class flat_multiset {
   }
 
   // iterators
-  _LIBCPP_HIDE_FROM_ABI iterator begin() noexcept { return iterator(std::as_const(__keys_).begin()); }
-  _LIBCPP_HIDE_FROM_ABI const_iterator begin() const noexcept { return const_iterator(__keys_.begin()); }
-  _LIBCPP_HIDE_FROM_ABI iterator end() noexcept { return iterator(std::as_const(__keys_).end()); }
-  _LIBCPP_HIDE_FROM_ABI const_iterator end() const noexcept { return const_iterator(__keys_.end()); }
+  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX26 iterator begin() noexcept {
+    return iterator(std::as_const(__keys_).begin());
+  }
+  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX26 const_iterator begin() const noexcept {
+    return const_iterator(__keys_.begin());
+  }
+  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX26 iterator end() noexcept {
+    return iterator(std::as_const(__keys_).end());
+  }
+  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX26 const_iterator end() const noexcept {
+    return const_iterator(__keys_.end());
+  }
 
-  _LIBCPP_HIDE_FROM_ABI reverse_iterator rbegin() noexcept { return reverse_iterator(end()); }
-  _LIBCPP_HIDE_FROM_ABI const_reverse_iterator rbegin() const noexcept { return const_reverse_iterator(end()); }
-  _LIBCPP_HIDE_FROM_ABI reverse_iterator rend() noexcept { return reverse_iterator(begin()); }
-  _LIBCPP_HIDE_FROM_ABI const_reverse_iterator rend() const noexcept { return const_reverse_iterator(begin()); }
+  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX26 reverse_iterator rbegin() noexcept {
+    return reverse_iterator(end());
+  }
+  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX26 const_reverse_iterator rbegin() const noexcept {
+    return const_reverse_iterator(end());
+  }
+  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX26 reverse_iterator rend() noexcept {
+    return reverse_iterator(begin());
+  }
+  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX26 const_reverse_iterator rend() const noexcept {
+    return const_reverse_iterator(begin());
+  }
 
-  _LIBCPP_HIDE_FROM_ABI const_iterator cbegin() const noexcept { return begin(); }
-  _LIBCPP_HIDE_FROM_ABI const_iterator cend() const noexcept { return end(); }
-  _LIBCPP_HIDE_FROM_ABI const_reverse_iterator crbegin() const noexcept { return const_reverse_iterator(end()); }
-  _LIBCPP_HIDE_FROM_ABI const_reverse_iterator crend() const noexcept { return const_reverse_iterator(begin()); }
+  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX26 const_iterator cbegin() const noexcept { return begin(); }
+  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX26 const_iterator cend() const noexcept { return end(); }
+  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX26 const_reverse_iterator crbegin() const noexcept {
+    return const_reverse_iterator(end());
+  }
+  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX26 const_reverse_iterator crend() const noexcept {
+    return const_reverse_iterator(begin());
+  }
 
   // capacity
-  [[nodiscard]] _LIBCPP_HIDE_FROM_ABI bool empty() const noexcept { return __keys_.empty(); }
-  _LIBCPP_HIDE_FROM_ABI size_type size() const noexcept { return __keys_.size(); }
-  _LIBCPP_HIDE_FROM_ABI size_type max_size() const noexcept { return __keys_.max_size(); }
+  [[nodiscard]] _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX26 bool empty() const noexcept {
+    return __keys_.empty();
+  }
+  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX26 size_type size() const noexcept { return __keys_.size(); }
+  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX26 size_type max_size() const noexcept { return __keys_.max_size(); }
 
   // [flat.multiset.modifiers], modifiers
   template <class... _Args>
     requires is_constructible_v<value_type, _Args...>
-  _LIBCPP_HIDE_FROM_ABI iterator emplace(_Args&&... __args) {
+  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX26 iterator emplace(_Args&&... __args) {
     if constexpr (sizeof...(__args) == 1 && (is_same_v<remove_cvref_t<_Args>, _Key> && ...)) {
       return __emplace(std::forward<_Args>(__args)...);
     } else {
@@ -367,7 +403,7 @@ class flat_multiset {
 
   template <class... _Args>
     requires is_constructible_v<value_type, _Args...>
-  _LIBCPP_HIDE_FROM_ABI iterator emplace_hint(const_iterator __hint, _Args&&... __args) {
+  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX26 iterator emplace_hint(const_iterator __hint, _Args&&... __args) {
     if constexpr (sizeof...(__args) == 1 && (is_same_v<remove_cvref_t<_Args>, _Key> && ...)) {
       return __emplace_hint(std::move(__hint), std::forward<_Args>(__args)...);
     } else {
@@ -375,21 +411,23 @@ class flat_multiset {
     }
   }
 
-  _LIBCPP_HIDE_FROM_ABI iterator insert(const value_type& __x) { return emplace(__x); }
+  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX26 iterator insert(const value_type& __x) { return emplace(__x); }
 
-  _LIBCPP_HIDE_FROM_ABI iterator insert(value_type&& __x) { return emplace(std::move(__x)); }
+  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX26 iterator insert(value_type&& __x) {
+    return emplace(std::move(__x));
+  }
 
-  _LIBCPP_HIDE_FROM_ABI iterator insert(const_iterator __hint, const value_type& __x) {
+  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX26 iterator insert(const_iterator __hint, const value_type& __x) {
     return emplace_hint(__hint, __x);
   }
 
-  _LIBCPP_HIDE_FROM_ABI iterator insert(const_iterator __hint, value_type&& __x) {
+  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX26 iterator insert(const_iterator __hint, value_type&& __x) {
     return emplace_hint(__hint, std::move(__x));
   }
 
   template <class _InputIterator>
     requires __has_input_iterator_category<_InputIterator>::value
-  _LIBCPP_HIDE_FROM_ABI void insert(_InputIterator __first, _InputIterator __last) {
+  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX26 void insert(_InputIterator __first, _InputIterator __last) {
     if constexpr (sized_sentinel_for<_InputIterator, _InputIterator>) {
       __reserve(__last - __first);
     }
@@ -398,7 +436,8 @@ class flat_multiset {
 
   template <class _InputIterator>
     requires __has_input_iterator_category<_InputIterator>::value
-  _LIBCPP_HIDE_FROM_ABI void insert(sorted_equivalent_t, _InputIterator __first, _InputIterator __last) {
+  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX26 void
+  insert(sorted_equivalent_t, _InputIterator __first, _InputIterator __last) {
     if constexpr (sized_sentinel_for<_InputIterator, _InputIterator>) {
       __reserve(__last - __first);
     }
@@ -407,7 +446,7 @@ class flat_multiset {
   }
 
   template <_ContainerCompatibleRange<value_type> _Range>
-  _LIBCPP_HIDE_FROM_ABI void insert_range(_Range&& __range) {
+  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX26 void insert_range(_Range&& __range) {
     if constexpr (ranges::sized_range<_Range>) {
       __reserve(ranges::size(__range));
     }
@@ -415,26 +454,29 @@ class flat_multiset {
     __append_sort_merge</*WasSorted = */ false>(std::forward<_Range>(__range));
   }
 
-  _LIBCPP_HIDE_FROM_ABI void insert(initializer_list<value_type> __il) { insert(__il.begin(), __il.end()); }
+  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX26 void insert(initializer_list<value_type> __il) {
+    insert(__il.begin(), __il.end());
+  }
 
-  _LIBCPP_HIDE_FROM_ABI void insert(sorted_equivalent_t, initializer_list<value_type> __il) {
+  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX26 void
+  insert(sorted_equivalent_t, initializer_list<value_type> __il) {
     insert(sorted_equivalent, __il.begin(), __il.end());
   }
 
-  _LIBCPP_HIDE_FROM_ABI container_type extract() && {
+  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX26 container_type extract() && {
     auto __guard = std::__make_scope_guard([&]() noexcept { clear() /* noexcept */; });
     auto __ret   = std::move(__keys_);
     return __ret;
   }
 
-  _LIBCPP_HIDE_FROM_ABI void replace(container_type&& __keys) {
+  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX26 void replace(container_type&& __keys) {
     _LIBCPP_ASSERT_SEMANTIC_REQUIREMENT(ranges::is_sorted(__keys, __compare_), "Key container is not sorted");
     auto __guard = std::__make_exception_guard([&]() noexcept { clear() /* noexcept */; });
     __keys_      = std::move(__keys);
     __guard.__complete();
   }
 
-  _LIBCPP_HIDE_FROM_ABI iterator erase(iterator __position) {
+  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX26 iterator erase(iterator __position) {
     auto __on_failure = std::__make_exception_guard([&]() noexcept { clear() /* noexcept */; });
     auto __key_iter   = __keys_.erase(__position.__base());
     __on_failure.__complete();
@@ -444,7 +486,7 @@ class flat_multiset {
   // The following overload is the same as the iterator overload
   // iterator erase(const_iterator __position);
 
-  _LIBCPP_HIDE_FROM_ABI size_type erase(const key_type& __x) {
+  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX26 size_type erase(const key_type& __x) {
     auto [__first, __last] = equal_range(__x);
     auto __res             = __last - __first;
     erase(__first, __last);
@@ -454,21 +496,21 @@ class flat_multiset {
   template <class _Kp>
     requires(__is_transparent_v<_Compare> && !is_convertible_v<_Kp &&, iterator> &&
              !is_convertible_v<_Kp &&, const_iterator>)
-  _LIBCPP_HIDE_FROM_ABI size_type erase(_Kp&& __x) {
+  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX26 size_type erase(_Kp&& __x) {
     auto [__first, __last] = equal_range(__x);
     auto __res             = __last - __first;
     erase(__first, __last);
     return __res;
   }
 
-  _LIBCPP_HIDE_FROM_ABI iterator erase(const_iterator __first, const_iterator __last) {
+  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX26 iterator erase(const_iterator __first, const_iterator __last) {
     auto __on_failure = std::__make_exception_guard([&]() noexcept { clear() /* noexcept */; });
     auto __key_it     = __keys_.erase(__first.__base(), __last.__base());
     __on_failure.__complete();
     return iterator(std::move(__key_it));
   }
 
-  _LIBCPP_HIDE_FROM_ABI void swap(flat_multiset& __y) noexcept {
+  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX26 void swap(flat_multiset& __y) noexcept {
     // warning: The spec has unconditional noexcept, which means that
     // if any of the following functions throw an exception,
     // std::terminate will be called
@@ -477,126 +519,139 @@ class flat_multiset {
     ranges::swap(__keys_, __y.__keys_);
   }
 
-  _LIBCPP_HIDE_FROM_ABI void clear() noexcept { __keys_.clear(); }
+  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX26 void clear() noexcept { __keys_.clear(); }
 
   // observers
-  _LIBCPP_HIDE_FROM_ABI key_compare key_comp() const { return __compare_; }
-  _LIBCPP_HIDE_FROM_ABI value_compare value_comp() const { return __compare_; }
+  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX26 key_compare key_comp() const { return __compare_; }
+  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX26 value_compare value_comp() const { return __compare_; }
 
   // map operations
-  _LIBCPP_HIDE_FROM_ABI iterator find(const key_type& __x) { return __find_impl(*this, __x); }
+  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX26 iterator find(const key_type& __x) {
+    return __find_impl(*this, __x);
+  }
 
-  _LIBCPP_HIDE_FROM_ABI const_iterator find(const key_type& __x) const { return __find_impl(*this, __x); }
+  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX26 const_iterator find(const key_type& __x) const {
+    return __find_impl(*this, __x);
+  }
 
   template <class _Kp>
     requires __is_transparent_v<_Compare>
-  _LIBCPP_HIDE_FROM_ABI iterator find(const _Kp& __x) {
+  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX26 iterator find(const _Kp& __x) {
     return __find_impl(*this, __x);
   }
 
   template <class _Kp>
     requires __is_transparent_v<_Compare>
-  _LIBCPP_HIDE_FROM_ABI const_iterator find(const _Kp& __x) const {
+  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX26 const_iterator find(const _Kp& __x) const {
     return __find_impl(*this, __x);
   }
 
-  _LIBCPP_HIDE_FROM_ABI size_type count(const key_type& __x) const {
+  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX26 size_type count(const key_type& __x) const {
     auto [__first, __last] = equal_range(__x);
     return __last - __first;
   }
 
   template <class _Kp>
     requires __is_transparent_v<_Compare>
-  _LIBCPP_HIDE_FROM_ABI size_type count(const _Kp& __x) const {
+  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX26 size_type count(const _Kp& __x) const {
     auto [__first, __last] = equal_range(__x);
     return __last - __first;
   }
 
-  _LIBCPP_HIDE_FROM_ABI bool contains(const key_type& __x) const { return find(__x) != end(); }
+  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX26 bool contains(const key_type& __x) const {
+    return find(__x) != end();
+  }
 
   template <class _Kp>
     requires __is_transparent_v<_Compare>
-  _LIBCPP_HIDE_FROM_ABI bool contains(const _Kp& __x) const {
+  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX26 bool contains(const _Kp& __x) const {
     return find(__x) != end();
   }
 
-  _LIBCPP_HIDE_FROM_ABI iterator lower_bound(const key_type& __x) {
+  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX26 iterator lower_bound(const key_type& __x) {
     const auto& __keys = __keys_;
     return iterator(std::lower_bound(__keys.begin(), __keys.end(), __x, __compare_));
   }
 
-  _LIBCPP_HIDE_FROM_ABI const_iterator lower_bound(const key_type& __x) const {
+  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX26 const_iterator lower_bound(const key_type& __x) const {
     return const_iterator(std::lower_bound(__keys_.begin(), __keys_.end(), __x, __compare_));
   }
 
   template <class _Kp>
     requires __is_transparent_v<_Compare>
-  _LIBCPP_HIDE_FROM_ABI iterator lower_bound(const _Kp& __x) {
+  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX26 iterator lower_bound(const _Kp& __x) {
     const auto& __keys = __keys_;
     return iterator(std::lower_bound(__keys.begin(), __keys.end(), __x, __compare_));
   }
 
   template <class _Kp>
     requires __is_transparent_v<_Compare>
-  _LIBCPP_HIDE_FROM_ABI const_iterator lower_bound(const _Kp& __x) const {
+  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX26 const_iterator lower_bound(const _Kp& __x) const {
     return const_iterator(std::lower_bound(__keys_.begin(), __keys_.end(), __x, __compare_));
   }
 
-  _LIBCPP_HIDE_FROM_ABI iterator upper_bound(const key_type& __x) {
+  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX26 iterator upper_bound(const key_type& __x) {
     const auto& __keys = __keys_;
     return iterator(std::upper_bound(__keys.begin(), __keys.end(), __x, __compare_));
   }
 
-  _LIBCPP_HIDE_FROM_ABI const_iterator upper_bound(const key_type& __x) const {
+  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX26 const_iterator upper_bound(const key_type& __x) const {
     return const_iterator(std::upper_bound(__keys_.begin(), __keys_.end(), __x, __compare_));
   }
 
   template <class _Kp>
     requires __is_transparent_v<_Compare>
-  _LIBCPP_HIDE_FROM_ABI iterator upper_bound(const _Kp& __x) {
+  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX26 iterator upper_bound(const _Kp& __x) {
     const auto& __keys = __keys_;
     return iterator(std::upper_bound(__keys.begin(), __keys.end(), __x, __compare_));
   }
 
   template <class _Kp>
     requires __is_transparent_v<_Compare>
-  _LIBCPP_HIDE_FROM_ABI const_iterator upper_bound(const _Kp& __x) const {
+  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX26 const_iterator upper_bound(const _Kp& __x) const {
     return const_iterator(std::upper_bound(__keys_.begin(), __keys_.end(), __x, __compare_));
   }
 
-  _LIBCPP_HIDE_FROM_ABI pair<iterator, iterator> equal_range(const key_type& __x) {
+  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX26 pair<iterator, iterator> equal_range(const key_type& __x) {
     return __equal_range_impl(*this, __x);
   }
 
-  _LIBCPP_HIDE_FROM_ABI pair<const_iterator, const_iterator> equal_range(const key_type& __x) const {
+  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX26 pair<const_iterator, const_iterator>
+  equal_range(const key_type& __x) const {
     return __equal_range_impl(*this, __x);
   }
 
   template <class _Kp>
     requires __is_transparent_v<_Compare>
-  _LIBCPP_HIDE_FROM_ABI pair<iterator, iterator> equal_range(const _Kp& __x) {
+  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX26 pair<iterator, iterator> equal_range(const _Kp& __x) {
     return __equal_range_impl(*this, __x);
   }
   template <class _Kp>
     requires __is_transparent_v<_Compare>
-  _LIBCPP_HIDE_FROM_ABI pair<const_iterator, const_iterator> equal_range(const _Kp& __x) const {
+  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX26 pair<const_iterator, const_iterator>
+  equal_range(const _Kp& __x) const {
     return __equal_range_impl(*this, __x);
   }
 
-  friend _LIBCPP_HIDE_FROM_ABI bool operator==(const flat_multiset& __x, const flat_multiset& __y) {
+  friend _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX26 bool
+  operator==(const flat_multiset& __x, const flat_multiset& __y) {
     return ranges::equal(__x, __y);
   }
 
-  friend _LIBCPP_HIDE_FROM_ABI auto operator<=>(const flat_multiset& __x, const flat_multiset& __y) {
+  friend _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX26 auto
+  operator<=>(const flat_multiset& __x, const flat_multiset& __y) {
     return std::lexicographical_compare_three_way(
         __x.begin(), __x.end(), __y.begin(), __y.end(), std::__synth_three_way);
   }
 
-  friend _LIBCPP_HIDE_FROM_ABI void swap(flat_multiset& __x, flat_multiset& __y) noexcept { __x.swap(__y); }
+  friend _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX26 void
+  swap(flat_multiset& __x, flat_multiset& __y) noexcept {
+    __x.swap(__y);
+  }
 
 private:
   template <bool _WasSorted, class... _Args>
-  _LIBCPP_HIDE_FROM_ABI void __append_sort_merge(_Args&&... __args) {
+  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX26 void __append_sort_merge(_Args&&... __args) {
     auto __on_failure    = std::__make_exception_guard([&]() noexcept { clear() /* noexcept */; });
     size_type __old_size = size();
     __flat_set_utils::__append(*this, std::forward<_Args>(__args)...);
@@ -611,13 +666,13 @@ class flat_multiset {
   }
 
   template <class _Kp>
-  _LIBCPP_HIDE_FROM_ABI iterator __emplace(_Kp&& __key) {
+  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX26 iterator __emplace(_Kp&& __key) {
     auto __it = upper_bound(__key);
     return __flat_set_utils::__emplace_exact_pos(*this, __it, std::forward<_Kp>(__key));
   }
 
   template <class _Kp>
-  _LIBCPP_HIDE_FROM_ABI iterator __emplace_hint(const_iterator __hint, _Kp&& __key) {
+  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX26 iterator __emplace_hint(const_iterator __hint, _Kp&& __key) {
     auto __prev_larger  = __hint != cbegin() && __compare_(__key, *std::prev(__hint));
     auto __next_smaller = __hint != cend() && __compare_(*__hint, __key);
 
@@ -649,7 +704,7 @@ class flat_multiset {
   }
 
   template <class _Self, class _Kp>
-  _LIBCPP_HIDE_FROM_ABI static auto __find_impl(_Self&& __self, const _Kp& __key) {
+  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX26 static auto __find_impl(_Self&& __self, const _Kp& __key) {
     auto __it   = __self.lower_bound(__key);
     auto __last = __self.end();
     if (__it == __last || __self.__compare_(__key, *__it)) {
@@ -659,29 +714,30 @@ class flat_multiset {
   }
 
   template <class _Self, class _Kp>
-  _LIBCPP_HIDE_FROM_ABI static auto __equal_range_impl(_Self&& __self, const _Kp& __key) {
+  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX26 static auto __equal_range_impl(_Self&& __self, const _Kp& __key) {
     using __iter = _If<is_const_v<__libcpp_remove_reference_t<_Self>>, const_iterator, iterator>;
     auto [__key_first, __key_last] =
         std::equal_range(__self.__keys_.begin(), __self.__keys_.end(), __key, __self.__compare_);
     return std::make_pair(__iter(__key_first), __iter(__key_last));
   }
 
-  _LIBCPP_HIDE_FROM_ABI void __reserve(size_t __size) {
+  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX26 void __reserve(size_t __size) {
     if constexpr (__container_traits<_KeyContainer>::__reservable) {
       __keys_.reserve(__size);
     }
   }
 
   template <class _Key2, class _Compare2, class _KeyContainer2, class _Predicate>
-  friend typename flat_multiset<_Key2, _Compare2, _KeyContainer2>::size_type
+  friend typename flat_multiset<_Key2, _Compare2, _KeyContainer2>::size_type _LIBCPP_CONSTEXPR_SINCE_CXX26
   erase_if(flat_multiset<_Key2, _Compare2, _KeyContainer2>&, _Predicate);
 
   _KeyContainer __keys_;
   _LIBCPP_NO_UNIQUE_ADDRESS key_compare __compare_;
 
   struct __key_equiv {
-    _LIBCPP_HIDE_FROM_ABI __key_equiv(key_compare __c) : __comp_(__c) {}
-    _LIBCPP_HIDE_FROM_ABI bool operator()(const_reference __x, const_reference __y) const {
+    _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX26 __key_equiv(key_compare __c) : __comp_(__c) {}
+    _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX26 bool
+    operator()(const_reference __x, const_reference __y) const {
       return !__comp_(std::get<0>(__x), std::get<0>(__y)) && !__comp_(std::get<0>(__y), std::get<0>(__x));
     }
     key_compare __comp_;
@@ -770,7 +826,7 @@ struct uses_allocator<flat_multiset<_Key, _Compare, _KeyContainer>, _Allocator>
     : bool_constant<uses_allocator_v<_KeyContainer, _Allocator> > {};
 
 template <class _Key, class _Compare, class _KeyContainer, class _Predicate>
-_LIBCPP_HIDE_FROM_ABI typename flat_multiset<_Key, _Compare, _KeyContainer>::size_type
+_LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX26 typename flat_multiset<_Key, _Compare, _KeyContainer>::size_type
 erase_if(flat_multiset<_Key, _Compare, _KeyContainer>& __flat_multiset, _Predicate __pred) {
   auto __guard = std::__make_exception_guard([&] { __flat_multiset.clear(); });
   auto __it =
diff --git a/libcxx/include/version b/libcxx/include/version
index 16917a3bd9ddd..edd0c7c4fa663 100644
--- a/libcxx/include/version
+++ b/libcxx/include/version
@@ -70,6 +70,8 @@ __cpp_lib_constexpr_charconv                            202207L <charconv>
 __cpp_lib_constexpr_cmath                               202202L <cmath> <cstdlib>
 __cpp_lib_constexpr_complex                             201711L <complex>
 __cpp_lib_constexpr_dynamic_alloc                       201907L <memory>
+__cpp_lib_constexpr_flat_map                            202502L <flat_map>
+__cpp_lib_constexpr_flat_set                            202502L <flat_set>
 __cpp_lib_constexpr_forward_list                        202502L <forward_list>
 __cpp_lib_constexpr_functional                          201907L <functional>
 __cpp_lib_constexpr_iterator                            201811L <iterator>
@@ -549,6 +551,8 @@ __cpp_lib_void_t                                        201411L <type_traits>
 # define __cpp_lib_bitset                               202306L
 # undef  __cpp_lib_constexpr_algorithms
 # define __cpp_lib_constexpr_algorithms                 202306L
+# define __cpp_lib_constexpr_flat_map                   202502L
+# define __cpp_lib_constexpr_flat_set                   202502L
 # define __cpp_lib_constexpr_forward_list               202502L
 # define __cpp_lib_constexpr_list                       202502L
 # if !defined(_LIBCPP_ABI_VCRUNTIME)
diff --git a/libcxx/test/libcxx/containers/container.adaptors/flat.multiset/insert.temporary.pass.cpp b/libcxx/test/libcxx/containers/container.adaptors/flat.multiset/insert.temporary.pass.cpp
index 248f282209fd7..acd20ce525a0d 100644
--- a/libcxx/test/libcxx/containers/container.adaptors/flat.multiset/insert.temporary.pass.cpp
+++ b/libcxx/test/libcxx/containers/container.adaptors/flat.multiset/insert.temporary.pass.cpp
@@ -21,7 +21,7 @@
 #include "../flat_helpers.h"
 #include "test_macros.h"
 
-bool test() {
+constexpr bool test() {
   using M = std::flat_multiset<TrackCopyMove>;
   {
     M m;
@@ -43,6 +43,9 @@ bool test() {
 
 int main(int, char**) {
   test();
+#if TEST_STD_VER >= 26
+  static_assert(test());
+#endif
 
   return 0;
 }
diff --git a/libcxx/test/libcxx/containers/container.adaptors/flat.multiset/insert_range.pass.cpp b/libcxx/test/libcxx/containers/container.adaptors/flat.multiset/insert_range.pass.cpp
index 57a581c6c5cb9..9197aec1b7a7e 100644
--- a/libcxx/test/libcxx/containers/container.adaptors/flat.multiset/insert_range.pass.cpp
+++ b/libcxx/test/libcxx/containers/container.adaptors/flat.multiset/insert_range.pass.cpp
@@ -24,23 +24,33 @@
 #include <vector>
 
 #include "../flat_helpers.h"
+#include "test_iterators.h"
 #include "test_macros.h"
 
-void test() {
+constexpr bool test() {
   NotQuiteSequenceContainer<int> v;
   std::flat_multiset s(v);
-  std::istringstream ints("0 1 1 0");
-  auto r = std::ranges::subrange(std::istream_iterator<int>(ints), std::istream_iterator<int>()) |
-           std::views::transform([](int i) { return i * i; });
+
+  int ar[]   = {0, 1, 1, 0};
+  using Iter = cpp20_input_iterator<const int*>;
+  using Sent = sentinel_wrapper<Iter>;
+  using R    = std::ranges::subrange<Iter, Sent>;
+  auto r     = R(Iter(ar), Sent(Iter(ar + 4)));
+
   static_assert(
       ![](auto& t) { return requires { t.insert_range(t.end(), r); }; }(v),
       "This test is to test the case where the underlying container does not provide insert_range");
   s.insert_range(r);
   assert(std::ranges::equal(s, std::vector<int>{0, 0, 1, 1}));
+
+  return true;
 }
 
 int main(int, char**) {
   test();
+#if TEST_STD_VER >= 26
+  static_assert(test());
+#endif
 
   return 0;
 }
diff --git a/libcxx/test/std/containers/container.adaptors/flat.multiset/flat.multiset.capacity/empty.pass.cpp b/libcxx/test/std/containers/container.adaptors/flat.multiset/flat.multiset.capacity/empty.pass.cpp
index 52f77438df2ce..88a76d3c1c8b8 100644
--- a/libcxx/test/std/containers/container.adaptors/flat.multiset/flat.multiset.capacity/empty.pass.cpp
+++ b/libcxx/test/std/containers/container.adaptors/flat.multiset/flat.multiset.capacity/empty.pass.cpp
@@ -24,7 +24,7 @@
 #include "min_allocator.h"
 
 template <class KeyContainer>
-void test_one() {
+constexpr void test_one() {
   using Key = typename KeyContainer::value_type;
   using M   = std::flat_multiset<Key, std::less<int>, KeyContainer>;
   M m;
@@ -38,15 +38,23 @@ void test_one() {
   assert(m.empty());
 }
 
-void test() {
+constexpr bool test() {
   test_one<std::vector<int>>();
-  test_one<std::deque<int>>();
+#ifndef __cpp_lib_constexpr_deque
+  if (!TEST_IS_CONSTANT_EVALUATED)
+#endif
+    test_one<std::deque<int>>();
   test_one<MinSequenceContainer<int>>();
   test_one<std::vector<int, min_allocator<int>>>();
+
+  return true;
 }
 
 int main(int, char**) {
   test();
+#if TEST_STD_VER >= 26
+  static_assert(test());
+#endif
 
   return 0;
 }
diff --git a/libcxx/test/std/containers/container.adaptors/flat.multiset/flat.multiset.capacity/max_size.pass.cpp b/libcxx/test/std/containers/container.adaptors/flat.multiset/flat.multiset.capacity/max_size.pass.cpp
index 4e3d1414b28af..fb9c38f592262 100644
--- a/libcxx/test/std/containers/container.adaptors/flat.multiset/flat.multiset.capacity/max_size.pass.cpp
+++ b/libcxx/test/std/containers/container.adaptors/flat.multiset/flat.multiset.capacity/max_size.pass.cpp
@@ -24,7 +24,7 @@
 #include "test_allocator.h"
 #include "test_macros.h"
 
-void test() {
+constexpr bool test() {
   {
     using A1 = limited_allocator<int, 10>;
     using C  = std::flat_multiset<int, std::less<int>, std::vector<int, A1>>;
@@ -59,10 +59,15 @@ void test() {
     assert(c.max_size() <= max_dist);
     assert(c.max_size() <= alloc_max_size(std::allocator<char>()));
   }
+
+  return true;
 }
 
 int main(int, char**) {
   test();
+#if TEST_STD_VER >= 26
+  static_assert(test());
+#endif
 
   return 0;
 }
diff --git a/libcxx/test/std/containers/container.adaptors/flat.multiset/flat.multiset.capacity/size.pass.cpp b/libcxx/test/std/containers/container.adaptors/flat.multiset/flat.multiset.capacity/size.pass.cpp
index 4aff08b8127b6..156bb27fae992 100644
--- a/libcxx/test/std/containers/container.adaptors/flat.multiset/flat.multiset.capacity/size.pass.cpp
+++ b/libcxx/test/std/containers/container.adaptors/flat.multiset/flat.multiset.capacity/size.pass.cpp
@@ -7,6 +7,8 @@
 //===----------------------------------------------------------------------===//
 
 // UNSUPPORTED: c++03, c++11, c++14, c++17, c++20
+// ADDITIONAL_COMPILE_FLAGS(has-fconstexpr-steps): -fconstexpr-steps=200000000
+// ADDITIONAL_COMPILE_FLAGS(has-fconstexpr-ops-limit): -fconstexpr-ops-limit=800000000
 
 // <flat_set>
 
@@ -23,7 +25,7 @@
 #include "min_allocator.h"
 
 template <class KeyContainer>
-void test_one() {
+constexpr void test_one() {
   using M = std::flat_multiset<int, std::less<int>, KeyContainer>;
   using S = typename M::size_type;
   {
@@ -46,7 +48,7 @@ void test_one() {
   }
   {
     M m;
-    S s = 500000;
+    S s = 5000;
     for (std::size_t i = 0u; i < s; ++i) {
       m.emplace(i);
       m.emplace(i);
@@ -57,15 +59,23 @@ void test_one() {
   }
 }
 
-void test() {
+constexpr bool test() {
   test_one<std::vector<int>>();
-  test_one<std::deque<int>>();
+#ifndef __cpp_lib_constexpr_deque
+  if (!TEST_IS_CONSTANT_EVALUATED)
+#endif
+    test_one<std::deque<int>>();
   test_one<MinSequenceContainer<int>>();
   test_one<std::vector<int, min_allocator<int>>>();
+
+  return true;
 }
 
 int main(int, char**) {
   test();
+#if TEST_STD_VER >= 26
+  static_assert(test());
+#endif
 
   return 0;
 }
diff --git a/libcxx/test/std/containers/container.adaptors/flat.multiset/flat.multiset.cons/alloc.pass.cpp b/libcxx/test/std/containers/container.adaptors/flat.multiset/flat.multiset.cons/alloc.pass.cpp
index 4fffcb304d20a..1bfd13e0d3cf2 100644
--- a/libcxx/test/std/containers/container.adaptors/flat.multiset/flat.multiset.cons/alloc.pass.cpp
+++ b/libcxx/test/std/containers/container.adaptors/flat.multiset/flat.multiset.cons/alloc.pass.cpp
@@ -14,6 +14,7 @@
 //   explicit flat_multiset(const Allocator& a);
 
 #include <cassert>
+#include <deque>
 #include <flat_set>
 #include <functional>
 #include <vector>
@@ -22,7 +23,19 @@
 #include "test_allocator.h"
 #include "../../../test_compare.h"
 
-void test() {
+template <template <class...> class KeyContainer>
+constexpr void test() {
+  {
+    using A = test_allocator<short>;
+    using M = std::flat_multiset<int, std::less<int>, KeyContainer<int, test_allocator<int>>>;
+    M m(A(0, 5));
+    assert(m.empty());
+    assert(m.begin() == m.end());
+    assert(std::move(m).extract().get_allocator().get_id() == 5);
+  }
+}
+
+constexpr bool test() {
   {
     // The constructors in this subclause shall not participate in overload
     // resolution unless uses_allocator_v<container_type, Alloc> is true
@@ -46,18 +59,21 @@ void test() {
     static_assert(std::is_constructible_v<M, test_allocator<int>>);
     static_assert(!std::is_convertible_v<test_allocator<int>, M>);
   }
-  {
-    using A = test_allocator<short>;
-    using M = std::flat_multiset<int, std::less<int>, std::vector<int, test_allocator<int>>>;
-    M m(A(0, 5));
-    assert(m.empty());
-    assert(m.begin() == m.end());
-    assert(std::move(m).extract().get_allocator().get_id() == 5);
-  }
+
+  test<std::vector>();
+#ifndef __cpp_lib_constexpr_deque
+  if (!TEST_IS_CONSTANT_EVALUATED)
+#endif
+    test<std::deque>();
+
+  return true;
 }
 
 int main(int, char**) {
   test();
+#if TEST_STD_VER >= 26
+  static_assert(test());
+#endif
 
   return 0;
 }
diff --git a/libcxx/test/std/containers/container.adaptors/flat.multiset/flat.multiset.cons/assign_initializer_list.pass.cpp b/libcxx/test/std/containers/container.adaptors/flat.multiset/flat.multiset.cons/assign_initializer_list.pass.cpp
index ae81ab044932d..a895117517ef4 100644
--- a/libcxx/test/std/containers/container.adaptors/flat.multiset/flat.multiset.cons/assign_initializer_list.pass.cpp
+++ b/libcxx/test/std/containers/container.adaptors/flat.multiset/flat.multiset.cons/assign_initializer_list.pass.cpp
@@ -26,7 +26,7 @@
 #include "test_allocator.h"
 
 template <class KeyContainer>
-void test() {
+constexpr void test() {
   using Key = typename KeyContainer::value_type;
   using M   = std::flat_multiset<Key, std::less<Key>, KeyContainer>;
   {
@@ -53,16 +53,24 @@ void test() {
   }
 }
 
-void test() {
+constexpr bool test() {
   test<std::vector<int>>();
   test<std::vector<double>>();
-  test<std::deque<int>>();
+#ifndef __cpp_lib_constexpr_deque
+  if (!TEST_IS_CONSTANT_EVALUATED)
+#endif
+    test<std::deque<int>>();
   test<MinSequenceContainer<int>>();
   test<std::vector<int, min_allocator<int>>>();
+
+  return true;
 }
 
 int main(int, char**) {
   test();
+#if TEST_STD_VER >= 26
+  static_assert(test());
+#endif
 
   return 0;
 }
diff --git a/libcxx/test/std/containers/container.adaptors/flat.multiset/flat.multiset.cons/compare.pass.cpp b/libcxx/test/std/containers/container.adaptors/flat.multiset/flat.multiset.cons/compare.pass.cpp
index 6b68589e6814f..64c833620e7b7 100644
--- a/libcxx/test/std/containers/container.adaptors/flat.multiset/flat.multiset.cons/compare.pass.cpp
+++ b/libcxx/test/std/containers/container.adaptors/flat.multiset/flat.multiset.cons/compare.pass.cpp
@@ -20,47 +20,39 @@
 #include <type_traits>
 #include <vector>
 
+#include "MinSequenceContainer.h"
+#include "min_allocator.h"
 #include "test_macros.h"
 #include "../../../test_compare.h"
 #include "test_allocator.h"
 
-void test() {
+template <class KeyContainer>
+constexpr void test_compare() {
+  using Key = typename KeyContainer::value_type;
   {
-    // The constructors in this subclause shall not participate in overload
-    // resolution unless uses_allocator_v<container_type, Alloc> is true
+    // The one-argument ctor is explicit.
+    using C = test_less<Key>;
+    static_assert(std::is_constructible_v<std::flat_multiset<Key, C>, C>);
+    static_assert(!std::is_convertible_v<C, std::flat_multiset<Key, C>>);
 
-    using C  = test_less<int>;
-    using A1 = test_allocator<int>;
-    using A2 = other_allocator<int>;
-    using V1 = std::vector<int, A1>;
-    using V2 = std::vector<int, A2>;
-    using M1 = std::flat_multiset<int, C, V1>;
-    using M2 = std::flat_multiset<int, C, V2>;
-    static_assert(std::is_constructible_v<M1, const C&, const A1&>);
-    static_assert(std::is_constructible_v<M2, const C&, const A2&>);
-    static_assert(!std::is_constructible_v<M1, const C&, const A2&>);
-    static_assert(!std::is_constructible_v<M2, const C&, const A1&>);
+    static_assert(std::is_constructible_v<std::flat_multiset<Key>, std::less<Key>>);
+    static_assert(!std::is_convertible_v<std::less<Key>, std::flat_multiset<Key>>);
   }
   {
-    using C = test_less<int>;
-    auto m  = std::flat_multiset<int, C>(C(3));
+    using C = test_less<Key>;
+    auto m  = std::flat_multiset<Key, C>(C(3));
     assert(m.empty());
     assert(m.begin() == m.end());
     assert(m.key_comp() == C(3));
   }
-  {
-    // The one-argument ctor is explicit.
-    using C = test_less<int>;
-    static_assert(std::is_constructible_v<std::flat_multiset<int, C>, C>);
-    static_assert(!std::is_convertible_v<C, std::flat_multiset<int, C>>);
+}
 
-    static_assert(std::is_constructible_v<std::flat_multiset<int>, std::less<int>>);
-    static_assert(!std::is_convertible_v<std::less<int>, std::flat_multiset<int>>);
-  }
+template <template <class...> class KeyContainer>
+constexpr void test_compare_alloc() {
   {
     using C  = test_less<int>;
     using A1 = test_allocator<int>;
-    auto m   = std::flat_multiset<int, C, std::vector<int, A1>>(C(4), A1(5));
+    auto m   = std::flat_multiset<int, C, KeyContainer<int, A1>>(C(4), A1(5));
     assert(m.empty());
     assert(m.begin() == m.end());
     assert(m.key_comp() == C(4));
@@ -68,9 +60,9 @@ void test() {
   }
   {
     // explicit(false)
-    using C                                           = test_less<int>;
-    using A1                                          = test_allocator<int>;
-    std::flat_multiset<int, C, std::deque<int, A1>> m = {C(4), A1(5)};
+    using C                                             = test_less<int>;
+    using A1                                            = test_allocator<int>;
+    std::flat_multiset<int, C, KeyContainer<int, A1>> m = {C(4), A1(5)};
     assert(m.empty());
     assert(m.begin() == m.end());
     assert(m.key_comp() == C(4));
@@ -78,8 +70,46 @@ void test() {
   }
 }
 
+constexpr bool test() {
+  {
+    // The constructors in this subclause shall not participate in overload
+    // resolution unless uses_allocator_v<container_type, Alloc> is true
+
+    using C  = test_less<int>;
+    using A1 = test_allocator<int>;
+    using A2 = other_allocator<int>;
+    using V1 = std::vector<int, A1>;
+    using V2 = std::vector<int, A2>;
+    using M1 = std::flat_multiset<int, C, V1>;
+    using M2 = std::flat_multiset<int, C, V2>;
+    static_assert(std::is_constructible_v<M1, const C&, const A1&>);
+    static_assert(std::is_constructible_v<M2, const C&, const A2&>);
+    static_assert(!std::is_constructible_v<M1, const C&, const A2&>);
+    static_assert(!std::is_constructible_v<M2, const C&, const A1&>);
+  }
+
+  test_compare<std::vector<int>>();
+  test_compare<MinSequenceContainer<int>>();
+  test_compare<std::vector<int, min_allocator<int>>>();
+
+  test_compare_alloc<std::vector>();
+
+#ifndef __cpp_lib_constexpr_deque
+  if (!TEST_IS_CONSTANT_EVALUATED)
+#endif
+  {
+    test_compare<std::deque<int>>();
+    test_compare_alloc<std::deque>();
+  }
+
+  return true;
+}
+
 int main(int, char**) {
   test();
+#if TEST_STD_VER >= 26
+  static_assert(test());
+#endif
 
   return 0;
 }
diff --git a/libcxx/test/std/containers/container.adaptors/flat.multiset/flat.multiset.cons/containers.pass.cpp b/libcxx/test/std/containers/container.adaptors/flat.multiset/flat.multiset.cons/containers.pass.cpp
index 78eac420a8f22..ae448818829bd 100644
--- a/libcxx/test/std/containers/container.adaptors/flat.multiset/flat.multiset.cons/containers.pass.cpp
+++ b/libcxx/test/std/containers/container.adaptors/flat.multiset/flat.multiset.cons/containers.pass.cpp
@@ -35,39 +35,19 @@ void conversion_test(T);
 template <class T, class... Args>
 concept ImplicitlyConstructible = requires(Args&&... args) { conversion_test<T>({std::forward<Args>(args)...}); };
 
-void test() {
-  {
-    // The constructors in this subclause shall not participate in overload
-    // resolution unless uses_allocator_v<container_type, Alloc> is true
-
-    using C  = test_less<int>;
-    using A1 = test_allocator<int>;
-    using A2 = other_allocator<int>;
-    using V1 = std::vector<int, A1>;
-    using V2 = std::vector<int, A2>;
-    using M1 = std::flat_multiset<int, C, V1>;
-    using M2 = std::flat_multiset<int, C, V2>;
-    static_assert(std::is_constructible_v<M1, const V1&, const A1&>);
-    static_assert(std::is_constructible_v<M2, const V2&, const A2&>);
-    static_assert(!std::is_constructible_v<M1, const V1&, const A2&>);
-    static_assert(!std::is_constructible_v<M2, const V2&, const A1&>);
-
-    static_assert(std::is_constructible_v<M1, const V1&, const C&, const A1&>);
-    static_assert(std::is_constructible_v<M2, const V2&, const C&, const A2&>);
-    static_assert(!std::is_constructible_v<M1, const V1&, const C&, const A2&>);
-    static_assert(!std::is_constructible_v<M2, const V2&, const C&, const A1&>);
-  }
+template <template <class...> class KeyContainer>
+constexpr void test() {
   {
     // flat_multiset(container_type)
-    using M             = std::flat_multiset<int>;
-    std::vector<int> ks = {1, 1, 1, 2, 2, 3, 2, 3, 3};
-    auto m              = M(ks);
-    int expected[]      = {1, 1, 1, 2, 2, 2, 3, 3, 3};
+    using M              = std::flat_multiset<int, std::less<int>, KeyContainer<int>>;
+    KeyContainer<int> ks = {1, 1, 1, 2, 2, 3, 2, 3, 3};
+    auto m               = M(ks);
+    int expected[]       = {1, 1, 1, 2, 2, 2, 3, 3, 3};
     assert(std::ranges::equal(m, expected));
 
     // explicit(false)
-    static_assert(std::is_constructible_v<M, const std::vector<int>&>);
-    static_assert(!ImplicitlyConstructible<M, const std::vector<int>&>);
+    static_assert(std::is_constructible_v<M, const KeyContainer<int>&>);
+    static_assert(!ImplicitlyConstructible<M, const KeyContainer<int>&>);
 
     m = M(std::move(ks));
     assert(ks.empty()); // it was moved-from
@@ -77,7 +57,7 @@ void test() {
     // flat_multiset(container_type)
     // move-only
     int expected[] = {3, 3, 2, 1};
-    using Ks       = std::deque<MoveOnly, min_allocator<MoveOnly>>;
+    using Ks       = KeyContainer<MoveOnly, min_allocator<MoveOnly>>;
     using M        = std::flat_multiset<MoveOnly, std::greater<MoveOnly>, Ks>;
     Ks ks;
     ks.push_back(1);
@@ -92,8 +72,8 @@ void test() {
     // flat_multiset(container_type)
     // container's allocators are used
     using A = test_allocator<int>;
-    using M = std::flat_multiset<int, std::less<int>, std::deque<int, A>>;
-    auto ks = std::deque<int, A>({1, 1, 1, 2, 2, 3, 2, 3, 3}, A(5));
+    using M = std::flat_multiset<int, std::less<int>, KeyContainer<int, A>>;
+    auto ks = KeyContainer<int, A>({1, 1, 1, 2, 2, 3, 2, 3, 3}, A(5));
     auto m  = M(std::move(ks));
     assert(ks.empty()); // it was moved-from
     assert((m == M{1, 1, 1, 2, 2, 2, 3, 3, 3}));
@@ -102,22 +82,22 @@ void test() {
   }
   {
     // flat_multiset(container_type, key_compare)
-    using C             = test_less<int>;
-    using M             = std::flat_multiset<int, C>;
-    std::vector<int> ks = {1, 1, 1, 2, 2, 3, 2, 3, 3};
-    auto m              = M(ks, C(4));
+    using C              = test_less<int>;
+    using M              = std::flat_multiset<int, C, KeyContainer<int>>;
+    KeyContainer<int> ks = {1, 1, 1, 2, 2, 3, 2, 3, 3};
+    auto m               = M(ks, C(4));
     assert(std::ranges::equal(m, std::vector<int>{1, 1, 1, 2, 2, 2, 3, 3, 3}));
     assert(m.key_comp() == C(4));
 
     // explicit
-    static_assert(std::is_constructible_v<M, const std::vector<int>&, const C&>);
-    static_assert(!ImplicitlyConstructible<M, const std::vector<int>&, const C&>);
+    static_assert(std::is_constructible_v<M, const KeyContainer<int>&, const C&>);
+    static_assert(!ImplicitlyConstructible<M, const KeyContainer<int>&, const C&>);
   }
   {
     // flat_multiset(container_type , const Allocator&)
     using A = test_allocator<int>;
-    using M = std::flat_multiset<int, std::less<int>, std::deque<int, A>>;
-    auto ks = std::deque<int, A>({1, 1, 1, 2, 2, 3, 2, 3, 3}, A(5));
+    using M = std::flat_multiset<int, std::less<int>, KeyContainer<int, A>>;
+    auto ks = KeyContainer<int, A>({1, 1, 1, 2, 2, 3, 2, 3, 3}, A(5));
     auto m  = M(ks, A(4)); // replaces the allocators
     assert(!ks.empty());   // it was an lvalue above
     assert((m == M{1, 1, 1, 2, 2, 2, 3, 3, 3}));
@@ -125,7 +105,7 @@ void test() {
     assert(keys.get_allocator() == A(4));
 
     // explicit(false)
-    static_assert(ImplicitlyConstructible<M, const std::deque<int, A>&, const A&>);
+    static_assert(ImplicitlyConstructible<M, const KeyContainer<int, A>&, const A&>);
     M m2 = {ks, A(4)};   // implicit ctor
     assert(!ks.empty()); // it was an lvalue above
     assert(m2 == m);
@@ -134,19 +114,19 @@ void test() {
   }
   {
     // flat_multiset(container_type , const Allocator&)
-    using C                = test_less<int>;
-    using A                = test_allocator<int>;
-    using M                = std::flat_multiset<int, C, std::vector<int, A>>;
-    std::vector<int, A> ks = {1, 1, 1, 2, 2, 3, 2, 3, 3};
-    auto m                 = M(ks, C(4), A(5));
-    assert(std::ranges::equal(m, std::vector<int, A>{1, 1, 1, 2, 2, 2, 3, 3, 3}));
+    using C                 = test_less<int>;
+    using A                 = test_allocator<int>;
+    using M                 = std::flat_multiset<int, C, KeyContainer<int, A>>;
+    KeyContainer<int, A> ks = {1, 1, 1, 2, 2, 3, 2, 3, 3};
+    auto m                  = M(ks, C(4), A(5));
+    assert(std::ranges::equal(m, KeyContainer<int, A>{1, 1, 1, 2, 2, 2, 3, 3, 3}));
     assert(m.key_comp() == C(4));
     auto m_copy = m;
     auto keys   = std::move(m_copy).extract();
     assert(keys.get_allocator() == A(5));
 
     // explicit(false)
-    static_assert(ImplicitlyConstructible<M, const std::vector<int, A>&, const A&>);
+    static_assert(ImplicitlyConstructible<M, const KeyContainer<int, A>&, const A&>);
     M m2 = {ks, C(4), A(5)};
     assert(m2 == m);
     assert(m2.key_comp() == C(4));
@@ -155,8 +135,44 @@ void test() {
   }
 }
 
+constexpr bool test() {
+  {
+    // The constructors in this subclause shall not participate in overload
+    // resolution unless uses_allocator_v<container_type, Alloc> is true
+
+    using C  = test_less<int>;
+    using A1 = test_allocator<int>;
+    using A2 = other_allocator<int>;
+    using V1 = std::vector<int, A1>;
+    using V2 = std::vector<int, A2>;
+    using M1 = std::flat_multiset<int, C, V1>;
+    using M2 = std::flat_multiset<int, C, V2>;
+    static_assert(std::is_constructible_v<M1, const V1&, const A1&>);
+    static_assert(std::is_constructible_v<M2, const V2&, const A2&>);
+    static_assert(!std::is_constructible_v<M1, const V1&, const A2&>);
+    static_assert(!std::is_constructible_v<M2, const V2&, const A1&>);
+
+    static_assert(std::is_constructible_v<M1, const V1&, const C&, const A1&>);
+    static_assert(std::is_constructible_v<M2, const V2&, const C&, const A2&>);
+    static_assert(!std::is_constructible_v<M1, const V1&, const C&, const A2&>);
+    static_assert(!std::is_constructible_v<M2, const V2&, const C&, const A1&>);
+  }
+
+  test<std::vector>();
+
+#ifndef __cpp_lib_constexpr_deque
+  if (!TEST_IS_CONSTANT_EVALUATED)
+#endif
+    test<std::deque>();
+
+  return true;
+}
+
 int main(int, char**) {
   test();
+#if TEST_STD_VER >= 26
+  static_assert(test());
+#endif
 
   return 0;
 }
diff --git a/libcxx/test/std/containers/container.adaptors/flat.multiset/flat.multiset.cons/copy.pass.cpp b/libcxx/test/std/containers/container.adaptors/flat.multiset/flat.multiset.cons/copy.pass.cpp
index b4f7220e1bac7..55f3defc5ddff 100644
--- a/libcxx/test/std/containers/container.adaptors/flat.multiset/flat.multiset.cons/copy.pass.cpp
+++ b/libcxx/test/std/containers/container.adaptors/flat.multiset/flat.multiset.cons/copy.pass.cpp
@@ -14,6 +14,7 @@
 
 #include <algorithm>
 #include <cassert>
+#include <deque>
 #include <flat_set>
 #include <vector>
 
@@ -21,10 +22,11 @@
 #include "../../../test_compare.h"
 #include "test_allocator.h"
 
-void test() {
+template <template <class...> class KeyContainer>
+constexpr void test() {
   {
     using C = test_less<int>;
-    std::vector<int, test_allocator<int>> ks({1, 3, 5, 3, 1}, test_allocator<int>(6));
+    KeyContainer<int, test_allocator<int>> ks({1, 3, 5, 3, 1}, test_allocator<int>(6));
     const int expected[] = {1, 1, 3, 3, 5};
     using M              = std::flat_multiset<int, C, decltype(ks)>;
     auto mo              = M(ks, C(5));
@@ -43,7 +45,7 @@ void test() {
   }
   {
     using C              = test_less<int>;
-    using Ks             = std::vector<int, other_allocator<int>>;
+    using Ks             = KeyContainer<int, other_allocator<int>>;
     auto ks              = Ks({1, 3, 5, 3, 1}, other_allocator<int>(6));
     const int expected[] = {1, 1, 3, 3, 5};
     using M              = std::flat_multiset<int, C, Ks>;
@@ -63,8 +65,22 @@ void test() {
   }
 }
 
+constexpr bool test() {
+  test<std::vector>();
+
+#ifndef __cpp_lib_constexpr_deque
+  if (!TEST_IS_CONSTANT_EVALUATED)
+#endif
+    test<std::deque>();
+
+  return true;
+}
+
 int main(int, char**) {
   test();
+#if TEST_STD_VER >= 26
+  static_assert(test());
+#endif
 
   return 0;
 }
diff --git a/libcxx/test/std/containers/container.adaptors/flat.multiset/flat.multiset.cons/copy_alloc.pass.cpp b/libcxx/test/std/containers/container.adaptors/flat.multiset/flat.multiset.cons/copy_alloc.pass.cpp
index ec8ad824ea14b..9862b4074f80f 100644
--- a/libcxx/test/std/containers/container.adaptors/flat.multiset/flat.multiset.cons/copy_alloc.pass.cpp
+++ b/libcxx/test/std/containers/container.adaptors/flat.multiset/flat.multiset.cons/copy_alloc.pass.cpp
@@ -23,7 +23,27 @@
 #include "../../../test_compare.h"
 #include "test_allocator.h"
 
-void test() {
+template <template <class...> class KeyContainer>
+constexpr void test() {
+  using C = test_less<int>;
+  KeyContainer<int, test_allocator<int>> ks({1, 3, 5, 5}, test_allocator<int>(6));
+  using M = std::flat_multiset<int, C, decltype(ks)>;
+  auto mo = M(ks, C(5));
+  auto m  = M(mo, test_allocator<int>(3));
+
+  assert(m.key_comp() == C(5));
+  assert(std::ranges::equal(m, ks));
+  auto keys = std::move(m).extract();
+  assert(keys.get_allocator() == test_allocator<int>(3));
+
+  // mo is unchanged
+  assert(mo.key_comp() == C(5));
+  assert(std::ranges::equal(mo, ks));
+  auto keys2 = std::move(mo).extract();
+  assert(keys2.get_allocator() == test_allocator<int>(6));
+}
+
+constexpr bool test() {
   {
     // The constructors in this subclause shall not participate in overload
     // resolution unless uses_allocator_v<container_type, Alloc> is true.
@@ -40,27 +60,22 @@ void test() {
     static_assert(!std::is_constructible_v<M1, const M1&, const A2&>);
     static_assert(!std::is_constructible_v<M2, const M2&, const A1&>);
   }
-  {
-    using C = test_less<int>;
-    std::vector<int, test_allocator<int>> ks({1, 3, 5, 5}, test_allocator<int>(6));
-    using M = std::flat_multiset<int, C, decltype(ks)>;
-    auto mo = M(ks, C(5));
-    auto m  = M(mo, test_allocator<int>(3));
 
-    assert(m.key_comp() == C(5));
-    assert(std::ranges::equal(m, ks));
-    auto keys = std::move(m).extract();
-    assert(keys.get_allocator() == test_allocator<int>(3));
+  test<std::vector>();
 
-    // mo is unchanged
-    assert(mo.key_comp() == C(5));
-    assert(std::ranges::equal(mo, ks));
-    auto keys2 = std::move(mo).extract();
-    assert(keys2.get_allocator() == test_allocator<int>(6));
-  }
+#ifndef __cpp_lib_constexpr_deque
+  if (!TEST_IS_CONSTANT_EVALUATED)
+#endif
+    test<std::deque>();
+
+  return true;
 }
+
 int main(int, char**) {
   test();
+#if TEST_STD_VER >= 26
+  static_assert(test());
+#endif
 
   return 0;
 }
diff --git a/libcxx/test/std/containers/container.adaptors/flat.multiset/flat.multiset.cons/copy_assign.pass.cpp b/libcxx/test/std/containers/container.adaptors/flat.multiset/flat.multiset.cons/copy_assign.pass.cpp
index 2b6176ac915a7..2e63a004ffa88 100644
--- a/libcxx/test/std/containers/container.adaptors/flat.multiset/flat.multiset.cons/copy_assign.pass.cpp
+++ b/libcxx/test/std/containers/container.adaptors/flat.multiset/flat.multiset.cons/copy_assign.pass.cpp
@@ -13,6 +13,7 @@
 // flat_multiset& operator=(const flat_multiset& m);
 
 #include <algorithm>
+#include <deque>
 #include <flat_set>
 #include <functional>
 #include <vector>
@@ -22,11 +23,12 @@
 #include "../../../test_compare.h"
 #include "test_allocator.h"
 
-void test() {
+template <template <class...> class KeyContainer>
+constexpr void test() {
   {
     // test_allocator is not propagated
     using C = test_less<int>;
-    std::vector<int, test_allocator<int>> ks({1, 3, 5, 5}, test_allocator<int>(6));
+    KeyContainer<int, test_allocator<int>> ks({1, 3, 5, 5}, test_allocator<int>(6));
     using M = std::flat_multiset<int, C, decltype(ks)>;
     auto mo = M(ks, C(5));
     auto m  = M({{3, 4, 5, 4}}, C(3), test_allocator<int>(2));
@@ -46,7 +48,7 @@ void test() {
   {
     // other_allocator is propagated
     using C              = test_less<int>;
-    using Ks             = std::vector<int, other_allocator<int>>;
+    using Ks             = KeyContainer<int, other_allocator<int>>;
     auto ks              = Ks({1, 3, 5, 3}, other_allocator<int>(6));
     const int expected[] = {1, 3, 3, 5};
     using M              = std::flat_multiset<int, C, Ks>;
@@ -65,7 +67,7 @@ void test() {
     auto keys2 = std::move(mo).extract();
     assert(keys2.get_allocator() == other_allocator<int>(6));
   }
-  {
+  if (!TEST_IS_CONSTANT_EVALUATED) {
     // comparator is copied and invariant is preserved
     using M = std::flat_multiset<int, std::function<bool(int, int)>>;
     M mo    = M({1, 2}, std::less<int>());
@@ -103,8 +105,22 @@ void test() {
   }
 }
 
+constexpr bool test() {
+  test<std::vector>();
+
+#ifndef __cpp_lib_constexpr_deque
+  if (!TEST_IS_CONSTANT_EVALUATED)
+#endif
+    test<std::deque>();
+
+  return true;
+}
+
 int main(int, char**) {
   test();
+#if TEST_STD_VER >= 26
+  static_assert(test());
+#endif
 
   return 0;
 }
diff --git a/libcxx/test/std/containers/container.adaptors/flat.multiset/flat.multiset.cons/default.pass.cpp b/libcxx/test/std/containers/container.adaptors/flat.multiset/flat.multiset.cons/default.pass.cpp
index 16f90322cd31a..3a7ff86c6c040 100644
--- a/libcxx/test/std/containers/container.adaptors/flat.multiset/flat.multiset.cons/default.pass.cpp
+++ b/libcxx/test/std/containers/container.adaptors/flat.multiset/flat.multiset.cons/default.pass.cpp
@@ -25,28 +25,29 @@
 #include "test_macros.h"
 
 struct DefaultCtableComp {
-  explicit DefaultCtableComp() { default_constructed_ = true; }
-  bool operator()(int, int) const { return false; }
+  constexpr explicit DefaultCtableComp() { default_constructed_ = true; }
+  constexpr bool operator()(int, int) const { return false; }
   bool default_constructed_ = false;
 };
 
 struct ThrowingCtorComp {
-  ThrowingCtorComp() noexcept(false) {}
-  bool operator()(const auto&, const auto&) const { return false; }
+  constexpr ThrowingCtorComp() noexcept(false) {}
+  constexpr bool operator()(const auto&, const auto&) const { return false; }
 };
 
-void test() {
+template <template <class...> class KeyContainer>
+constexpr void test() {
   {
-    std::flat_multiset<int> m;
+    std::flat_multiset<int, std::less<int>, KeyContainer<int>> m;
     assert(m.empty());
   }
   {
     // explicit(false)
-    std::flat_multiset<int> m = {};
+    std::flat_multiset<int, std::less<int>, KeyContainer<int>> m = {};
     assert(m.empty());
   }
   {
-    std::flat_multiset<int, DefaultCtableComp, std::deque<int, min_allocator<int>>> m;
+    std::flat_multiset<int, DefaultCtableComp, KeyContainer<int, min_allocator<int>>> m;
     assert(m.empty());
     assert(m.begin() == m.end());
     assert(m.key_comp().default_constructed_);
@@ -54,7 +55,7 @@ void test() {
   {
     using A1 = explicit_allocator<int>;
     {
-      std::flat_multiset<int, DefaultCtableComp, std::vector<int, A1>> m;
+      std::flat_multiset<int, DefaultCtableComp, KeyContainer<int, A1>> m;
       assert(m.empty());
       assert(m.key_comp().default_constructed_);
     }
@@ -67,30 +68,46 @@ void test() {
   }
 #if defined(_LIBCPP_VERSION)
   {
-    using C = std::flat_multiset<MoveOnly>;
+    using C = std::flat_multiset<MoveOnly, std::less<MoveOnly>>;
     static_assert(std::is_nothrow_default_constructible_v<C>);
     C c;
   }
   {
-    using C = std::flat_multiset<MoveOnly, std::less<MoveOnly>, std::vector<MoveOnly, test_allocator<MoveOnly>>>;
+    using C = std::flat_multiset<MoveOnly, std::less<MoveOnly>, KeyContainer<MoveOnly, test_allocator<MoveOnly>>>;
     static_assert(std::is_nothrow_default_constructible_v<C>);
     C c;
   }
 #endif // _LIBCPP_VERSION
   {
-    using C = std::flat_multiset<MoveOnly, std::less<MoveOnly>, std::vector<MoveOnly, other_allocator<MoveOnly>>>;
+    using C = std::flat_multiset<MoveOnly, std::less<MoveOnly>, KeyContainer<MoveOnly, other_allocator<MoveOnly>>>;
     static_assert(!std::is_nothrow_default_constructible_v<C>);
     C c;
   }
   {
-    using C = std::flat_multiset<MoveOnly, ThrowingCtorComp>;
+    using C = std::flat_multiset<MoveOnly, ThrowingCtorComp, KeyContainer<MoveOnly>>;
     static_assert(!std::is_nothrow_default_constructible_v<C>);
     C c;
   }
 }
 
+constexpr bool test() {
+  test<std::vector>();
+
+#ifndef __cpp_lib_constexpr_deque
+  if (!TEST_IS_CONSTANT_EVALUATED)
+#endif
+  {
+    test<std::deque>();
+  }
+
+  return true;
+}
+
 int main(int, char**) {
   test();
+#if TEST_STD_VER >= 26
+  static_assert(test());
+#endif
 
   return 0;
 }
diff --git a/libcxx/test/std/containers/container.adaptors/flat.multiset/flat.multiset.cons/dtor_noexcept.pass.cpp b/libcxx/test/std/containers/container.adaptors/flat.multiset/flat.multiset.cons/dtor_noexcept.pass.cpp
index f852f2f85572c..f7243fa7e7fb3 100644
--- a/libcxx/test/std/containers/container.adaptors/flat.multiset/flat.multiset.cons/dtor_noexcept.pass.cpp
+++ b/libcxx/test/std/containers/container.adaptors/flat.multiset/flat.multiset.cons/dtor_noexcept.pass.cpp
@@ -23,39 +23,56 @@
 #include "test_allocator.h"
 
 struct ThrowingDtorComp {
-  bool operator()(const auto&, const auto&) const;
-  ~ThrowingDtorComp() noexcept(false) {}
+  constexpr bool operator()(const auto&, const auto&) const;
+  constexpr ~ThrowingDtorComp() noexcept(false) {}
 };
 
-void test() {
+template <template <class...> class KeyContainer>
+constexpr void test() {
   {
-    using C = std::flat_multiset<MoveOnly, MoveOnly>;
+    using C = std::flat_multiset<MoveOnly, std::less<MoveOnly>, KeyContainer<MoveOnly>>;
     static_assert(std::is_nothrow_destructible_v<C>);
     C c;
   }
   {
-    using V = std::vector<MoveOnly, test_allocator<MoveOnly>>;
+    using V = KeyContainer<MoveOnly, test_allocator<MoveOnly>>;
     using C = std::flat_multiset<MoveOnly, std::less<MoveOnly>, V>;
     static_assert(std::is_nothrow_destructible_v<C>);
     C c;
   }
   {
-    using V = std::deque<MoveOnly, other_allocator<MoveOnly>>;
+    using V = KeyContainer<MoveOnly, other_allocator<MoveOnly>>;
     using C = std::flat_multiset<MoveOnly, std::greater<MoveOnly>, V>;
     static_assert(std::is_nothrow_destructible_v<C>);
     C c;
   }
 #if defined(_LIBCPP_VERSION)
   {
-    using C = std::flat_multiset<MoveOnly, ThrowingDtorComp>;
+    using C = std::flat_multiset<MoveOnly, ThrowingDtorComp, KeyContainer<MoveOnly>>;
     static_assert(!std::is_nothrow_destructible_v<C>);
     C c;
   }
 #endif // _LIBCPP_VERSION
 }
 
+constexpr bool test() {
+  test<std::vector>();
+
+#ifndef __cpp_lib_constexpr_deque
+  if (!TEST_IS_CONSTANT_EVALUATED)
+#endif
+  {
+    test<std::deque>();
+  }
+
+  return true;
+}
+
 int main(int, char**) {
   test();
+#if TEST_STD_VER >= 26
+  static_assert(test());
+#endif
 
   return 0;
 }
diff --git a/libcxx/test/std/containers/container.adaptors/flat.multiset/flat.multiset.cons/initializer_list.pass.cpp b/libcxx/test/std/containers/container.adaptors/flat.multiset/flat.multiset.cons/initializer_list.pass.cpp
index 10638d75bbd14..9d486b6b6876a 100644
--- a/libcxx/test/std/containers/container.adaptors/flat.multiset/flat.multiset.cons/initializer_list.pass.cpp
+++ b/libcxx/test/std/containers/container.adaptors/flat.multiset/flat.multiset.cons/initializer_list.pass.cpp
@@ -32,57 +32,17 @@
 #include "../../../test_compare.h"
 
 struct DefaultCtableComp {
-  explicit DefaultCtableComp() { default_constructed_ = true; }
-  bool operator()(int, int) const { return false; }
+  constexpr explicit DefaultCtableComp() { default_constructed_ = true; }
+  constexpr bool operator()(int, int) const { return false; }
   bool default_constructed_ = false;
 };
 
-void test() {
-  {
-    // The constructors in this subclause shall not participate in overload
-    // resolution unless uses_allocator_v<container_type, Alloc> is true.
-
-    using C  = test_less<int>;
-    using A1 = test_allocator<int>;
-    using A2 = other_allocator<int>;
-    using V1 = std::vector<int, A1>;
-    using V2 = std::vector<int, A2>;
-    using M1 = std::flat_multiset<int, C, V1>;
-    using M2 = std::flat_multiset<int, C, V2>;
-    using IL = std::initializer_list<int>;
-    static_assert(std::is_constructible_v<M1, IL, const A1&>);
-    static_assert(std::is_constructible_v<M2, IL, const A2&>);
-    static_assert(!std::is_constructible_v<M1, IL, const A2&>);
-    static_assert(!std::is_constructible_v<M2, IL, const A1&>);
-
-    static_assert(std::is_constructible_v<M1, IL, const C&, const A1&>);
-    static_assert(std::is_constructible_v<M2, IL, const C&, const A2&>);
-    static_assert(!std::is_constructible_v<M1, IL, const C&, const A2&>);
-    static_assert(!std::is_constructible_v<M2, IL, const C&, const A1&>);
-  }
-
-  {
-    // initializer_list<value_type> needs to match exactly
-    using M = std::flat_multiset<int>;
-    using C = typename M::key_compare;
-    static_assert(std::is_constructible_v<M, std::initializer_list<int>>);
-    static_assert(std::is_constructible_v<M, std::initializer_list<int>, C>);
-    static_assert(std::is_constructible_v<M, std::initializer_list<int>, C, std::allocator<int>>);
-    static_assert(std::is_constructible_v<M, std::initializer_list<int>, std::allocator<int>>);
-    static_assert(!std::is_constructible_v<M, std::initializer_list<const int>>);
-    static_assert(!std::is_constructible_v<M, std::initializer_list<const int>, C>);
-    static_assert(!std::is_constructible_v<M, std::initializer_list<const int>, C, std::allocator<int>>);
-    static_assert(!std::is_constructible_v<M, std::initializer_list<const int>, std::allocator<int>>);
-    static_assert(!std::is_constructible_v<M, std::initializer_list<const int>>);
-    static_assert(!std::is_constructible_v<M, std::initializer_list<const int>, C>);
-    static_assert(!std::is_constructible_v<M, std::initializer_list<const int>, C, std::allocator<int>>);
-    static_assert(!std::is_constructible_v<M, std::initializer_list<const int>, std::allocator<int>>);
-  }
-
+template <template <class...> class KeyContainer>
+constexpr void test() {
   int expected[] = {1, 2, 2, 3, 3, 5};
   {
     // flat_multiset(initializer_list<value_type>);
-    using M                       = std::flat_multiset<int>;
+    using M                       = std::flat_multiset<int, std::less<int>, KeyContainer<int>>;
     std::initializer_list<int> il = {5, 2, 2, 3, 1, 3};
     M m(il);
     assert(std::ranges::equal(m, expected));
@@ -90,13 +50,13 @@ void test() {
   {
     // flat_multiset(initializer_list<value_type>);
     // explicit(false)
-    using M = std::flat_multiset<int>;
+    using M = std::flat_multiset<int, std::less<int>, KeyContainer<int>>;
     M m     = {5, 2, 2, 3, 1, 3};
     assert(std::ranges::equal(m, expected));
   }
   {
     // flat_multiset(initializer_list<value_type>);
-    using M = std::flat_multiset<int, std::greater<int>, std::deque<int, min_allocator<int>>>;
+    using M = std::flat_multiset<int, std::greater<int>, KeyContainer<int, min_allocator<int>>>;
     M m     = {5, 2, 2, 3, 1, 3};
     assert(std::ranges::equal(m, expected | std::views::reverse));
   }
@@ -105,15 +65,14 @@ void test() {
     {
       // flat_multiset(initializer_list<value_type>);
       // different comparator
-      using M = std::flat_multiset<int, DefaultCtableComp, std::vector<int, A>>;
+      using M = std::flat_multiset<int, DefaultCtableComp, KeyContainer<int, A>>;
       M m     = {1, 2, 3};
       assert(m.size() == 3);
-      LIBCPP_ASSERT(*m.begin() == 1);
       assert(m.key_comp().default_constructed_);
     }
     {
       // flat_multiset(initializer_list<value_type>, const Allocator&);
-      using M = std::flat_multiset<int, std::greater<int>, std::deque<int, A>>;
+      using M = std::flat_multiset<int, std::greater<int>, KeyContainer<int, A>>;
       A a;
       M m({5, 2, 2, 3, 1, 3}, a);
       assert(std::ranges::equal(m, expected | std::views::reverse));
@@ -122,7 +81,7 @@ void test() {
   {
     // flat_multiset(initializer_list<value_type>, const key_compare&);
     using C = test_less<int>;
-    using M = std::flat_multiset<int, C>;
+    using M = std::flat_multiset<int, C, KeyContainer<int>>;
     auto m  = M({5, 2, 2, 3, 1, 3}, C(10));
     assert(std::ranges::equal(m, expected));
     assert(m.key_comp() == C(10));
@@ -132,10 +91,10 @@ void test() {
     assert(m2 == m);
     assert(m2.key_comp() == C(10));
   }
-  {
+  if (!TEST_IS_CONSTANT_EVALUATED) {
     // flat_multiset(initializer_list<value_type>, const key_compare&);
     // Sorting uses the comparator that was passed in
-    using M = std::flat_multiset<int, std::function<bool(int, int)>, std::deque<int, min_allocator<int>>>;
+    using M = std::flat_multiset<int, std::function<bool(int, int)>, KeyContainer<int, min_allocator<int>>>;
     auto m  = M({5, 2, 2, 1, 3, 3}, std::greater<int>());
     assert(std::ranges::equal(m, expected | std::views::reverse));
     assert(m.key_comp()(2, 1) == true);
@@ -143,15 +102,72 @@ void test() {
   {
     // flat_multiset(initializer_list<value_type> il, const key_compare& comp, const Alloc& a);
     using A = explicit_allocator<int>;
-    using M = std::flat_multiset<int, std::greater<int>, std::deque<int, A>>;
+    using M = std::flat_multiset<int, std::greater<int>, KeyContainer<int, A>>;
     A a;
     M m({5, 2, 2, 3, 1, 3}, {}, a);
     assert(std::ranges::equal(m, expected | std::views::reverse));
   }
 }
 
+constexpr bool test() {
+  {
+    // The constructors in this subclause shall not participate in overload
+    // resolution unless uses_allocator_v<container_type, Alloc> is true.
+
+    using C  = test_less<int>;
+    using A1 = test_allocator<int>;
+    using A2 = other_allocator<int>;
+    using V1 = std::vector<int, A1>;
+    using V2 = std::vector<int, A2>;
+    using M1 = std::flat_multiset<int, C, V1>;
+    using M2 = std::flat_multiset<int, C, V2>;
+    using IL = std::initializer_list<int>;
+    static_assert(std::is_constructible_v<M1, IL, const A1&>);
+    static_assert(std::is_constructible_v<M2, IL, const A2&>);
+    static_assert(!std::is_constructible_v<M1, IL, const A2&>);
+    static_assert(!std::is_constructible_v<M2, IL, const A1&>);
+
+    static_assert(std::is_constructible_v<M1, IL, const C&, const A1&>);
+    static_assert(std::is_constructible_v<M2, IL, const C&, const A2&>);
+    static_assert(!std::is_constructible_v<M1, IL, const C&, const A2&>);
+    static_assert(!std::is_constructible_v<M2, IL, const C&, const A1&>);
+  }
+
+  {
+    // initializer_list<value_type> needs to match exactly
+    using M = std::flat_multiset<int>;
+    using C = typename M::key_compare;
+    static_assert(std::is_constructible_v<M, std::initializer_list<int>>);
+    static_assert(std::is_constructible_v<M, std::initializer_list<int>, C>);
+    static_assert(std::is_constructible_v<M, std::initializer_list<int>, C, std::allocator<int>>);
+    static_assert(std::is_constructible_v<M, std::initializer_list<int>, std::allocator<int>>);
+    static_assert(!std::is_constructible_v<M, std::initializer_list<const int>>);
+    static_assert(!std::is_constructible_v<M, std::initializer_list<const int>, C>);
+    static_assert(!std::is_constructible_v<M, std::initializer_list<const int>, C, std::allocator<int>>);
+    static_assert(!std::is_constructible_v<M, std::initializer_list<const int>, std::allocator<int>>);
+    static_assert(!std::is_constructible_v<M, std::initializer_list<const int>>);
+    static_assert(!std::is_constructible_v<M, std::initializer_list<const int>, C>);
+    static_assert(!std::is_constructible_v<M, std::initializer_list<const int>, C, std::allocator<int>>);
+    static_assert(!std::is_constructible_v<M, std::initializer_list<const int>, std::allocator<int>>);
+  }
+
+  test<std::vector>();
+
+#ifndef __cpp_lib_constexpr_deque
+  if (!TEST_IS_CONSTANT_EVALUATED)
+#endif
+  {
+    test<std::deque>();
+  }
+
+  return true;
+}
+
 int main(int, char**) {
   test();
+#if TEST_STD_VER >= 26
+  static_assert(test());
+#endif
 
   return 0;
 }
diff --git a/libcxx/test/std/containers/container.adaptors/flat.multiset/flat.multiset.cons/iter_iter.pass.cpp b/libcxx/test/std/containers/container.adaptors/flat.multiset/flat.multiset.cons/iter_iter.pass.cpp
index da9aef3dc36cd..3a3362862e634 100644
--- a/libcxx/test/std/containers/container.adaptors/flat.multiset/flat.multiset.cons/iter_iter.pass.cpp
+++ b/libcxx/test/std/containers/container.adaptors/flat.multiset/flat.multiset.cons/iter_iter.pass.cpp
@@ -30,37 +30,14 @@
 #include "test_macros.h"
 #include "../../../test_compare.h"
 
-void test() {
-  {
-    // The constructors in this subclause shall not participate in overload
-    // resolution unless uses_allocator_v<container_type, Alloc> is true.
-
-    using C     = test_less<int>;
-    using A1    = test_allocator<int>;
-    using A2    = other_allocator<int>;
-    using V1    = std::vector<int, A1>;
-    using V2    = std::vector<int, A2>;
-    using M1    = std::flat_multiset<int, C, V1>;
-    using M2    = std::flat_multiset<int, C, V2>;
-    using Iter1 = typename M1::iterator;
-    using Iter2 = typename M2::iterator;
-    static_assert(std::is_constructible_v<M1, Iter1, Iter1, const A1&>);
-    static_assert(std::is_constructible_v<M2, Iter2, Iter2, const A2&>);
-    static_assert(!std::is_constructible_v<M1, Iter1, Iter1, const A2&>);
-    static_assert(!std::is_constructible_v<M2, Iter2, Iter2, const A1&>);
-
-    static_assert(std::is_constructible_v<M1, Iter1, Iter1, const C&, const A1&>);
-    static_assert(std::is_constructible_v<M2, Iter2, Iter2, const C&, const A2&>);
-    static_assert(!std::is_constructible_v<M1, Iter1, Iter1, const C&, const A2&>);
-    static_assert(!std::is_constructible_v<M2, Iter2, Iter2, const C&, const A1&>);
-  }
-
+template <template <class...> class KeyContainer>
+constexpr void test() {
   int ar[]       = {1, 1, 1, 2, 2, 3, 2, 3, 3};
   int expected[] = {1, 1, 1, 2, 2, 2, 3, 3, 3};
   {
     // flat_multiset(InputIterator , InputIterator)
     // cpp17_input_iterator
-    using M = std::flat_multiset<int>;
+    using M = std::flat_multiset<int, std::less<int>, KeyContainer<int>>;
     auto m  = M(cpp17_input_iterator<const int*>(ar), cpp17_input_iterator<const int*>(ar + 9));
     assert(std::ranges::equal(m, expected));
 
@@ -71,21 +48,21 @@ void test() {
   {
     // flat_multiset(InputIterator , InputIterator)
     // greater
-    using M = std::flat_multiset<int, std::greater<int>, std::deque<int, min_allocator<int>>>;
+    using M = std::flat_multiset<int, std::greater<int>, KeyContainer<int, min_allocator<int>>>;
     auto m  = M(cpp17_input_iterator<const int*>(ar), cpp17_input_iterator<const int*>(ar + 9));
     assert(std::ranges::equal(m, expected | std::views::reverse));
   }
   {
     // flat_multiset(InputIterator , InputIterator)
     // Test when the operands are of array type (also contiguous iterator type)
-    using M = std::flat_multiset<int, std::greater<int>, std::vector<int, min_allocator<int>>>;
+    using M = std::flat_multiset<int, std::greater<int>, KeyContainer<int, min_allocator<int>>>;
     auto m  = M(ar, ar);
     assert(m.empty());
   }
   {
     // flat_multiset(InputIterator , InputIterator, const key_compare&)
     using C = test_less<int>;
-    using M = std::flat_multiset<int, C, std::vector<int>>;
+    using M = std::flat_multiset<int, C, KeyContainer<int>>;
     auto m  = M(ar, ar + 9, C(3));
     assert(std::ranges::equal(m, expected));
     assert(m.key_comp() == C(3));
@@ -98,7 +75,7 @@ void test() {
   {
     // flat_multiset(InputIterator , InputIterator, const Allocator&)
     using A1 = test_allocator<int>;
-    using M  = std::flat_multiset<int, std::less<int>, std::vector<int, A1>>;
+    using M  = std::flat_multiset<int, std::less<int>, KeyContainer<int, A1>>;
     auto m   = M(ar, ar + 9, A1(5));
     assert(std::ranges::equal(m, expected));
     assert(std::move(m).extract().get_allocator() == A1(5));
@@ -107,7 +84,7 @@ void test() {
     // flat_multiset(InputIterator , InputIterator, const Allocator&)
     // explicit(false)
     using A1 = test_allocator<int>;
-    using M  = std::flat_multiset<int, std::less<int>, std::vector<int, A1>>;
+    using M  = std::flat_multiset<int, std::less<int>, KeyContainer<int, A1>>;
     M m      = {ar, ar + 9, A1(5)}; // implicit ctor
     assert(std::ranges::equal(m, expected));
     assert(std::move(m).extract().get_allocator() == A1(5));
@@ -116,7 +93,7 @@ void test() {
     // flat_multiset(InputIterator , InputIterator, const key_compare&, const Allocator&)
     using C  = test_less<int>;
     using A1 = test_allocator<int>;
-    using M  = std::flat_multiset<int, C, std::vector<int, A1>>;
+    using M  = std::flat_multiset<int, C, KeyContainer<int, A1>>;
     auto m   = M(ar, ar + 9, C(3), A1(5));
     assert(std::ranges::equal(m, expected));
     assert(m.key_comp() == C(3));
@@ -126,7 +103,7 @@ void test() {
     // flat_multiset(InputIterator , InputIterator, const key_compare&, const Allocator&)
     // explicit(false)
     using A1 = test_allocator<int>;
-    using M  = std::flat_multiset<int, std::less<int>, std::deque<int, A1>>;
+    using M  = std::flat_multiset<int, std::less<int>, KeyContainer<int, A1>>;
     M m      = {ar, ar + 9, {}, A1(5)}; // implicit ctor
     assert(std::ranges::equal(m, expected));
     LIBCPP_ASSERT(std::ranges::equal(m, expected));
@@ -134,8 +111,45 @@ void test() {
   }
 }
 
+constexpr bool test() {
+  {
+    // The constructors in this subclause shall not participate in overload
+    // resolution unless uses_allocator_v<container_type, Alloc> is true.
+
+    using C     = test_less<int>;
+    using A1    = test_allocator<int>;
+    using A2    = other_allocator<int>;
+    using V1    = std::vector<int, A1>;
+    using V2    = std::vector<int, A2>;
+    using M1    = std::flat_multiset<int, C, V1>;
+    using M2    = std::flat_multiset<int, C, V2>;
+    using Iter1 = typename M1::iterator;
+    using Iter2 = typename M2::iterator;
+    static_assert(std::is_constructible_v<M1, Iter1, Iter1, const A1&>);
+    static_assert(std::is_constructible_v<M2, Iter2, Iter2, const A2&>);
+    static_assert(!std::is_constructible_v<M1, Iter1, Iter1, const A2&>);
+    static_assert(!std::is_constructible_v<M2, Iter2, Iter2, const A1&>);
+
+    static_assert(std::is_constructible_v<M1, Iter1, Iter1, const C&, const A1&>);
+    static_assert(std::is_constructible_v<M2, Iter2, Iter2, const C&, const A2&>);
+    static_assert(!std::is_constructible_v<M1, Iter1, Iter1, const C&, const A2&>);
+    static_assert(!std::is_constructible_v<M2, Iter2, Iter2, const C&, const A1&>);
+  }
+
+  test<std::vector>();
+#ifndef __cpp_lib_constexpr_deque
+  if (!TEST_IS_CONSTANT_EVALUATED)
+#endif
+    test<std::deque>();
+
+  return true;
+}
+
 int main(int, char**) {
   test();
+#if TEST_STD_VER >= 26
+  static_assert(test());
+#endif
 
   return 0;
 }
diff --git a/libcxx/test/std/containers/container.adaptors/flat.multiset/flat.multiset.cons/move.pass.cpp b/libcxx/test/std/containers/container.adaptors/flat.multiset/flat.multiset.cons/move.pass.cpp
index 825ad75cc8f4c..7fb0c0e9c3fd0 100644
--- a/libcxx/test/std/containers/container.adaptors/flat.multiset/flat.multiset.cons/move.pass.cpp
+++ b/libcxx/test/std/containers/container.adaptors/flat.multiset/flat.multiset.cons/move.pass.cpp
@@ -25,11 +25,12 @@
 #include "test_allocator.h"
 #include "min_allocator.h"
 
-void test() {
+template <template <class...> class KeyContainer>
+constexpr void test() {
   {
     using C = test_less<int>;
     using A = test_allocator<int>;
-    using M = std::flat_multiset<int, C, std::deque<int, A>>;
+    using M = std::flat_multiset<int, C, KeyContainer<int, A>>;
     M mo    = M({1, 2, 1, 3}, C(5), A(7));
     M m     = std::move(mo);
     assert((m == M{1, 1, 2, 3}));
@@ -43,7 +44,7 @@ void test() {
   {
     using C = test_less<int>;
     using A = min_allocator<int>;
-    using M = std::flat_multiset<int, C, std::vector<int, A>>;
+    using M = std::flat_multiset<int, C, KeyContainer<int, A>>;
     M mo    = M({1, 2, 1, 3}, C(5), A());
     M m     = std::move(mo);
     assert((m == M{1, 1, 2, 3}));
@@ -54,9 +55,9 @@ void test() {
     assert(mo.key_comp() == C(5));
     assert(std::move(mo).extract().get_allocator() == A());
   }
-  {
+  if (!TEST_IS_CONSTANT_EVALUATED) {
     // A moved-from flat_multiset maintains its class invariant in the presence of moved-from comparators.
-    using M = std::flat_multiset<int, std::function<bool(int, int)>>;
+    using M = std::flat_multiset<int, std::function<bool(int, int)>, KeyContainer<int>>;
     M mo    = M({1, 2, 1, 3}, std::less<int>());
     M m     = std::move(mo);
     assert(m.size() == 4);
@@ -81,6 +82,16 @@ void test() {
   }
 }
 
+constexpr bool test() {
+  test<std::vector>();
+#ifndef __cpp_lib_constexpr_deque
+  if (!TEST_IS_CONSTANT_EVALUATED)
+#endif
+    test<std::deque>();
+
+  return true;
+}
+
 template <class T>
 struct ThrowingMoveAllocator {
   using value_type                                    = T;
@@ -179,6 +190,9 @@ void test_move_exception() {
 
 int main(int, char**) {
   test();
+#if TEST_STD_VER >= 26
+  static_assert(test());
+#endif
   test_move_noexcept();
 #if !defined(TEST_HAS_NO_EXCEPTIONS)
   test_move_exception();
diff --git a/libcxx/test/std/containers/container.adaptors/flat.multiset/flat.multiset.cons/move_alloc.pass.cpp b/libcxx/test/std/containers/container.adaptors/flat.multiset/flat.multiset.cons/move_alloc.pass.cpp
index ee8258e5ac846..4c3787645cd14 100644
--- a/libcxx/test/std/containers/container.adaptors/flat.multiset/flat.multiset.cons/move_alloc.pass.cpp
+++ b/libcxx/test/std/containers/container.adaptors/flat.multiset/flat.multiset.cons/move_alloc.pass.cpp
@@ -24,28 +24,13 @@
 #include "../../../test_compare.h"
 #include "test_allocator.h"
 
-void test() {
-  {
-    // The constructors in this subclause shall not participate in overload
-    // resolution unless uses_allocator_v<container_type, Alloc> is true.
-
-    using C  = test_less<int>;
-    using A1 = test_allocator<int>;
-    using A2 = other_allocator<int>;
-    using V1 = std::vector<int, A1>;
-    using V2 = std::vector<int, A2>;
-    using M1 = std::flat_multiset<int, C, V1>;
-    using M2 = std::flat_multiset<int, C, V2>;
-    static_assert(std::is_constructible_v<M1, M1&&, const A1&>);
-    static_assert(std::is_constructible_v<M2, M2&&, const A2&>);
-    static_assert(!std::is_constructible_v<M1, M1&&, const A2&>);
-    static_assert(!std::is_constructible_v<M2, M2&&, const A1&>);
-  }
+template <template <class...> class KeyContainer>
+constexpr void test() {
   {
     int expected[] = {1, 1, 2, 2, 3};
     using C        = test_less<int>;
     using A        = test_allocator<int>;
-    using M        = std::flat_multiset<int, C, std::deque<int, A>>;
+    using M        = std::flat_multiset<int, C, KeyContainer<int, A>>;
     auto mo        = M(expected, expected + 5, C(5), A(7));
     auto m         = M(std::move(mo), A(3));
 
@@ -72,8 +57,38 @@ void test() {
   }
 }
 
+constexpr bool test() {
+  {
+    // The constructors in this subclause shall not participate in overload
+    // resolution unless uses_allocator_v<container_type, Alloc> is true.
+
+    using C  = test_less<int>;
+    using A1 = test_allocator<int>;
+    using A2 = other_allocator<int>;
+    using V1 = std::vector<int, A1>;
+    using V2 = std::vector<int, A2>;
+    using M1 = std::flat_multiset<int, C, V1>;
+    using M2 = std::flat_multiset<int, C, V2>;
+    static_assert(std::is_constructible_v<M1, M1&&, const A1&>);
+    static_assert(std::is_constructible_v<M2, M2&&, const A2&>);
+    static_assert(!std::is_constructible_v<M1, M1&&, const A2&>);
+    static_assert(!std::is_constructible_v<M2, M2&&, const A1&>);
+  }
+
+  test<std::vector>();
+#ifndef __cpp_lib_constexpr_deque
+  if (!TEST_IS_CONSTANT_EVALUATED)
+#endif
+    test<std::deque>();
+
+  return true;
+}
+
 int main(int, char**) {
   test();
+#if TEST_STD_VER >= 26
+  static_assert(test());
+#endif
 
   return 0;
 }
diff --git a/libcxx/test/std/containers/container.adaptors/flat.multiset/flat.multiset.cons/move_assign.pass.cpp b/libcxx/test/std/containers/container.adaptors/flat.multiset/flat.multiset.cons/move_assign.pass.cpp
index 96e046e38668f..df85fa46ad796 100644
--- a/libcxx/test/std/containers/container.adaptors/flat.multiset/flat.multiset.cons/move_assign.pass.cpp
+++ b/libcxx/test/std/containers/container.adaptors/flat.multiset/flat.multiset.cons/move_assign.pass.cpp
@@ -187,25 +187,12 @@ void test_move_assign_no_except() {
   }
 }
 
-void test() {
-  {
-    using C                           = test_less<int>;
-    using A1                          = test_allocator<int>;
-    using M                           = std::flat_multiset<int, C, std::vector<int, A1>>;
-    M mo                              = M({1, 1, 2, 3}, C(5), A1(7));
-    M m                               = M({}, C(3), A1(7));
-    std::same_as<M&> decltype(auto) r = m = std::move(mo);
-    assert(&r == &m);
-    assert((m == M{1, 1, 2, 3}));
-    assert(m.key_comp() == C(5));
-    auto ks = std::move(m).extract();
-    assert(ks.get_allocator() == A1(7));
-    assert(mo.empty());
-  }
+template <template <class...> class KeyContainer>
+constexpr void test() {
   {
     using C                           = test_less<int>;
     using A1                          = other_allocator<int>;
-    using M                           = std::flat_multiset<int, C, std::deque<int, A1>>;
+    using M                           = std::flat_multiset<int, C, KeyContainer<int, A1>>;
     M mo                              = M({4, 4, 5}, C(5), A1(7));
     M m                               = M({1, 1, 2, 3, 4}, C(3), A1(7));
     std::same_as<M&> decltype(auto) r = m = std::move(mo);
@@ -218,7 +205,7 @@ void test() {
   }
   {
     using A                           = min_allocator<int>;
-    using M                           = std::flat_multiset<int, std::greater<int>, std::vector<int, A>>;
+    using M                           = std::flat_multiset<int, std::greater<int>, KeyContainer<int, A>>;
     M mo                              = M({5, 3, 4, 3}, A());
     M m                               = M({4, 1, 3, 2, 1}, A());
     std::same_as<M&> decltype(auto) r = m = std::move(mo);
@@ -230,8 +217,36 @@ void test() {
   }
 }
 
+constexpr bool test() {
+  {
+    using C                           = test_less<int>;
+    using A1                          = test_allocator<int>;
+    using M                           = std::flat_multiset<int, C, std::vector<int, A1>>;
+    M mo                              = M({1, 1, 2, 3}, C(5), A1(7));
+    M m                               = M({}, C(3), A1(7));
+    std::same_as<M&> decltype(auto) r = m = std::move(mo);
+    assert(&r == &m);
+    assert((m == M{1, 1, 2, 3}));
+    assert(m.key_comp() == C(5));
+    auto ks = std::move(m).extract();
+    assert(ks.get_allocator() == A1(7));
+    assert(mo.empty());
+  }
+
+  test<std::vector>();
+#ifndef __cpp_lib_constexpr_deque
+  if (!TEST_IS_CONSTANT_EVALUATED)
+#endif
+    test<std::deque>();
+
+  return true;
+}
+
 int main(int, char**) {
   test();
+#if TEST_STD_VER >= 26
+  static_assert(test());
+#endif
   test_move_assign_clears();
   test_move_assign_no_except();
 
diff --git a/libcxx/test/std/containers/container.adaptors/flat.multiset/flat.multiset.cons/range.pass.cpp b/libcxx/test/std/containers/container.adaptors/flat.multiset/flat.multiset.cons/range.pass.cpp
index 76485b47ec5ea..a71d303d7ff05 100644
--- a/libcxx/test/std/containers/container.adaptors/flat.multiset/flat.multiset.cons/range.pass.cpp
+++ b/libcxx/test/std/containers/container.adaptors/flat.multiset/flat.multiset.cons/range.pass.cpp
@@ -56,35 +56,14 @@ static_assert(
     !std::
         is_constructible_v<Set, std::from_range_t, RangeOf<std::pair<int, int>>, std::less<int>, std::allocator<int>>);
 
-void test() {
-  {
-    // The constructors in this subclause shall not participate in overload
-    // resolution unless uses_allocator_v<container_type, Alloc> is true.
-
-    using C  = test_less<int>;
-    using A1 = test_allocator<int>;
-    using A2 = other_allocator<int>;
-    using V1 = std::vector<int, A1>;
-    using V2 = std::vector<int, A2>;
-    using M1 = std::flat_multiset<int, C, V1>;
-    using M2 = std::flat_multiset<int, C, V2>;
-    static_assert(std::is_constructible_v<M1, std::from_range_t, M1, const A1&>);
-    static_assert(std::is_constructible_v<M2, std::from_range_t, M2, const A2&>);
-    static_assert(!std::is_constructible_v<M1, std::from_range_t, M1, const A2&>);
-    static_assert(!std::is_constructible_v<M2, std::from_range_t, M2, const A1&>);
-
-    static_assert(std::is_constructible_v<M1, std::from_range_t, M1, const C&, const A1&>);
-    static_assert(std::is_constructible_v<M2, std::from_range_t, M2, const C&, const A2&>);
-    static_assert(!std::is_constructible_v<M1, std::from_range_t, M1, const C&, const A2&>);
-    static_assert(!std::is_constructible_v<M2, std::from_range_t, M2, const C&, const A1&>);
-  }
-
+template <template <class...> class KeyContainer>
+constexpr void test() {
   int ar[]       = {1, 1, 1, 2, 2, 3, 2, 3, 3};
   int expected[] = {1, 1, 1, 2, 2, 2, 3, 3, 3};
   {
     // flat_multiset(from_range_t, R&&)
     // input_range && !common
-    using M    = std::flat_multiset<int>;
+    using M    = std::flat_multiset<int, std::less<int>, KeyContainer<int>>;
     using Iter = cpp20_input_iterator<const int*>;
     using Sent = sentinel_wrapper<Iter>;
     using R    = std::ranges::subrange<Iter, Sent>;
@@ -98,17 +77,17 @@ void test() {
   {
     // flat_multiset(from_range_t, R&&)
     // greater
-    using M    = std::flat_multiset<int, std::greater<int>, std::deque<int, min_allocator<int>>>;
+    using M    = std::flat_multiset<int, std::greater<int>, KeyContainer<int, min_allocator<int>>>;
     using Iter = cpp20_input_iterator<const int*>;
     using Sent = sentinel_wrapper<Iter>;
     using R    = std::ranges::subrange<Iter, Sent>;
     auto m     = M(std::from_range, R(Iter(ar), Sent(Iter(ar + 9))));
-    assert(std::ranges::equal(m, std::deque<int, min_allocator<int>>{3, 3, 3, 2, 2, 2, 1, 1, 1}));
+    assert(std::ranges::equal(m, KeyContainer<int, min_allocator<int>>{3, 3, 3, 2, 2, 2, 1, 1, 1}));
   }
   {
     // flat_multiset(from_range_t, R&&)
     // contiguous range
-    using M = std::flat_multiset<int>;
+    using M = std::flat_multiset<int, std::less<int>, KeyContainer<int>>;
     using R = std::ranges::subrange<const int*>;
     auto m  = M(std::from_range, R(ar, ar + 9));
     assert(std::ranges::equal(m, expected));
@@ -116,7 +95,7 @@ void test() {
   {
     // flat_multiset(from_range_t, R&&, const key_compare&)
     using C = test_less<int>;
-    using M = std::flat_multiset<int, C, std::vector<int>>;
+    using M = std::flat_multiset<int, C, KeyContainer<int>>;
     using R = std::ranges::subrange<const int*>;
     auto m  = M(std::from_range, R(ar, ar + 9), C(3));
     assert(std::ranges::equal(m, expected));
@@ -130,7 +109,7 @@ void test() {
   {
     // flat_multiset(from_range_t, R&&, const Allocator&)
     using A1 = test_allocator<int>;
-    using M  = std::flat_multiset<int, std::less<int>, std::vector<int, A1>>;
+    using M  = std::flat_multiset<int, std::less<int>, KeyContainer<int, A1>>;
     using R  = std::ranges::subrange<const int*>;
     auto m   = M(std::from_range, R(ar, ar + 9), A1(5));
     assert(std::ranges::equal(m, expected));
@@ -140,7 +119,7 @@ void test() {
     // flat_multiset(from_range_t, R&&, const Allocator&)
     // explicit(false)
     using A1 = test_allocator<int>;
-    using M  = std::flat_multiset<int, std::less<int>, std::deque<int, A1>>;
+    using M  = std::flat_multiset<int, std::less<int>, KeyContainer<int, A1>>;
     using R  = std::ranges::subrange<const int*>;
     M m      = {std::from_range, R(ar, ar + 9), A1(5)}; // implicit ctor
     assert(std::ranges::equal(m, expected));
@@ -150,7 +129,7 @@ void test() {
     // flat_multiset(from_range_t, R&&, const key_compare&, const Allocator&)
     using C  = test_less<int>;
     using A1 = test_allocator<int>;
-    using M  = std::flat_multiset<int, C, std::vector<int, A1>>;
+    using M  = std::flat_multiset<int, C, KeyContainer<int, A1>>;
     using R  = std::ranges::subrange<const int*>;
     auto m   = M(std::from_range, R(ar, ar + 9), C(3), A1(5));
     assert(std::ranges::equal(m, expected));
@@ -161,7 +140,7 @@ void test() {
     // flat_multiset(from_range_t, R&&, const key_compare&, const Allocator&)
     // explicit(false)
     using A1 = test_allocator<int>;
-    using M  = std::flat_multiset<int, std::less<int>, std::deque<int, A1>>;
+    using M  = std::flat_multiset<int, std::less<int>, KeyContainer<int, A1>>;
     using R  = std::ranges::subrange<const int*>;
     M m      = {std::from_range, R(ar, ar + 9), {}, A1(5)}; // implicit ctor
     assert(std::ranges::equal(m, expected));
@@ -169,8 +148,43 @@ void test() {
   }
 }
 
+constexpr bool test() {
+  {
+    // The constructors in this subclause shall not participate in overload
+    // resolution unless uses_allocator_v<container_type, Alloc> is true.
+
+    using C  = test_less<int>;
+    using A1 = test_allocator<int>;
+    using A2 = other_allocator<int>;
+    using V1 = std::vector<int, A1>;
+    using V2 = std::vector<int, A2>;
+    using M1 = std::flat_multiset<int, C, V1>;
+    using M2 = std::flat_multiset<int, C, V2>;
+    static_assert(std::is_constructible_v<M1, std::from_range_t, M1, const A1&>);
+    static_assert(std::is_constructible_v<M2, std::from_range_t, M2, const A2&>);
+    static_assert(!std::is_constructible_v<M1, std::from_range_t, M1, const A2&>);
+    static_assert(!std::is_constructible_v<M2, std::from_range_t, M2, const A1&>);
+
+    static_assert(std::is_constructible_v<M1, std::from_range_t, M1, const C&, const A1&>);
+    static_assert(std::is_constructible_v<M2, std::from_range_t, M2, const C&, const A2&>);
+    static_assert(!std::is_constructible_v<M1, std::from_range_t, M1, const C&, const A2&>);
+    static_assert(!std::is_constructible_v<M2, std::from_range_t, M2, const C&, const A1&>);
+  }
+
+  test<std::vector>();
+#ifndef __cpp_lib_constexpr_deque
+  if (!TEST_IS_CONSTANT_EVALUATED)
+#endif
+    test<std::deque>();
+
+  return true;
+}
+
 int main(int, char**) {
   test();
+#if TEST_STD_VER >= 26
+  static_assert(test());
+#endif
 
   return 0;
 }
diff --git a/libcxx/test/std/containers/container.adaptors/flat.multiset/flat.multiset.cons/sorted_container.pass.cpp b/libcxx/test/std/containers/container.adaptors/flat.multiset/flat.multiset.cons/sorted_container.pass.cpp
index 76759be7da8e3..72fe911f54dab 100644
--- a/libcxx/test/std/containers/container.adaptors/flat.multiset/flat.multiset.cons/sorted_container.pass.cpp
+++ b/libcxx/test/std/containers/container.adaptors/flat.multiset/flat.multiset.cons/sorted_container.pass.cpp
@@ -30,33 +30,13 @@
 #include "test_macros.h"
 #include "../../../test_compare.h"
 
-void test() {
-  {
-    // The constructors in this subclause shall not participate in overload
-    // resolution unless uses_allocator_v<container_type, Alloc> is true.
-
-    using C  = test_less<int>;
-    using A1 = test_allocator<int>;
-    using A2 = other_allocator<int>;
-    using V1 = std::vector<int, A1>;
-    using V2 = std::vector<int, A2>;
-    using M1 = std::flat_multiset<int, C, V1>;
-    using M2 = std::flat_multiset<int, C, V2>;
-    static_assert(std::is_constructible_v<M1, std::sorted_equivalent_t, const V1&, const A1&>);
-    static_assert(std::is_constructible_v<M2, std::sorted_equivalent_t, const V2&, const A2&>);
-    static_assert(!std::is_constructible_v<M1, std::sorted_equivalent_t, const V1&, const A2&>);
-    static_assert(!std::is_constructible_v<M2, std::sorted_equivalent_t, const V2&, const A1&>);
-
-    static_assert(std::is_constructible_v<M1, std::sorted_equivalent_t, const V1&, const C&, const A1&>);
-    static_assert(std::is_constructible_v<M2, std::sorted_equivalent_t, const V2&, const C&, const A2&>);
-    static_assert(!std::is_constructible_v<M1, std::sorted_equivalent_t, const V1&, const C&, const A2&>);
-    static_assert(!std::is_constructible_v<M2, std::sorted_equivalent_t, const V2&, const C&, const A1&>);
-  }
+template <template <class...> class KeyContainer>
+constexpr void test() {
   {
     // flat_multiset(sorted_equivalent_t, container_type)
-    using M             = std::flat_multiset<int>;
-    std::vector<int> ks = {1, 2, 2, 4, 10};
-    auto ks2            = ks;
+    using M              = std::flat_multiset<int, std::less<int>, KeyContainer<int>>;
+    KeyContainer<int> ks = {1, 2, 2, 4, 10};
+    auto ks2             = ks;
 
     auto m = M(std::sorted_equivalent, ks);
     assert((m == M{1, 2, 2, 4, 10}));
@@ -71,7 +51,7 @@ void test() {
   {
     // flat_multiset(sorted_equivalent_t, container_type)
     // non-default container, comparator and allocator type
-    using Ks = std::deque<int, min_allocator<int>>;
+    using Ks = KeyContainer<int, min_allocator<int>>;
     using M  = std::flat_multiset<int, std::greater<int>, Ks>;
     Ks ks    = {10, 4, 4, 2, 1};
     auto m   = M(std::sorted_equivalent, ks);
@@ -84,8 +64,8 @@ void test() {
     // flat_multiset(sorted_equivalent_t, container_type)
     // allocator copied into the containers
     using A = test_allocator<int>;
-    using M = std::flat_multiset<int, std::less<int>, std::deque<int, A>>;
-    auto ks = std::deque<int, A>({1, 2, 2, 4, 10}, A(4));
+    using M = std::flat_multiset<int, std::less<int>, KeyContainer<int, A>>;
+    auto ks = KeyContainer<int, A>({1, 2, 2, 4, 10}, A(4));
     auto m  = M(std::sorted_equivalent, std::move(ks));
     assert(ks.empty()); // it was moved-from
     assert((m == M{1, 2, 2, 4, 10}));
@@ -93,9 +73,9 @@ void test() {
   }
   {
     // flat_multiset(sorted_equivalent_t, container_type ,  key_compare)
-    using C             = test_less<int>;
-    using M             = std::flat_multiset<int, C>;
-    std::vector<int> ks = {1, 2, 2, 4, 10};
+    using C              = test_less<int>;
+    using M              = std::flat_multiset<int, C, KeyContainer<int>>;
+    KeyContainer<int> ks = {1, 2, 2, 4, 10};
 
     auto m = M(std::sorted_equivalent, ks, C(4));
     assert((m == M{1, 2, 2, 4, 10}));
@@ -108,11 +88,11 @@ void test() {
   }
   {
     // flat_multiset(sorted_equivalent_t, container_type , key_compare, const Allocator&)
-    using C                = test_less<int>;
-    using A                = test_allocator<int>;
-    using M                = std::flat_multiset<int, C, std::vector<int, A>>;
-    std::vector<int, A> ks = {1, 2, 2, 4, 10};
-    auto m                 = M(std::sorted_equivalent, ks, C(4), A(5));
+    using C                 = test_less<int>;
+    using A                 = test_allocator<int>;
+    using M                 = std::flat_multiset<int, C, KeyContainer<int, A>>;
+    KeyContainer<int, A> ks = {1, 2, 2, 4, 10};
+    auto m                  = M(std::sorted_equivalent, ks, C(4), A(5));
     assert((m == M{1, 2, 2, 4, 10}));
     assert(m.key_comp() == C(4));
     assert(M(m).extract().get_allocator() == A(5));
@@ -126,8 +106,8 @@ void test() {
   {
     // flat_multiset(sorted_equivalent_t, container_type , const Allocator&)
     using A = test_allocator<int>;
-    using M = std::flat_multiset<int, std::less<int>, std::deque<int, A>>;
-    auto ks = std::deque<int, A>({1, 2, 2, 4, 10}, A(4));
+    using M = std::flat_multiset<int, std::less<int>, KeyContainer<int, A>>;
+    auto ks = KeyContainer<int, A>({1, 2, 2, 4, 10}, A(4));
     auto m  = M(std::sorted_equivalent, ks, A(6)); // replaces the allocators
     assert(!ks.empty());                           // it was an lvalue above
     assert((m == M{1, 2, 2, 4, 10}));
@@ -140,8 +120,44 @@ void test() {
   }
 }
 
+constexpr bool test() {
+  {
+    // The constructors in this subclause shall not participate in overload
+    // resolution unless uses_allocator_v<container_type, Alloc> is true.
+
+    using C  = test_less<int>;
+    using A1 = test_allocator<int>;
+    using A2 = other_allocator<int>;
+    using V1 = std::vector<int, A1>;
+    using V2 = std::vector<int, A2>;
+    using M1 = std::flat_multiset<int, C, V1>;
+    using M2 = std::flat_multiset<int, C, V2>;
+    static_assert(std::is_constructible_v<M1, std::sorted_equivalent_t, const V1&, const A1&>);
+    static_assert(std::is_constructible_v<M2, std::sorted_equivalent_t, const V2&, const A2&>);
+    static_assert(!std::is_constructible_v<M1, std::sorted_equivalent_t, const V1&, const A2&>);
+    static_assert(!std::is_constructible_v<M2, std::sorted_equivalent_t, const V2&, const A1&>);
+
+    static_assert(std::is_constructible_v<M1, std::sorted_equivalent_t, const V1&, const C&, const A1&>);
+    static_assert(std::is_constructible_v<M2, std::sorted_equivalent_t, const V2&, const C&, const A2&>);
+    static_assert(!std::is_constructible_v<M1, std::sorted_equivalent_t, const V1&, const C&, const A2&>);
+    static_assert(!std::is_constructible_v<M2, std::sorted_equivalent_t, const V2&, const C&, const A1&>);
+  }
+
+  test<std::vector>();
+
+#ifndef __cpp_lib_constexpr_deque
+  if (!TEST_IS_CONSTANT_EVALUATED)
+#endif
+    test<std::deque>();
+
+  return true;
+}
+
 int main(int, char**) {
   test();
+#if TEST_STD_VER >= 26
+  static_assert(test());
+#endif
 
   return 0;
 }
diff --git a/libcxx/test/std/containers/container.adaptors/flat.multiset/flat.multiset.cons/sorted_initializer_list.pass.cpp b/libcxx/test/std/containers/container.adaptors/flat.multiset/flat.multiset.cons/sorted_initializer_list.pass.cpp
index 955662dd233ef..1b44552351f34 100644
--- a/libcxx/test/std/containers/container.adaptors/flat.multiset/flat.multiset.cons/sorted_initializer_list.pass.cpp
+++ b/libcxx/test/std/containers/container.adaptors/flat.multiset/flat.multiset.cons/sorted_initializer_list.pass.cpp
@@ -31,64 +31,16 @@
 #include "../../../test_compare.h"
 
 template <class T>
-std::initializer_list<T> il = {1, 2, 4, 4, 5};
+constexpr std::initializer_list<T> il = {1, 2, 4, 4, 5};
 
-void test() {
-  const auto il1 = il<int>;
-  const auto il2 = il<short>;
-
-  {
-    // The constructors in this subclause shall not participate in overload
-    // resolution unless uses_allocator_v<container_type, Alloc> is true.
-
-    using C  = test_less<int>;
-    using A1 = test_allocator<int>;
-    using A2 = other_allocator<int>;
-    using V1 = std::vector<int, A1>;
-    using V2 = std::vector<int, A2>;
-    using M1 = std::flat_multiset<int, C, V1>;
-    using M2 = std::flat_multiset<int, C, V2>;
-    using IL = std::initializer_list<int>;
-
-    static_assert(std::is_constructible_v<M1, std::sorted_equivalent_t, IL, const A1&>);
-    static_assert(std::is_constructible_v<M2, std::sorted_equivalent_t, IL, const A2&>);
-    static_assert(!std::is_constructible_v<M1, std::sorted_equivalent_t, IL, const A2&>);
-    static_assert(!std::is_constructible_v<M2, std::sorted_equivalent_t, IL, const A1&>);
-
-    static_assert(std::is_constructible_v<M1, std::sorted_equivalent_t, IL, const C&, const A1&>);
-    static_assert(std::is_constructible_v<M2, std::sorted_equivalent_t, IL, const C&, const A2&>);
-    static_assert(!std::is_constructible_v<M1, std::sorted_equivalent_t, IL, const C&, const A2&>);
-    static_assert(!std::is_constructible_v<M2, std::sorted_equivalent_t, IL, const C&, const A1&>);
-  }
-  {
-    // initializer_list<value_type> needs to match exactly
-    using M = std::flat_multiset<int>;
-    using C = typename M::key_compare;
-    static_assert(std::is_constructible_v<M, std::sorted_equivalent_t, std::initializer_list<int>>);
-    static_assert(std::is_constructible_v<M, std::sorted_equivalent_t, std::initializer_list<int>, C>);
-    static_assert(
-        std::is_constructible_v<M, std::sorted_equivalent_t, std::initializer_list<int>, C, std::allocator<int>>);
-    static_assert(
-        std::is_constructible_v<M, std::sorted_equivalent_t, std::initializer_list<int>, std::allocator<int>>);
-    static_assert(!std::is_constructible_v<M, std::sorted_equivalent_t, std::initializer_list<const int>>);
-    static_assert(!std::is_constructible_v<M, std::sorted_equivalent_t, std::initializer_list<const int>, C>);
-    static_assert(
-        !std::
-            is_constructible_v<M, std::sorted_equivalent_t, std::initializer_list<const int>, C, std::allocator<int>>);
-    static_assert(
-        !std::is_constructible_v<M, std::sorted_equivalent_t, std::initializer_list<const int>, std::allocator<int>>);
-    static_assert(!std::is_constructible_v<M, std::sorted_equivalent_t, std::initializer_list<const int>>);
-    static_assert(!std::is_constructible_v<M, std::sorted_equivalent_t, std::initializer_list<const int>, C>);
-    static_assert(
-        !std::
-            is_constructible_v<M, std::sorted_equivalent_t, std::initializer_list<const int>, C, std::allocator<int>>);
-    static_assert(
-        !std::is_constructible_v<M, std::sorted_equivalent_t, std::initializer_list<const int>, std::allocator<int>>);
-  }
+constexpr auto il1 = il<int>;
+constexpr auto il2 = il<short>;
 
+template <template <class...> class KeyContainer>
+constexpr void test() {
   {
     // flat_multiset(sorted_equivalent_t, initializer_list<value_type>);
-    using M       = std::flat_multiset<int>;
+    using M       = std::flat_multiset<int, std::less<int>, KeyContainer<int>>;
     auto m        = M(std::sorted_equivalent, il1);
     auto expected = M{1, 2, 4, 4, 5};
     assert(m == expected);
@@ -97,9 +49,9 @@ void test() {
     M m2 = {std::sorted_equivalent, il1};
     assert(m2 == m);
   }
-  {
+  if (!TEST_IS_CONSTANT_EVALUATED) {
     // flat_multiset(sorted_equivalent_t, initializer_list<value_type>, const key_compare&);
-    using M = std::flat_multiset<int, std::function<bool(int, int)>>;
+    using M = std::flat_multiset<int, std::function<bool(int, int)>, KeyContainer<int>>;
     auto m  = M(std::sorted_equivalent, il1, std::less<int>());
     assert(m == M({1, 2, 4, 4, 5}, std::less<>()));
     assert(m.key_comp()(1, 2) == true);
@@ -111,7 +63,7 @@ void test() {
   {
     // flat_multiset(sorted_equivalent_t, initializer_list<value_type>, const key_compare&);
     // greater
-    using M = std::flat_multiset<int, std::greater<int>, std::deque<int, min_allocator<int>>>;
+    using M = std::flat_multiset<int, std::greater<int>, KeyContainer<int, min_allocator<int>>>;
     std::initializer_list<int> il4{5, 4, 4, 2, 1};
     auto m = M(std::sorted_equivalent, il4, std::greater<int>());
     assert((m == M{5, 4, 4, 2, 1}));
@@ -119,7 +71,7 @@ void test() {
   {
     // flat_multiset(sorted_equivalent_t, initializer_list<value_type>,  const Allocator&)
     using A1      = test_allocator<short>;
-    using M       = std::flat_multiset<short, std::less<int>, std::deque<short, A1>>;
+    using M       = std::flat_multiset<short, std::less<int>, KeyContainer<short, A1>>;
     auto m        = M(std::sorted_equivalent, il2, A1(5));
     auto expected = M{1, 2, 4, 4, 5};
     assert(m == expected);
@@ -134,7 +86,7 @@ void test() {
     // flat_multiset(sorted_equivalent_t, initializer_list<value_type>, const key_compare&, const Allocator&);
     using C  = test_less<int>;
     using A1 = test_allocator<short>;
-    using M  = std::flat_multiset<short, C, std::vector<short, A1>>;
+    using M  = std::flat_multiset<short, C, KeyContainer<short, A1>>;
     auto m   = M(std::sorted_equivalent, il2, C(3), A1(5));
     assert((m == M{1, 2, 4, 4, 5}));
     assert(m.key_comp() == C(3));
@@ -144,15 +96,78 @@ void test() {
     // flat_multiset(sorted_equivalent_t, initializer_list<value_type>, const key_compare&, const Allocator&);
     // explicit(false)
     using A1 = test_allocator<short>;
-    using M  = std::flat_multiset<short, std::less<int>, std::deque<short, A1>>;
+    using M  = std::flat_multiset<short, std::less<int>, KeyContainer<short, A1>>;
     M m      = {std::sorted_equivalent, il2, {}, A1(5)}; // implicit ctor
     assert((m == M{1, 2, 4, 4, 5}));
     assert(std::move(m).extract().get_allocator() == A1(5));
   }
 }
 
+constexpr bool test() {
+  {
+    // The constructors in this subclause shall not participate in overload
+    // resolution unless uses_allocator_v<container_type, Alloc> is true.
+
+    using C  = test_less<int>;
+    using A1 = test_allocator<int>;
+    using A2 = other_allocator<int>;
+    using V1 = std::vector<int, A1>;
+    using V2 = std::vector<int, A2>;
+    using M1 = std::flat_multiset<int, C, V1>;
+    using M2 = std::flat_multiset<int, C, V2>;
+    using IL = std::initializer_list<int>;
+
+    static_assert(std::is_constructible_v<M1, std::sorted_equivalent_t, IL, const A1&>);
+    static_assert(std::is_constructible_v<M2, std::sorted_equivalent_t, IL, const A2&>);
+    static_assert(!std::is_constructible_v<M1, std::sorted_equivalent_t, IL, const A2&>);
+    static_assert(!std::is_constructible_v<M2, std::sorted_equivalent_t, IL, const A1&>);
+
+    static_assert(std::is_constructible_v<M1, std::sorted_equivalent_t, IL, const C&, const A1&>);
+    static_assert(std::is_constructible_v<M2, std::sorted_equivalent_t, IL, const C&, const A2&>);
+    static_assert(!std::is_constructible_v<M1, std::sorted_equivalent_t, IL, const C&, const A2&>);
+    static_assert(!std::is_constructible_v<M2, std::sorted_equivalent_t, IL, const C&, const A1&>);
+  }
+  {
+    // initializer_list<value_type> needs to match exactly
+    using M = std::flat_multiset<int>;
+    using C = typename M::key_compare;
+    static_assert(std::is_constructible_v<M, std::sorted_equivalent_t, std::initializer_list<int>>);
+    static_assert(std::is_constructible_v<M, std::sorted_equivalent_t, std::initializer_list<int>, C>);
+    static_assert(
+        std::is_constructible_v<M, std::sorted_equivalent_t, std::initializer_list<int>, C, std::allocator<int>>);
+    static_assert(
+        std::is_constructible_v<M, std::sorted_equivalent_t, std::initializer_list<int>, std::allocator<int>>);
+    static_assert(!std::is_constructible_v<M, std::sorted_equivalent_t, std::initializer_list<const int>>);
+    static_assert(!std::is_constructible_v<M, std::sorted_equivalent_t, std::initializer_list<const int>, C>);
+    static_assert(
+        !std::
+            is_constructible_v<M, std::sorted_equivalent_t, std::initializer_list<const int>, C, std::allocator<int>>);
+    static_assert(
+        !std::is_constructible_v<M, std::sorted_equivalent_t, std::initializer_list<const int>, std::allocator<int>>);
+    static_assert(!std::is_constructible_v<M, std::sorted_equivalent_t, std::initializer_list<const int>>);
+    static_assert(!std::is_constructible_v<M, std::sorted_equivalent_t, std::initializer_list<const int>, C>);
+    static_assert(
+        !std::
+            is_constructible_v<M, std::sorted_equivalent_t, std::initializer_list<const int>, C, std::allocator<int>>);
+    static_assert(
+        !std::is_constructible_v<M, std::sorted_equivalent_t, std::initializer_list<const int>, std::allocator<int>>);
+  }
+
+  test<std::vector>();
+
+#ifndef __cpp_lib_constexpr_deque
+  if (!TEST_IS_CONSTANT_EVALUATED)
+#endif
+    test<std::deque>();
+
+  return true;
+}
+
 int main(int, char**) {
   test();
+#if TEST_STD_VER >= 26
+  static_assert(test());
+#endif
 
   return 0;
 }
diff --git a/libcxx/test/std/containers/container.adaptors/flat.multiset/flat.multiset.cons/sorted_iter_iter.pass.cpp b/libcxx/test/std/containers/container.adaptors/flat.multiset/flat.multiset.cons/sorted_iter_iter.pass.cpp
index 9ebe45d71d667..979a9c30199bd 100644
--- a/libcxx/test/std/containers/container.adaptors/flat.multiset/flat.multiset.cons/sorted_iter_iter.pass.cpp
+++ b/libcxx/test/std/containers/container.adaptors/flat.multiset/flat.multiset.cons/sorted_iter_iter.pass.cpp
@@ -28,34 +28,13 @@
 #include "test_macros.h"
 #include "../../../test_compare.h"
 
-void test() {
-  {
-    // The constructors in this subclause shall not participate in overload
-    // resolution unless uses_allocator_v<container_type, Alloc> is true.
-
-    using C     = test_less<int>;
-    using A1    = test_allocator<int>;
-    using A2    = other_allocator<int>;
-    using V1    = std::vector<int, A1>;
-    using V2    = std::vector<int, A2>;
-    using M1    = std::flat_multiset<int, C, V1>;
-    using M2    = std::flat_multiset<int, C, V2>;
-    using Iter1 = typename M1::iterator;
-    using Iter2 = typename M2::iterator;
-    static_assert(std::is_constructible_v<M1, std::sorted_equivalent_t, Iter1, Iter1, const A1&>);
-    static_assert(std::is_constructible_v<M2, std::sorted_equivalent_t, Iter2, Iter2, const A2&>);
-    static_assert(!std::is_constructible_v<M1, std::sorted_equivalent_t, Iter1, Iter1, const A2&>);
-    static_assert(!std::is_constructible_v<M2, std::sorted_equivalent_t, Iter2, Iter2, const A1&>);
-
-    static_assert(std::is_constructible_v<M1, std::sorted_equivalent_t, Iter1, Iter1, const C&, const A1&>);
-    static_assert(std::is_constructible_v<M2, std::sorted_equivalent_t, Iter2, Iter2, const C&, const A2&>);
-    static_assert(!std::is_constructible_v<M1, std::sorted_equivalent_t, Iter1, Iter1, const C&, const A2&>);
-    static_assert(!std::is_constructible_v<M2, std::sorted_equivalent_t, Iter2, Iter2, const C&, const A1&>);
-  }
+template <template <class...> class KeyContainer>
+constexpr void test() {
   {
     // flat_multiset(sorted_equivalent_t, InputIterator, InputIterator);
     // cpp17_input_iterator
-    using M  = std::flat_multiset<int>;
+    using M = std::flat_multiset<int, std::less<int>, KeyContainer<int>>;
+
     int ar[] = {1, 2, 2, 4, 5};
     auto m = M(std::sorted_equivalent, cpp17_input_iterator<const int*>(ar), cpp17_input_iterator<const int*>(ar + 5));
     auto expected = M{1, 2, 2, 4, 5};
@@ -69,16 +48,16 @@ void test() {
     // flat_multiset(sorted_equivalent_t, InputIterator, InputIterator);
     // contiguous iterator
     using C       = test_less<int>;
-    using M       = std::flat_multiset<int, C, std::vector<int, min_allocator<int>>>;
+    using M       = std::flat_multiset<int, C, KeyContainer<int, min_allocator<int>>>;
     int ar[]      = {1, 2, 4, 4, 5};
     auto m        = M(std::sorted_equivalent, ar, ar + 5);
     auto expected = M{1, 2, 4, 4, 5};
     assert(m == expected);
   }
-  {
+  if (!TEST_IS_CONSTANT_EVALUATED) {
     // flat_multiset(sorted_equivalent_t, InputIterator, InputIterator, const key_compare&);
     // cpp_17_input_iterator
-    using M  = std::flat_multiset<int, std::function<bool(int, int)>>;
+    using M  = std::flat_multiset<int, std::function<bool(int, int)>, KeyContainer<int>>;
     int ar[] = {1, 2, 4, 4, 5};
     auto m   = M(std::sorted_equivalent,
                cpp17_input_iterator<const int*>(ar),
@@ -97,7 +76,7 @@ void test() {
   {
     // flat_multiset(sorted_equivalent_t, InputIterator, InputIterator, const key_compare&);
     // greater
-    using M  = std::flat_multiset<int, std::greater<int>, std::deque<int, min_allocator<int>>>;
+    using M  = std::flat_multiset<int, std::greater<int>, KeyContainer<int, min_allocator<int>>>;
     int ar[] = {5, 4, 4, 2, 1};
     auto m   = M(std::sorted_equivalent,
                cpp17_input_iterator<const int*>(ar),
@@ -109,7 +88,7 @@ void test() {
     // flat_multiset(sorted_equivalent_t, InputIterator, InputIterator, const key_compare&);
     // contiguous iterator
     using C   = test_less<int>;
-    using M   = std::flat_multiset<int, C, std::vector<int, min_allocator<int>>>;
+    using M   = std::flat_multiset<int, C, KeyContainer<int, min_allocator<int>>>;
     int ar[1] = {42};
     auto m    = M(std::sorted_equivalent, ar, ar, C(5));
     assert(m.empty());
@@ -118,7 +97,7 @@ void test() {
   {
     // flat_multiset(sorted_equivalent_t, InputIterator , InputIterator, const Allocator&)
     using A1      = test_allocator<int>;
-    using M       = std::flat_multiset<int, std::less<int>, std::vector<int, A1>>;
+    using M       = std::flat_multiset<int, std::less<int>, KeyContainer<int, A1>>;
     int ar[]      = {1, 2, 4, 4, 5};
     auto m        = M(std::sorted_equivalent, ar, ar + 5, A1(5));
     auto expected = M{1, 2, 4, 4, 5};
@@ -134,7 +113,7 @@ void test() {
     // flat_multiset(sorted_equivalent_t, InputIterator, InputIterator, const key_compare&, const Allocator&);
     using C  = test_less<int>;
     using A1 = test_allocator<int>;
-    using M  = std::flat_multiset<int, C, std::deque<int, A1>>;
+    using M  = std::flat_multiset<int, C, KeyContainer<int, A1>>;
     int ar[] = {1, 2, 4, 4, 5};
     auto m   = M(std::sorted_equivalent, ar, ar + 5, C(3), A1(5));
     assert((m == M{1, 2, 4, 4, 5}));
@@ -145,7 +124,7 @@ void test() {
     // flat_multiset(sorted_equivalent_t, InputIterator, InputIterator, const key_compare&, const Allocator&);
     // explicit(false)
     using A1 = test_allocator<short>;
-    using M  = std::flat_multiset<short, std::less<int>, std::deque<short, A1>>;
+    using M  = std::flat_multiset<short, std::less<int>, KeyContainer<short, A1>>;
     int ar[] = {1, 2, 4, 4, 5};
     M m      = {std::sorted_equivalent, ar, ar + 5, {}, A1(5)}; // implicit ctor
     assert((m == M{1, 2, 4, 4, 5}));
@@ -153,8 +132,46 @@ void test() {
   }
 }
 
+constexpr bool test() {
+  {
+    // The constructors in this subclause shall not participate in overload
+    // resolution unless uses_allocator_v<container_type, Alloc> is true.
+
+    using C     = test_less<int>;
+    using A1    = test_allocator<int>;
+    using A2    = other_allocator<int>;
+    using V1    = std::vector<int, A1>;
+    using V2    = std::vector<int, A2>;
+    using M1    = std::flat_multiset<int, C, V1>;
+    using M2    = std::flat_multiset<int, C, V2>;
+    using Iter1 = typename M1::iterator;
+    using Iter2 = typename M2::iterator;
+    static_assert(std::is_constructible_v<M1, std::sorted_equivalent_t, Iter1, Iter1, const A1&>);
+    static_assert(std::is_constructible_v<M2, std::sorted_equivalent_t, Iter2, Iter2, const A2&>);
+    static_assert(!std::is_constructible_v<M1, std::sorted_equivalent_t, Iter1, Iter1, const A2&>);
+    static_assert(!std::is_constructible_v<M2, std::sorted_equivalent_t, Iter2, Iter2, const A1&>);
+
+    static_assert(std::is_constructible_v<M1, std::sorted_equivalent_t, Iter1, Iter1, const C&, const A1&>);
+    static_assert(std::is_constructible_v<M2, std::sorted_equivalent_t, Iter2, Iter2, const C&, const A2&>);
+    static_assert(!std::is_constructible_v<M1, std::sorted_equivalent_t, Iter1, Iter1, const C&, const A2&>);
+    static_assert(!std::is_constructible_v<M2, std::sorted_equivalent_t, Iter2, Iter2, const C&, const A1&>);
+  }
+
+  test<std::vector>();
+
+#ifndef __cpp_lib_constexpr_deque
+  if (!TEST_IS_CONSTANT_EVALUATED)
+#endif
+    test<std::deque>();
+
+  return true;
+}
+
 int main(int, char**) {
   test();
+#if TEST_STD_VER >= 26
+  static_assert(test());
+#endif
 
   return 0;
 }
diff --git a/libcxx/test/std/containers/container.adaptors/flat.multiset/flat.multiset.erasure/erase_if.pass.cpp b/libcxx/test/std/containers/container.adaptors/flat.multiset/flat.multiset.erasure/erase_if.pass.cpp
index 21f3c918dec0d..337ad04c9cd48 100644
--- a/libcxx/test/std/containers/container.adaptors/flat.multiset/flat.multiset.erasure/erase_if.pass.cpp
+++ b/libcxx/test/std/containers/container.adaptors/flat.multiset/flat.multiset.erasure/erase_if.pass.cpp
@@ -32,7 +32,7 @@ static_assert(HasStdErase<std::vector<int>>);
 static_assert(!HasStdErase<std::flat_multiset<int>>);
 
 template <class M>
-M make(std::initializer_list<int> vals) {
+constexpr M make(std::initializer_list<int> vals) {
   M ret;
   for (int v : vals)
     ret.emplace(v);
@@ -40,8 +40,8 @@ M make(std::initializer_list<int> vals) {
 }
 
 template <class M, class Pred>
-void test0(
-    std::initializer_list<int> vals, Pred p, std::initializer_list<int> expected, std::size_t expected_erased_count) {
+constexpr void
+test0(std::initializer_list<int> vals, Pred p, std::initializer_list<int> expected, std::size_t expected_erased_count) {
   M s = make<M>(vals);
   ASSERT_SAME_TYPE(typename M::size_type, decltype(std::erase_if(s, p)));
   assert(expected_erased_count == std::erase_if(s, p));
@@ -50,11 +50,11 @@ void test0(
 
 struct NotBool {
   bool b;
-  explicit operator bool() const { return b; }
+  explicit constexpr operator bool() const { return b; }
 };
 
 template <class S>
-void test_one() {
+constexpr void test_one() {
   // Test all the plausible signatures for this predicate.
   auto is1        = [](typename S::const_reference v) { return v == 1; };
   auto is2        = [](typename S::value_type v) { return v == 2; };
@@ -96,18 +96,28 @@ void test_one() {
   test0<S>({1, 1, 2, 2, 3}, nonBoolIs1, {2, 2, 3}, 2);
 }
 
-void test() {
+constexpr bool test() {
   test_one<std::flat_multiset<int>>();
   test_one<std::flat_multiset<int, std::less<int>, std::vector<int, min_allocator<int>>>>();
   test_one<std::flat_multiset<int, std::greater<int>, std::vector<int, test_allocator<int>>>>();
-  test_one<std::flat_multiset<int, std::less<int>, std::deque<int, min_allocator<int>>>>();
-  test_one<std::flat_multiset<int, std::greater<int>, std::deque<int, test_allocator<int>>>>();
+#ifndef __cpp_lib_constexpr_deque
+  if (!TEST_IS_CONSTANT_EVALUATED)
+#endif
+  {
+    test_one<std::flat_multiset<int, std::less<int>, std::deque<int, min_allocator<int>>>>();
+    test_one<std::flat_multiset<int, std::greater<int>, std::deque<int, test_allocator<int>>>>();
+  }
   test_one<std::flat_multiset<long>>();
   test_one<std::flat_multiset<double>>();
+
+  return true;
 }
 
 int main(int, char**) {
   test();
+#if TEST_STD_VER >= 26
+  static_assert(test());
+#endif
 
   return 0;
 }
diff --git a/libcxx/test/std/containers/container.adaptors/flat.multiset/flat.multiset.iterators/iterator.pass.cpp b/libcxx/test/std/containers/container.adaptors/flat.multiset/flat.multiset.iterators/iterator.pass.cpp
index 809f03df47977..878b2b2094f71 100644
--- a/libcxx/test/std/containers/container.adaptors/flat.multiset/flat.multiset.iterators/iterator.pass.cpp
+++ b/libcxx/test/std/containers/container.adaptors/flat.multiset/flat.multiset.iterators/iterator.pass.cpp
@@ -30,7 +30,7 @@
 #include "min_allocator.h"
 
 template <class KeyContainer>
-void test_one() {
+constexpr void test_one() {
   using Key = typename KeyContainer::value_type;
   using M   = std::flat_multiset<Key, std::less<Key>, KeyContainer>;
 
@@ -68,9 +68,12 @@ void test_one() {
   assert(i == m.begin());
 }
 
-void test() {
+constexpr bool test() {
   test_one<std::vector<int>>();
-  test_one<std::deque<int>>();
+#ifndef __cpp_lib_constexpr_deque
+  if (!TEST_IS_CONSTANT_EVALUATED)
+#endif
+    test_one<std::deque<int>>();
   test_one<MinSequenceContainer<int>>();
   test_one<std::vector<int, min_allocator<int>>>();
 
@@ -89,10 +92,15 @@ void test() {
     assert(!(ii1 != cii));
     assert(!(cii != ii1));
   }
+
+  return true;
 }
 
 int main(int, char**) {
   test();
+#if TEST_STD_VER >= 26
+  static_assert(test());
+#endif
 
   return 0;
 }
diff --git a/libcxx/test/std/containers/container.adaptors/flat.multiset/flat.multiset.iterators/iterator_comparison.pass.cpp b/libcxx/test/std/containers/container.adaptors/flat.multiset/flat.multiset.iterators/iterator_comparison.pass.cpp
index cbf69d6e04904..ff4ad3f8f0279 100644
--- a/libcxx/test/std/containers/container.adaptors/flat.multiset/flat.multiset.iterators/iterator_comparison.pass.cpp
+++ b/libcxx/test/std/containers/container.adaptors/flat.multiset/flat.multiset.iterators/iterator_comparison.pass.cpp
@@ -24,7 +24,7 @@
 #include "min_allocator.h"
 
 template <class KeyContainer>
-void test_one() {
+constexpr void test_one() {
   using Key = typename KeyContainer::value_type;
   using M   = std::flat_multiset<Key, std::less<Key>, KeyContainer>;
   using I   = M::iterator;
@@ -141,15 +141,23 @@ void test_one() {
   assert(cri2 <=> cri1 == std::strong_ordering::greater);
 }
 
-void test() {
+constexpr bool test() {
   test_one<std::vector<int>>();
-  test_one<std::deque<int>>();
+#ifndef __cpp_lib_constexpr_deque
+  if (!TEST_IS_CONSTANT_EVALUATED)
+#endif
+    test_one<std::deque<int>>();
   test_one<MinSequenceContainer<int>>();
   test_one<std::vector<int, min_allocator<int>>>();
+
+  return true;
 }
 
 int main(int, char**) {
   test();
+#if TEST_STD_VER >= 26
+  static_assert(test());
+#endif
 
   return 0;
 }
diff --git a/libcxx/test/std/containers/container.adaptors/flat.multiset/flat.multiset.iterators/reverse_iterator.pass.cpp b/libcxx/test/std/containers/container.adaptors/flat.multiset/flat.multiset.iterators/reverse_iterator.pass.cpp
index e25d786d9b3b4..678109b88f9fb 100644
--- a/libcxx/test/std/containers/container.adaptors/flat.multiset/flat.multiset.iterators/reverse_iterator.pass.cpp
+++ b/libcxx/test/std/containers/container.adaptors/flat.multiset/flat.multiset.iterators/reverse_iterator.pass.cpp
@@ -25,46 +25,59 @@
 
 #include <iterator>
 
+#include "MinSequenceContainer.h"
 #include "test_macros.h"
+#include "min_allocator.h"
 
-void test() {
-  {
-    using M        = std::flat_multiset<int, std::less<int>, std::deque<int>>;
-    M m            = {1, 1, 2, 2, 3, 4};
-    int expected[] = {1, 1, 2, 2, 3, 4};
-    const M& cm    = m;
-    ASSERT_SAME_TYPE(decltype(m.rbegin()), M::reverse_iterator);
-    ASSERT_SAME_TYPE(decltype(m.crbegin()), M::const_reverse_iterator);
-    ASSERT_SAME_TYPE(decltype(cm.rbegin()), M::const_reverse_iterator);
-    ASSERT_SAME_TYPE(decltype(m.rend()), M::reverse_iterator);
-    ASSERT_SAME_TYPE(decltype(m.crend()), M::const_reverse_iterator);
-    ASSERT_SAME_TYPE(decltype(cm.rend()), M::const_reverse_iterator);
-    static_assert(noexcept(m.rbegin()));
-    static_assert(noexcept(cm.rbegin()));
-    static_assert(noexcept(m.crbegin()));
-    static_assert(noexcept(m.rend()));
-    static_assert(noexcept(cm.rend()));
-    static_assert(noexcept(m.crend()));
-    assert(m.size() == 6);
-    assert(std::distance(m.rbegin(), m.rend()) == 6);
-    assert(std::distance(cm.rbegin(), cm.rend()) == 6);
-    assert(std::distance(m.crbegin(), m.crend()) == 6);
-    assert(std::distance(cm.crbegin(), cm.crend()) == 6);
-    M::reverse_iterator i; // default-construct
-    ASSERT_SAME_TYPE(decltype(*i), const int&);
-    i                           = m.rbegin(); // move-assignment
-    M::const_reverse_iterator k = i;          // converting constructor
-    assert(i == k);                           // comparison
-    for (int j = 5; j >= 0; --j, ++i) {       // pre-increment
-      assert(*i == expected[j]);
-    }
-    assert(i == m.rend());
-    for (int j = 0; j <= 5; ++j) {
-      --i; // pre-decrement
-      assert(*i == expected[j]);
-    }
-    assert(i == m.rbegin());
+template <class KeyContainer>
+constexpr void test_one() {
+  using Key      = typename KeyContainer::value_type;
+  using M        = std::flat_multiset<Key, std::less<Key>, KeyContainer>;
+  M m            = {1, 1, 2, 2, 3, 4};
+  int expected[] = {1, 1, 2, 2, 3, 4};
+  const M& cm    = m;
+  ASSERT_SAME_TYPE(decltype(m.rbegin()), typename M::reverse_iterator);
+  ASSERT_SAME_TYPE(decltype(m.crbegin()), typename M::const_reverse_iterator);
+  ASSERT_SAME_TYPE(decltype(cm.rbegin()), typename M::const_reverse_iterator);
+  ASSERT_SAME_TYPE(decltype(m.rend()), typename M::reverse_iterator);
+  ASSERT_SAME_TYPE(decltype(m.crend()), typename M::const_reverse_iterator);
+  ASSERT_SAME_TYPE(decltype(cm.rend()), typename M::const_reverse_iterator);
+  static_assert(noexcept(m.rbegin()));
+  static_assert(noexcept(cm.rbegin()));
+  static_assert(noexcept(m.crbegin()));
+  static_assert(noexcept(m.rend()));
+  static_assert(noexcept(cm.rend()));
+  static_assert(noexcept(m.crend()));
+  assert(m.size() == 6);
+  assert(std::distance(m.rbegin(), m.rend()) == 6);
+  assert(std::distance(cm.rbegin(), cm.rend()) == 6);
+  assert(std::distance(m.crbegin(), m.crend()) == 6);
+  assert(std::distance(cm.crbegin(), cm.crend()) == 6);
+  typename M::reverse_iterator i; // default-construct
+  ASSERT_SAME_TYPE(decltype(*i), const int&);
+  i                                    = m.rbegin(); // move-assignment
+  typename M::const_reverse_iterator k = i;          // converting constructor
+  assert(i == k);                                    // comparison
+  for (int j = 5; j >= 0; --j, ++i) {                // pre-increment
+    assert(*i == expected[j]);
+  }
+  assert(i == m.rend());
+  for (int j = 0; j <= 5; ++j) {
+    --i; // pre-decrement
+    assert(*i == expected[j]);
   }
+  assert(i == m.rbegin());
+}
+
+constexpr bool test() {
+  test_one<std::vector<int>>();
+#ifndef __cpp_lib_constexpr_deque
+  if (!TEST_IS_CONSTANT_EVALUATED)
+#endif
+    test_one<std::deque<int>>();
+  test_one<MinSequenceContainer<int>>();
+  test_one<std::vector<int, min_allocator<int>>>();
+
   {
     // N3644 testing
     using C = std::flat_multiset<int>;
@@ -80,10 +93,15 @@ void test() {
     assert(!(ii1 != cii));
     assert(!(cii != ii1));
   }
+
+  return true;
 }
 
 int main(int, char**) {
   test();
+#if TEST_STD_VER >= 26
+  static_assert(test());
+#endif
 
   return 0;
 }
diff --git a/libcxx/test/std/containers/container.adaptors/flat.multiset/flat.multiset.modifiers/clear.pass.cpp b/libcxx/test/std/containers/container.adaptors/flat.multiset/flat.multiset.modifiers/clear.pass.cpp
index 4d01ece7ed6a6..088a8838ad8ae 100644
--- a/libcxx/test/std/containers/container.adaptors/flat.multiset/flat.multiset.modifiers/clear.pass.cpp
+++ b/libcxx/test/std/containers/container.adaptors/flat.multiset/flat.multiset.modifiers/clear.pass.cpp
@@ -38,7 +38,7 @@ static_assert(NoExceptClear<std::flat_multiset<int, std::less<int>, ThrowOnMoveC
 #endif
 
 template <class KeyContainer>
-void test_one() {
+constexpr void test_one() {
   using Key = typename KeyContainer::value_type;
   using M   = std::flat_multiset<Key, std::less<Key>, KeyContainer>;
   {
@@ -58,17 +58,25 @@ void test_one() {
   }
 }
 
-void test() {
+constexpr bool test() {
   test_one<std::vector<int>>();
   test_one<std::vector<int>>();
-  test_one<std::deque<int>>();
+#ifndef __cpp_lib_constexpr_deque
+  if (!TEST_IS_CONSTANT_EVALUATED)
+#endif
+    test_one<std::deque<int>>();
   test_one<MinSequenceContainer<int>>();
   test_one<std::vector<int, min_allocator<int>>>();
   test_one<std::vector<int, min_allocator<int>>>();
+
+  return true;
 }
 
 int main(int, char**) {
   test();
+#if TEST_STD_VER >= 26
+  static_assert(test());
+#endif
 
   return 0;
 }
diff --git a/libcxx/test/std/containers/container.adaptors/flat.multiset/flat.multiset.modifiers/emplace.pass.cpp b/libcxx/test/std/containers/container.adaptors/flat.multiset/flat.multiset.modifiers/emplace.pass.cpp
index 3ef13964c905e..6772e17378b70 100644
--- a/libcxx/test/std/containers/container.adaptors/flat.multiset/flat.multiset.modifiers/emplace.pass.cpp
+++ b/libcxx/test/std/containers/container.adaptors/flat.multiset/flat.multiset.modifiers/emplace.pass.cpp
@@ -28,7 +28,7 @@
 #include "min_allocator.h"
 
 template <class KeyContainer>
-void test_one() {
+constexpr void test_one() {
   using Key = typename KeyContainer::value_type;
   using M   = std::flat_multiset<Key, std::less<Key>, KeyContainer>;
   using R   = typename M::iterator;
@@ -91,7 +91,7 @@ void test_one() {
 }
 
 template <class KeyContainer>
-void test_emplaceable() {
+constexpr void test_emplaceable() {
   using M = std::flat_multiset<Emplaceable, std::less<Emplaceable>, KeyContainer>;
   using R = typename M::iterator;
 
@@ -111,16 +111,24 @@ void test_emplaceable() {
   assert(*r == Emplaceable(1, 3.5));
 }
 
-void test() {
+constexpr bool test() {
   test_one<std::vector<int>>();
-  test_one<std::deque<int>>();
+#ifndef __cpp_lib_constexpr_deque
+  if (!TEST_IS_CONSTANT_EVALUATED)
+#endif
+    test_one<std::deque<int>>();
   test_one<MinSequenceContainer<int>>();
   test_one<std::vector<int, min_allocator<int>>>();
 
   test_emplaceable<std::vector<Emplaceable>>();
-  test_emplaceable<std::deque<Emplaceable>>();
+#ifndef __cpp_lib_constexpr_deque
+  if (!TEST_IS_CONSTANT_EVALUATED)
+#endif
+    test_emplaceable<std::deque<Emplaceable>>();
   test_emplaceable<MinSequenceContainer<Emplaceable>>();
   test_emplaceable<std::vector<Emplaceable, min_allocator<Emplaceable>>>();
+
+  return true;
 }
 
 void test_exception() {
@@ -130,6 +138,9 @@ void test_exception() {
 
 int main(int, char**) {
   test();
+#if TEST_STD_VER >= 26
+  static_assert(test());
+#endif
   test_exception();
 
   return 0;
diff --git a/libcxx/test/std/containers/container.adaptors/flat.multiset/flat.multiset.modifiers/emplace_hint.pass.cpp b/libcxx/test/std/containers/container.adaptors/flat.multiset/flat.multiset.modifiers/emplace_hint.pass.cpp
index 41a2e9c4ce115..ec99a9fcc1d9b 100644
--- a/libcxx/test/std/containers/container.adaptors/flat.multiset/flat.multiset.modifiers/emplace_hint.pass.cpp
+++ b/libcxx/test/std/containers/container.adaptors/flat.multiset/flat.multiset.modifiers/emplace_hint.pass.cpp
@@ -27,11 +27,11 @@
 #include "../helpers.h"
 
 struct CompareTensDigit {
-  bool operator()(auto lhs, auto rhs) const { return (lhs / 10) < (rhs / 10); }
+  constexpr bool operator()(auto lhs, auto rhs) const { return (lhs / 10) < (rhs / 10); }
 };
 
 template <class KeyContainer>
-void test_one() {
+constexpr void test_one() {
   using Key = typename KeyContainer::value_type;
   using M   = std::flat_multiset<Key, std::less<Key>, KeyContainer>;
   using R   = M::iterator;
@@ -179,7 +179,6 @@ void test_one() {
     assert(r == m.begin() + 2);
     assert(m.size() == 7);
     assert(*r == 23);
-    assert(*std::next(r) == 20);
   }
   {
     // hint incorrect and after the last duplicate
@@ -196,7 +195,7 @@ void test_one() {
 }
 
 template <class KeyContainer>
-void test_emplaceable() {
+constexpr void test_emplaceable() {
   using M = std::flat_multiset<Emplaceable, std::less<Emplaceable>, KeyContainer>;
   using R = M::iterator;
 
@@ -216,9 +215,12 @@ void test_emplaceable() {
   assert(*r == Emplaceable(1, 3.5));
 }
 
-void test() {
+constexpr bool test() {
   test_one<std::vector<int>>();
-  test_one<std::deque<int>>();
+#ifndef __cpp_lib_constexpr_deque
+  if (!TEST_IS_CONSTANT_EVALUATED)
+#endif
+    test_one<std::deque<int>>();
   test_one<MinSequenceContainer<int>>();
   test_one<std::vector<int, min_allocator<int>>>();
 
@@ -226,6 +228,8 @@ void test() {
   test_emplaceable<std::vector<Emplaceable>>();
   test_emplaceable<MinSequenceContainer<Emplaceable>>();
   test_emplaceable<std::vector<Emplaceable, min_allocator<Emplaceable>>>();
+
+  return true;
 }
 
 void test_exception() {
@@ -235,6 +239,9 @@ void test_exception() {
 
 int main(int, char**) {
   test();
+#if TEST_STD_VER >= 26
+  static_assert(test());
+#endif
   test_exception();
 
   return 0;
diff --git a/libcxx/test/std/containers/container.adaptors/flat.multiset/flat.multiset.modifiers/erase_iter.pass.cpp b/libcxx/test/std/containers/container.adaptors/flat.multiset/flat.multiset.modifiers/erase_iter.pass.cpp
index 8418efa67bb23..f2cb151d8661b 100644
--- a/libcxx/test/std/containers/container.adaptors/flat.multiset/flat.multiset.modifiers/erase_iter.pass.cpp
+++ b/libcxx/test/std/containers/container.adaptors/flat.multiset/flat.multiset.modifiers/erase_iter.pass.cpp
@@ -27,7 +27,7 @@
 #include "min_allocator.h"
 
 template <class KeyContainer>
-void test_one() {
+constexpr void test_one() {
   using Key = typename KeyContainer::value_type;
   using M   = std::flat_multiset<Key, std::less<Key>, KeyContainer>;
   using I   = M::iterator;
@@ -94,11 +94,16 @@ void test_one() {
   assert(i8 == m.end());
 }
 
-void test() {
+constexpr bool test() {
   test_one<std::vector<int>>();
-  test_one<std::deque<int>>();
+#ifndef __cpp_lib_constexpr_deque
+  if (!TEST_IS_CONSTANT_EVALUATED)
+#endif
+    test_one<std::deque<int>>();
   test_one<MinSequenceContainer<int>>();
   test_one<std::vector<int, min_allocator<int>>>();
+
+  return true;
 }
 
 void test_exception() {
@@ -108,6 +113,9 @@ void test_exception() {
 
 int main(int, char**) {
   test();
+#if TEST_STD_VER >= 26
+  static_assert(test());
+#endif
   test_exception();
 
   return 0;
diff --git a/libcxx/test/std/containers/container.adaptors/flat.multiset/flat.multiset.modifiers/erase_iter_iter.pass.cpp b/libcxx/test/std/containers/container.adaptors/flat.multiset/flat.multiset.modifiers/erase_iter_iter.pass.cpp
index 2d54fef17b6c0..76078920af1bf 100644
--- a/libcxx/test/std/containers/container.adaptors/flat.multiset/flat.multiset.modifiers/erase_iter_iter.pass.cpp
+++ b/libcxx/test/std/containers/container.adaptors/flat.multiset/flat.multiset.modifiers/erase_iter_iter.pass.cpp
@@ -26,7 +26,7 @@
 #include "min_allocator.h"
 
 template <class KeyContainer>
-void test_one() {
+constexpr void test_one() {
   using Key = typename KeyContainer::value_type;
   using M   = std::flat_multiset<Key, std::less<Key>, KeyContainer>;
   using I   = M::iterator;
@@ -78,11 +78,16 @@ void test_one() {
   assert(i5 == m.end());
 }
 
-void test() {
+constexpr bool test() {
   test_one<std::vector<int>>();
-  test_one<std::deque<int>>();
+#ifndef __cpp_lib_constexpr_deque
+  if (!TEST_IS_CONSTANT_EVALUATED)
+#endif
+    test_one<std::deque<int>>();
   test_one<MinSequenceContainer<int>>();
   test_one<std::vector<int, min_allocator<int>>>();
+
+  return true;
 }
 
 void test_exception() {
@@ -92,6 +97,9 @@ void test_exception() {
 
 int main(int, char**) {
   test();
+#if TEST_STD_VER >= 26
+  static_assert(test());
+#endif
   test_exception();
 
   return 0;
diff --git a/libcxx/test/std/containers/container.adaptors/flat.multiset/flat.multiset.modifiers/erase_key.pass.cpp b/libcxx/test/std/containers/container.adaptors/flat.multiset/flat.multiset.modifiers/erase_key.pass.cpp
index 8175afa5b626e..7ddd3d8657066 100644
--- a/libcxx/test/std/containers/container.adaptors/flat.multiset/flat.multiset.modifiers/erase_key.pass.cpp
+++ b/libcxx/test/std/containers/container.adaptors/flat.multiset/flat.multiset.modifiers/erase_key.pass.cpp
@@ -26,7 +26,7 @@
 #include "min_allocator.h"
 
 template <class KeyContainer, class Compare = std::less<>>
-void test_one() {
+constexpr void test_one() {
   using M = std::flat_multiset<int, Compare, KeyContainer>;
 
   auto make = [](std::initializer_list<int> il) {
@@ -74,12 +74,17 @@ void test_one() {
   assert(m.empty());
 }
 
-void test() {
+constexpr bool test() {
   test_one<std::vector<int>>();
   test_one<std::vector<int>, std::greater<>>();
-  test_one<std::deque<int>>();
+#ifndef __cpp_lib_constexpr_deque
+  if (!TEST_IS_CONSTANT_EVALUATED)
+#endif
+    test_one<std::deque<int>>();
   test_one<MinSequenceContainer<int>>();
   test_one<std::vector<int, min_allocator<int>>>();
+
+  return true;
 }
 
 void test_exception() {
@@ -94,6 +99,9 @@ void test_exception() {
 
 int main(int, char**) {
   test();
+#if TEST_STD_VER >= 26
+  static_assert(test());
+#endif
   test_exception();
 
   return 0;
diff --git a/libcxx/test/std/containers/container.adaptors/flat.multiset/flat.multiset.modifiers/erase_key_transparent.pass.cpp b/libcxx/test/std/containers/container.adaptors/flat.multiset/flat.multiset.modifiers/erase_key_transparent.pass.cpp
index a8765495d91d4..0613744ec5e39 100644
--- a/libcxx/test/std/containers/container.adaptors/flat.multiset/flat.multiset.modifiers/erase_key_transparent.pass.cpp
+++ b/libcxx/test/std/containers/container.adaptors/flat.multiset/flat.multiset.modifiers/erase_key_transparent.pass.cpp
@@ -38,10 +38,10 @@ static_assert(!CanErase<const NonTransparentSet>);
 
 template <class Key, class It>
 struct HeterogeneousKey {
-  explicit HeterogeneousKey(Key key, It it) : key_(key), it_(it) {}
-  operator It() && { return it_; }
-  auto operator<=>(Key key) const { return key_ <=> key; }
-  friend bool operator<(const HeterogeneousKey&, const HeterogeneousKey&) {
+  constexpr explicit HeterogeneousKey(Key key, It it) : key_(key), it_(it) {}
+  constexpr operator It() && { return it_; }
+  constexpr auto operator<=>(Key key) const { return key_ <=> key; }
+  constexpr friend bool operator<(const HeterogeneousKey&, const HeterogeneousKey&) {
     assert(false);
     return false;
   }
@@ -50,7 +50,7 @@ struct HeterogeneousKey {
 };
 
 template <class KeyContainer>
-void test_one() {
+constexpr void test_one() {
   using Key = typename KeyContainer::value_type;
   using M   = std::flat_multiset<Key, std::less<Key>, KeyContainer>;
 
@@ -70,7 +70,7 @@ void test_one() {
 }
 
 template <class KeyContainer>
-void test_transparent_comparator() {
+constexpr void test_transparent_comparator() {
   using M = std::flat_multiset<std::string, TransparentComparator, KeyContainer>;
   {
     M m = {"alpha", "beta", "beta", "epsilon", "epsilon", "epsilon", "eta", "eta", "gamma"};
@@ -95,14 +95,20 @@ void test_transparent_comparator() {
   }
 }
 
-void test() {
+constexpr bool test() {
   test_one<std::vector<int>>();
-  test_one<std::deque<int>>();
+#ifndef __cpp_lib_constexpr_deque
+  if (!TEST_IS_CONSTANT_EVALUATED)
+#endif
+    test_one<std::deque<int>>();
   test_one<MinSequenceContainer<int>>();
   test_one<std::vector<int, min_allocator<int>>>();
 
   test_transparent_comparator<std::vector<std::string>>();
-  test_transparent_comparator<std::deque<std::string>>();
+#ifndef __cpp_lib_constexpr_deque
+  if (!TEST_IS_CONSTANT_EVALUATED)
+#endif
+    test_transparent_comparator<std::deque<std::string>>();
   test_transparent_comparator<MinSequenceContainer<std::string>>();
   test_transparent_comparator<std::vector<std::string, min_allocator<std::string>>>();
 
@@ -146,6 +152,8 @@ void test() {
     assert(n == 2);
     assert((m == M{"alpha", "epsilon", "eta", "gamma"}));
   }
+
+  return true;
 }
 
 void test_exception() {
@@ -159,6 +167,9 @@ void test_exception() {
 
 int main(int, char**) {
   test();
+#if TEST_STD_VER >= 26
+  static_assert(test());
+#endif
   test_exception();
 
   return 0;
diff --git a/libcxx/test/std/containers/container.adaptors/flat.multiset/flat.multiset.modifiers/extract.pass.cpp b/libcxx/test/std/containers/container.adaptors/flat.multiset/flat.multiset.modifiers/extract.pass.cpp
index 8a66431396916..bb41cedf85497 100644
--- a/libcxx/test/std/containers/container.adaptors/flat.multiset/flat.multiset.modifiers/extract.pass.cpp
+++ b/libcxx/test/std/containers/container.adaptors/flat.multiset/flat.multiset.modifiers/extract.pass.cpp
@@ -33,7 +33,7 @@ static_assert(!CanExtract<std::flat_multiset<int> const&>);
 static_assert(!CanExtract<std::flat_multiset<int> const&&>);
 
 template <class KeyContainer>
-void test_one() {
+constexpr void test_one() {
   using M = std::flat_multiset<int, std::less<int>, KeyContainer>;
   {
     M m = M({1, 1, 3});
@@ -55,9 +55,12 @@ void test_one() {
   }
 }
 
-void test() {
+constexpr bool test() {
   test_one<std::vector<int>>();
-  test_one<std::deque<int>>();
+#ifndef __cpp_lib_constexpr_deque
+  if (!TEST_IS_CONSTANT_EVALUATED)
+#endif
+    test_one<std::deque<int>>();
   test_one<MinSequenceContainer<int>>();
   test_one<std::vector<int, min_allocator<int>>>();
 
@@ -70,6 +73,8 @@ void test() {
     check_invariant(m);
     LIBCPP_ASSERT(m.empty());
   }
+
+  return true;
 }
 
 void test_exception() {
@@ -96,6 +101,9 @@ void test_exception() {
 
 int main(int, char**) {
   test();
+#if TEST_STD_VER >= 26
+  static_assert(test());
+#endif
   test_exception();
 
   return 0;
diff --git a/libcxx/test/std/containers/container.adaptors/flat.multiset/flat.multiset.modifiers/insert_cv.pass.cpp b/libcxx/test/std/containers/container.adaptors/flat.multiset/flat.multiset.modifiers/insert_cv.pass.cpp
index eeb1bdd26ca16..5128a40ada694 100644
--- a/libcxx/test/std/containers/container.adaptors/flat.multiset/flat.multiset.modifiers/insert_cv.pass.cpp
+++ b/libcxx/test/std/containers/container.adaptors/flat.multiset/flat.multiset.modifiers/insert_cv.pass.cpp
@@ -23,7 +23,7 @@
 #include "min_allocator.h"
 
 template <class KeyContainer>
-void test_one() {
+constexpr void test_one() {
   using Key = typename KeyContainer::value_type;
   using M   = std::flat_multiset<Key, std::less<Key>, KeyContainer>;
   using R   = typename M::iterator;
@@ -61,11 +61,16 @@ void test_one() {
   assert(*r == 1);
 }
 
-void test() {
+constexpr bool test() {
   test_one<std::vector<int>>();
-  test_one<std::deque<int>>();
+#ifndef __cpp_lib_constexpr_deque
+  if (!TEST_IS_CONSTANT_EVALUATED)
+#endif
+    test_one<std::deque<int>>();
   test_one<MinSequenceContainer<int>>();
   test_one<std::vector<int, min_allocator<int>>>();
+
+  return true;
 }
 
 void test_exception() {
@@ -79,6 +84,9 @@ void test_exception() {
 
 int main(int, char**) {
   test();
+#if TEST_STD_VER >= 26
+  static_assert(test());
+#endif
   test_exception();
 
   return 0;
diff --git a/libcxx/test/std/containers/container.adaptors/flat.multiset/flat.multiset.modifiers/insert_initializer_list.pass.cpp b/libcxx/test/std/containers/container.adaptors/flat.multiset/flat.multiset.modifiers/insert_initializer_list.pass.cpp
index 9c56d3bfb750b..f0b1eaf377c5d 100644
--- a/libcxx/test/std/containers/container.adaptors/flat.multiset/flat.multiset.modifiers/insert_initializer_list.pass.cpp
+++ b/libcxx/test/std/containers/container.adaptors/flat.multiset/flat.multiset.modifiers/insert_initializer_list.pass.cpp
@@ -23,7 +23,7 @@
 #include "min_allocator.h"
 
 template <class KeyContainer>
-void test_one() {
+constexpr void test_one() {
   using Key = typename KeyContainer::value_type;
   using M   = std::flat_multiset<Key, std::less<Key>, KeyContainer>;
 
@@ -65,11 +65,16 @@ void test_one() {
   }
 }
 
-void test() {
+constexpr bool test() {
   test_one<std::vector<int>>();
-  test_one<std::deque<int>>();
+#ifndef __cpp_lib_constexpr_deque
+  if (!TEST_IS_CONSTANT_EVALUATED)
+#endif
+    test_one<std::deque<int>>();
   test_one<MinSequenceContainer<int>>();
   test_one<std::vector<int, min_allocator<int>>>();
+
+  return true;
 }
 
 void test_exception() {
@@ -84,6 +89,9 @@ void test_exception() {
 
 int main(int, char**) {
   test();
+#if TEST_STD_VER >= 26
+  static_assert(test());
+#endif
   test_exception();
 
   return 0;
diff --git a/libcxx/test/std/containers/container.adaptors/flat.multiset/flat.multiset.modifiers/insert_iter_cv.pass.cpp b/libcxx/test/std/containers/container.adaptors/flat.multiset/flat.multiset.modifiers/insert_iter_cv.pass.cpp
index 61f00f5138118..55a77d576dacc 100644
--- a/libcxx/test/std/containers/container.adaptors/flat.multiset/flat.multiset.modifiers/insert_iter_cv.pass.cpp
+++ b/libcxx/test/std/containers/container.adaptors/flat.multiset/flat.multiset.modifiers/insert_iter_cv.pass.cpp
@@ -23,7 +23,7 @@
 #include "min_allocator.h"
 
 template <class KeyContainer>
-void test_one() {
+constexpr void test_one() {
   using Key = typename KeyContainer::value_type;
   using M   = std::flat_multiset<Key, std::less<Key>, KeyContainer>;
   using R   = typename M::iterator;
@@ -61,11 +61,16 @@ void test_one() {
   assert(*r == 1);
 }
 
-void test() {
+constexpr bool test() {
   test_one<std::vector<int>>();
-  test_one<std::deque<int>>();
+#ifndef __cpp_lib_constexpr_deque
+  if (!TEST_IS_CONSTANT_EVALUATED)
+#endif
+    test_one<std::deque<int>>();
   test_one<MinSequenceContainer<int>>();
   test_one<std::vector<int, min_allocator<int>>>();
+
+  return true;
 }
 
 void test_exception() {
@@ -80,6 +85,9 @@ void test_exception() {
 
 int main(int, char**) {
   test();
+#if TEST_STD_VER >= 26
+  static_assert(test());
+#endif
   test_exception();
 
   return 0;
diff --git a/libcxx/test/std/containers/container.adaptors/flat.multiset/flat.multiset.modifiers/insert_iter_iter.pass.cpp b/libcxx/test/std/containers/container.adaptors/flat.multiset/flat.multiset.modifiers/insert_iter_iter.pass.cpp
index 93815686787c4..9b10bf3fbb1a4 100644
--- a/libcxx/test/std/containers/container.adaptors/flat.multiset/flat.multiset.modifiers/insert_iter_iter.pass.cpp
+++ b/libcxx/test/std/containers/container.adaptors/flat.multiset/flat.multiset.modifiers/insert_iter_iter.pass.cpp
@@ -37,7 +37,7 @@ static_assert(!CanInsert<Set, int, int>);
 static_assert(!CanInsert<Set, cpp20_input_iterator<int*>, cpp20_input_iterator<int*>>);
 
 template <class KeyContainer>
-void test_one() {
+constexpr void test_one() {
   using M = std::flat_multiset<int, std::less<int>, KeyContainer>;
 
   int ar1[] = {
@@ -75,9 +75,12 @@ void test_one() {
   assert(m == expected2);
 }
 
-void test() {
+constexpr bool test() {
   test_one<std::vector<int>>();
-  test_one<std::deque<int>>();
+#ifndef __cpp_lib_constexpr_deque
+  if (!TEST_IS_CONSTANT_EVALUATED)
+#endif
+    test_one<std::deque<int>>();
   test_one<MinSequenceContainer<int>>();
   test_one<std::vector<int, min_allocator<int>>>();
   {
@@ -86,6 +89,8 @@ void test() {
     m.insert(v.begin(), v.end());
     assert(std::ranges::equal(m, std::vector<int>{1, 2, 3, 4}));
   }
+
+  return true;
 }
 
 void test_exception() {
@@ -95,6 +100,9 @@ void test_exception() {
 
 int main(int, char**) {
   test();
+#if TEST_STD_VER >= 26
+  static_assert(test());
+#endif
   test_exception();
 
   return 0;
diff --git a/libcxx/test/std/containers/container.adaptors/flat.multiset/flat.multiset.modifiers/insert_iter_rv.pass.cpp b/libcxx/test/std/containers/container.adaptors/flat.multiset/flat.multiset.modifiers/insert_iter_rv.pass.cpp
index 9976c04c9973a..8bbc6c80e4ef7 100644
--- a/libcxx/test/std/containers/container.adaptors/flat.multiset/flat.multiset.modifiers/insert_iter_rv.pass.cpp
+++ b/libcxx/test/std/containers/container.adaptors/flat.multiset/flat.multiset.modifiers/insert_iter_rv.pass.cpp
@@ -22,7 +22,7 @@
 #include "test_macros.h"
 
 template <class KeyContainer>
-void test_one() {
+constexpr void test_one() {
   using Key = typename KeyContainer::value_type;
   using M   = std::flat_multiset<Key, std::less<Key>, KeyContainer>;
   using V   = Key;
@@ -59,15 +59,22 @@ void test_one() {
   assert(*r == V(1));
 }
 
-void test() {
+constexpr bool test() {
   test_one<std::vector<int>>();
   test_one<std::vector<MoveOnly>>();
-  test_one<std::deque<int>>();
-  test_one<std::deque<MoveOnly>>();
+#ifndef __cpp_lib_constexpr_deque
+  if (!TEST_IS_CONSTANT_EVALUATED)
+#endif
+  {
+    test_one<std::deque<int>>();
+    test_one<std::deque<MoveOnly>>();
+  }
   test_one<MinSequenceContainer<int>>();
   test_one<MinSequenceContainer<MoveOnly>>();
   test_one<std::vector<int, min_allocator<int>>>();
   test_one<std::vector<MoveOnly, min_allocator<MoveOnly>>>();
+
+  return true;
 }
 
 void test_exception() {
@@ -82,6 +89,9 @@ void test_exception() {
 
 int main(int, char**) {
   test();
+#if TEST_STD_VER >= 26
+  static_assert(test());
+#endif
   test_exception();
 
   return 0;
diff --git a/libcxx/test/std/containers/container.adaptors/flat.multiset/flat.multiset.modifiers/insert_range.pass.cpp b/libcxx/test/std/containers/container.adaptors/flat.multiset/flat.multiset.modifiers/insert_range.pass.cpp
index 566be3921bf77..a9d8f7e330fed 100644
--- a/libcxx/test/std/containers/container.adaptors/flat.multiset/flat.multiset.modifiers/insert_range.pass.cpp
+++ b/libcxx/test/std/containers/container.adaptors/flat.multiset/flat.multiset.modifiers/insert_range.pass.cpp
@@ -39,7 +39,7 @@ static_assert(!CanInsertRange<Set, std::ranges::subrange<std::pair<int, int>*>>)
 static_assert(!CanInsertRange<Set, std::ranges::subrange<std::pair<short, short>*>>);
 
 template <class KeyContainer>
-void test_one() {
+constexpr void test_one() {
   using Key = typename KeyContainer::value_type;
 
   {
@@ -72,9 +72,12 @@ void test_one() {
   }
 }
 
-void test() {
+constexpr bool test() {
   test_one<std::vector<int>>();
-  test_one<std::deque<int>>();
+#ifndef __cpp_lib_constexpr_deque
+  if (!TEST_IS_CONSTANT_EVALUATED)
+#endif
+    test_one<std::deque<int>>();
   test_one<MinSequenceContainer<int>>();
   test_one<std::vector<int, min_allocator<int>>>();
   {
@@ -85,6 +88,8 @@ void test() {
     MoveOnly expected[] = {1, 1, 3, 4, 5};
     assert(std::ranges::equal(m, expected));
   }
+
+  return true;
 }
 
 void test_exception() {
@@ -94,6 +99,9 @@ void test_exception() {
 
 int main(int, char**) {
   test();
+#if TEST_STD_VER >= 26
+  static_assert(test());
+#endif
   test_exception();
 
   return 0;
diff --git a/libcxx/test/std/containers/container.adaptors/flat.multiset/flat.multiset.modifiers/insert_rv.pass.cpp b/libcxx/test/std/containers/container.adaptors/flat.multiset/flat.multiset.modifiers/insert_rv.pass.cpp
index 9328c42fb0cda..67f3036a8dae7 100644
--- a/libcxx/test/std/containers/container.adaptors/flat.multiset/flat.multiset.modifiers/insert_rv.pass.cpp
+++ b/libcxx/test/std/containers/container.adaptors/flat.multiset/flat.multiset.modifiers/insert_rv.pass.cpp
@@ -25,7 +25,7 @@
 #include "../helpers.h"
 
 template <class KeyContainer>
-void test_one() {
+constexpr void test_one() {
   using Key = typename KeyContainer::value_type;
   using M   = std::flat_multiset<Key, TransparentComparator, KeyContainer>;
   using R   = typename M::iterator;
@@ -63,15 +63,22 @@ void test_one() {
   assert(*r == V(1));
 }
 
-void test() {
+constexpr bool test() {
   test_one<std::vector<int>>();
   test_one<std::vector<MoveOnly>>();
-  test_one<std::deque<int>>();
-  test_one<std::deque<MoveOnly>>();
+#ifndef __cpp_lib_constexpr_deque
+  if (!TEST_IS_CONSTANT_EVALUATED)
+#endif
+  {
+    test_one<std::deque<int>>();
+    test_one<std::deque<MoveOnly>>();
+  }
   test_one<MinSequenceContainer<int>>();
   test_one<MinSequenceContainer<MoveOnly>>();
   test_one<std::vector<int, min_allocator<int>>>();
   test_one<std::vector<MoveOnly, min_allocator<MoveOnly>>>();
+
+  return true;
 }
 
 void test_exception() {
@@ -86,6 +93,9 @@ void test_exception() {
 
 int main(int, char**) {
   test();
+#if TEST_STD_VER >= 26
+  static_assert(test());
+#endif
   test_exception();
 
   return 0;
diff --git a/libcxx/test/std/containers/container.adaptors/flat.multiset/flat.multiset.modifiers/insert_sorted_initializer_list.pass.cpp b/libcxx/test/std/containers/container.adaptors/flat.multiset/flat.multiset.modifiers/insert_sorted_initializer_list.pass.cpp
index 11af199c3d1ee..81b7e4e196b30 100644
--- a/libcxx/test/std/containers/container.adaptors/flat.multiset/flat.multiset.modifiers/insert_sorted_initializer_list.pass.cpp
+++ b/libcxx/test/std/containers/container.adaptors/flat.multiset/flat.multiset.modifiers/insert_sorted_initializer_list.pass.cpp
@@ -23,7 +23,7 @@
 #include "min_allocator.h"
 
 template <class KeyContainer>
-void test_one() {
+constexpr void test_one() {
   using Key = typename KeyContainer::value_type;
   using M   = std::flat_multiset<Key, std::less<Key>, KeyContainer>;
   {
@@ -42,11 +42,16 @@ void test_one() {
   }
 }
 
-void test() {
+constexpr bool test() {
   test_one<std::vector<int>>();
-  test_one<std::deque<int>>();
+#ifndef __cpp_lib_constexpr_deque
+  if (!TEST_IS_CONSTANT_EVALUATED)
+#endif
+    test_one<std::deque<int>>();
   test_one<MinSequenceContainer<int>>();
   test_one<std::vector<int, min_allocator<int>>>();
+
+  return true;
 }
 
 void test_exception() {
@@ -61,6 +66,9 @@ void test_exception() {
 
 int main(int, char**) {
   test();
+#if TEST_STD_VER >= 26
+  static_assert(test());
+#endif
   test_exception();
 
   return 0;
diff --git a/libcxx/test/std/containers/container.adaptors/flat.multiset/flat.multiset.modifiers/insert_sorted_iter_iter.pass.cpp b/libcxx/test/std/containers/container.adaptors/flat.multiset/flat.multiset.modifiers/insert_sorted_iter_iter.pass.cpp
index 07b62d04e0ebc..bfb230718fb6f 100644
--- a/libcxx/test/std/containers/container.adaptors/flat.multiset/flat.multiset.modifiers/insert_sorted_iter_iter.pass.cpp
+++ b/libcxx/test/std/containers/container.adaptors/flat.multiset/flat.multiset.modifiers/insert_sorted_iter_iter.pass.cpp
@@ -36,7 +36,7 @@ static_assert(!CanInsert<Set, std::sorted_equivalent_t, int, int>);
 static_assert(!CanInsert<Set, std::sorted_equivalent_t, cpp20_input_iterator<int*>, cpp20_input_iterator<int*>>);
 
 template <class KeyContainer>
-void test_one() {
+constexpr void test_one() {
   using Key = typename KeyContainer::value_type;
   using M   = std::flat_multiset<Key, std::less<Key>, KeyContainer>;
 
@@ -60,11 +60,16 @@ void test_one() {
   assert(m == expected2);
 }
 
-void test() {
+constexpr bool test() {
   test_one<std::vector<int>>();
-  test_one<std::deque<int>>();
+#ifndef __cpp_lib_constexpr_deque
+  if (!TEST_IS_CONSTANT_EVALUATED)
+#endif
+    test_one<std::deque<int>>();
   test_one<MinSequenceContainer<int>>();
   test_one<std::vector<int, min_allocator<int>>>();
+
+  return true;
 }
 
 void test_exception() {
@@ -76,6 +81,9 @@ void test_exception() {
 
 int main(int, char**) {
   test();
+#if TEST_STD_VER >= 26
+  static_assert(test());
+#endif
   test_exception();
 
   return 0;
diff --git a/libcxx/test/std/containers/container.adaptors/flat.multiset/flat.multiset.modifiers/replace.pass.cpp b/libcxx/test/std/containers/container.adaptors/flat.multiset/flat.multiset.modifiers/replace.pass.cpp
index 5fe61389d72a1..3c74cf6ebe995 100644
--- a/libcxx/test/std/containers/container.adaptors/flat.multiset/flat.multiset.modifiers/replace.pass.cpp
+++ b/libcxx/test/std/containers/container.adaptors/flat.multiset/flat.multiset.modifiers/replace.pass.cpp
@@ -31,7 +31,7 @@ static_assert(CanReplace<Set, std::vector<int>>);
 static_assert(!CanReplace<Set, const std::vector<int>&>);
 
 template <class KeyContainer>
-void test_one() {
+constexpr void test_one() {
   using Key = typename KeyContainer::value_type;
   using M   = std::flat_multiset<Key, std::less<Key>, KeyContainer>;
   {
@@ -53,11 +53,16 @@ void test_one() {
   }
 }
 
-void test() {
+constexpr bool test() {
   test_one<std::vector<int>>();
-  test_one<std::deque<int>>();
+#ifndef __cpp_lib_constexpr_deque
+  if (!TEST_IS_CONSTANT_EVALUATED)
+#endif
+    test_one<std::deque<int>>();
   test_one<MinSequenceContainer<int>>();
   test_one<std::vector<int, min_allocator<int>>>();
+
+  return true;
 }
 
 void test_exception() {
@@ -82,6 +87,9 @@ void test_exception() {
 
 int main(int, char**) {
   test();
+#if TEST_STD_VER >= 26
+  static_assert(test());
+#endif
   test_exception();
 
   return 0;
diff --git a/libcxx/test/std/containers/container.adaptors/flat.multiset/flat.multiset.modifiers/swap_free.pass.cpp b/libcxx/test/std/containers/container.adaptors/flat.multiset/flat.multiset.modifiers/swap_free.pass.cpp
index 2e3ed02c3c00e..241f2cf9e0a73 100644
--- a/libcxx/test/std/containers/container.adaptors/flat.multiset/flat.multiset.modifiers/swap_free.pass.cpp
+++ b/libcxx/test/std/containers/container.adaptors/flat.multiset/flat.multiset.modifiers/swap_free.pass.cpp
@@ -38,7 +38,7 @@ static_assert(NoExceptAdlSwap<std::flat_multiset<int, std::less<int>, ThrowOnMov
 #endif
 
 template <class KeyContainer>
-void test_one() {
+constexpr void test_one() {
   using Key = typename KeyContainer::value_type;
   using M   = std::flat_multiset<Key, std::less<Key>, KeyContainer>;
 
@@ -84,15 +84,23 @@ void test_one() {
   }
 }
 
-void test() {
+constexpr bool test() {
   test_one<std::vector<int>>();
-  test_one<std::deque<int>>();
+#ifndef __cpp_lib_constexpr_deque
+  if (!TEST_IS_CONSTANT_EVALUATED)
+#endif
+    test_one<std::deque<int>>();
   test_one<MinSequenceContainer<int>>();
   test_one<std::vector<int, min_allocator<int>>>();
+
+  return true;
 }
 
 int main(int, char**) {
   test();
+#if TEST_STD_VER >= 26
+  static_assert(test());
+#endif
 
   return 0;
 }
diff --git a/libcxx/test/std/containers/container.adaptors/flat.multiset/flat.multiset.modifiers/swap_member.pass.cpp b/libcxx/test/std/containers/container.adaptors/flat.multiset/flat.multiset.modifiers/swap_member.pass.cpp
index 1d0d9152d1c1f..7ad96ed340955 100644
--- a/libcxx/test/std/containers/container.adaptors/flat.multiset/flat.multiset.modifiers/swap_member.pass.cpp
+++ b/libcxx/test/std/containers/container.adaptors/flat.multiset/flat.multiset.modifiers/swap_member.pass.cpp
@@ -37,7 +37,7 @@ static_assert(NoExceptMemberSwap<std::flat_multiset<int, std::less<int>, ThrowOn
 #endif
 
 template <class KeyContainer>
-void test_one() {
+constexpr void test_one() {
   using Key = typename KeyContainer::value_type;
   using M   = std::flat_multiset<Key, std::less<Key>, KeyContainer>;
   {
@@ -82,15 +82,23 @@ void test_one() {
   }
 }
 
-void test() {
+constexpr bool test() {
   test_one<std::vector<int>>();
-  test_one<std::deque<int>>();
+#ifndef __cpp_lib_constexpr_deque
+  if (!TEST_IS_CONSTANT_EVALUATED)
+#endif
+    test_one<std::deque<int>>();
   test_one<MinSequenceContainer<int>>();
   test_one<std::vector<int, min_allocator<int>>>();
+
+  return true;
 }
 
 int main(int, char**) {
   test();
+#if TEST_STD_VER >= 26
+  static_assert(test());
+#endif
 
   return 0;
 }
diff --git a/libcxx/test/std/containers/container.adaptors/flat.multiset/flat.multiset.observers/comp.pass.cpp b/libcxx/test/std/containers/container.adaptors/flat.multiset/flat.multiset.observers/comp.pass.cpp
index 4ca64516e242f..74c92f3a3f843 100644
--- a/libcxx/test/std/containers/container.adaptors/flat.multiset/flat.multiset.observers/comp.pass.cpp
+++ b/libcxx/test/std/containers/container.adaptors/flat.multiset/flat.multiset.observers/comp.pass.cpp
@@ -21,7 +21,7 @@
 
 #include "test_macros.h"
 
-void test() {
+constexpr bool test() {
   {
     using M    = std::flat_multiset<int>;
     using Comp = std::less<int>; // the default
@@ -36,7 +36,7 @@ void test() {
     assert(vc(1, 2));
     assert(!vc(2, 1));
   }
-  {
+  if (!TEST_IS_CONSTANT_EVALUATED) {
     using Comp = std::function<bool(int, int)>;
     using M    = std::flat_multiset<int, Comp>;
     Comp comp  = std::greater<int>();
@@ -67,10 +67,15 @@ void test() {
     assert(vc(1, 2));
     assert(!vc(2, 1));
   }
+
+  return true;
 }
 
 int main(int, char**) {
   test();
+#if TEST_STD_VER >= 26
+  static_assert(test());
+#endif
 
   return 0;
 }
diff --git a/libcxx/test/std/containers/container.adaptors/flat.multiset/flat.multiset.operations/contains.pass.cpp b/libcxx/test/std/containers/container.adaptors/flat.multiset/flat.multiset.operations/contains.pass.cpp
index 00fda6c2edd88..a178dfd3d0cb5 100644
--- a/libcxx/test/std/containers/container.adaptors/flat.multiset/flat.multiset.operations/contains.pass.cpp
+++ b/libcxx/test/std/containers/container.adaptors/flat.multiset/flat.multiset.operations/contains.pass.cpp
@@ -23,7 +23,7 @@
 #include "min_allocator.h"
 
 template <class KeyContainer>
-void test_one() {
+constexpr void test_one() {
   using Key = typename KeyContainer::value_type;
   {
     using M = std::flat_multiset<Key, std::less<>, KeyContainer>;
@@ -66,15 +66,23 @@ void test_one() {
   }
 }
 
-void test() {
+constexpr bool test() {
   test_one<std::vector<int>>();
-  test_one<std::deque<int>>();
+#ifndef __cpp_lib_constexpr_deque
+  if (!TEST_IS_CONSTANT_EVALUATED)
+#endif
+    test_one<std::deque<int>>();
   test_one<MinSequenceContainer<int>>();
   test_one<std::vector<int, min_allocator<int>>>();
+
+  return true;
 }
 
 int main(int, char**) {
   test();
+#if TEST_STD_VER >= 26
+  static_assert(test());
+#endif
 
   return 0;
 }
diff --git a/libcxx/test/std/containers/container.adaptors/flat.multiset/flat.multiset.operations/contains_transparent.pass.cpp b/libcxx/test/std/containers/container.adaptors/flat.multiset/flat.multiset.operations/contains_transparent.pass.cpp
index abee2b1bb12f9..3222762122f88 100644
--- a/libcxx/test/std/containers/container.adaptors/flat.multiset/flat.multiset.operations/contains_transparent.pass.cpp
+++ b/libcxx/test/std/containers/container.adaptors/flat.multiset/flat.multiset.operations/contains_transparent.pass.cpp
@@ -35,7 +35,7 @@ static_assert(!CanContains<NonTransparentSet>);
 static_assert(!CanContains<const NonTransparentSet>);
 
 template <class KeyContainer>
-void test_one() {
+constexpr void test_one() {
   using Key = typename KeyContainer::value_type;
   using M   = std::flat_multiset<Key, TransparentComparator, KeyContainer>;
 
@@ -60,9 +60,12 @@ void test_one() {
   }
 }
 
-void test() {
+constexpr bool test() {
   test_one<std::vector<std::string>>();
-  test_one<std::deque<std::string>>();
+#ifndef __cpp_lib_constexpr_deque
+  if (!TEST_IS_CONSTANT_EVALUATED)
+#endif
+    test_one<std::deque<std::string>>();
   test_one<MinSequenceContainer<std::string>>();
   test_one<std::vector<std::string, min_allocator<std::string>>>();
 
@@ -82,10 +85,15 @@ void test() {
     assert(m.contains("beta"));
     assert(!m.contains("charlie"));
   }
+
+  return true;
 }
 
 int main(int, char**) {
   test();
+#if TEST_STD_VER >= 26
+  static_assert(test());
+#endif
 
   return 0;
 }
diff --git a/libcxx/test/std/containers/container.adaptors/flat.multiset/flat.multiset.operations/count.pass.cpp b/libcxx/test/std/containers/container.adaptors/flat.multiset/flat.multiset.operations/count.pass.cpp
index 1752dab0e0e3a..8b034dfa1423c 100644
--- a/libcxx/test/std/containers/container.adaptors/flat.multiset/flat.multiset.operations/count.pass.cpp
+++ b/libcxx/test/std/containers/container.adaptors/flat.multiset/flat.multiset.operations/count.pass.cpp
@@ -23,7 +23,7 @@
 #include "min_allocator.h"
 
 template <class KeyContainer>
-void test_one() {
+constexpr void test_one() {
   using Key = typename KeyContainer::value_type;
   using S   = typename KeyContainer::size_type;
 
@@ -66,15 +66,23 @@ void test_one() {
   }
 }
 
-void test() {
+constexpr bool test() {
   test_one<std::vector<int>>();
-  test_one<std::deque<int>>();
+#ifndef __cpp_lib_constexpr_deque
+  if (!TEST_IS_CONSTANT_EVALUATED)
+#endif
+    test_one<std::deque<int>>();
   test_one<MinSequenceContainer<int>>();
   test_one<std::vector<int, min_allocator<int>>>();
+
+  return true;
 }
 
 int main(int, char**) {
   test();
+#if TEST_STD_VER >= 26
+  static_assert(test());
+#endif
 
   return 0;
 }
diff --git a/libcxx/test/std/containers/container.adaptors/flat.multiset/flat.multiset.operations/count_transparent.pass.cpp b/libcxx/test/std/containers/container.adaptors/flat.multiset/flat.multiset.operations/count_transparent.pass.cpp
index a9160aebb7517..a1a0d6b1f0310 100644
--- a/libcxx/test/std/containers/container.adaptors/flat.multiset/flat.multiset.operations/count_transparent.pass.cpp
+++ b/libcxx/test/std/containers/container.adaptors/flat.multiset/flat.multiset.operations/count_transparent.pass.cpp
@@ -35,7 +35,7 @@ static_assert(!CanCount<NonTransparentSet>);
 static_assert(!CanCount<const NonTransparentSet>);
 
 template <class KeyContainer>
-void test_one() {
+constexpr void test_one() {
   using Key = typename KeyContainer::value_type;
   using M   = std::flat_multiset<Key, TransparentComparator, KeyContainer>;
   {
@@ -59,9 +59,12 @@ void test_one() {
   }
 }
 
-void test() {
+constexpr bool test() {
   test_one<std::vector<std::string>>();
-  test_one<std::deque<std::string>>();
+#ifndef __cpp_lib_constexpr_deque
+  if (!TEST_IS_CONSTANT_EVALUATED)
+#endif
+    test_one<std::deque<std::string>>();
   test_one<MinSequenceContainer<std::string>>();
   test_one<std::vector<std::string, min_allocator<std::string>>>();
 
@@ -81,10 +84,15 @@ void test() {
     auto n  = m.count("beta");
     assert(n == 2);
   }
+
+  return true;
 }
 
 int main(int, char**) {
   test();
+#if TEST_STD_VER >= 26
+  static_assert(test());
+#endif
 
   return 0;
 }
diff --git a/libcxx/test/std/containers/container.adaptors/flat.multiset/flat.multiset.operations/equal_range.pass.cpp b/libcxx/test/std/containers/container.adaptors/flat.multiset/flat.multiset.operations/equal_range.pass.cpp
index 54ae27e9ba19c..b105d1914113a 100644
--- a/libcxx/test/std/containers/container.adaptors/flat.multiset/flat.multiset.operations/equal_range.pass.cpp
+++ b/libcxx/test/std/containers/container.adaptors/flat.multiset/flat.multiset.operations/equal_range.pass.cpp
@@ -24,7 +24,7 @@
 #include "min_allocator.h"
 
 template <class KeyContainer>
-void test_one() {
+constexpr void test_one() {
   using Key = typename KeyContainer::value_type;
   {
     using M  = std::flat_multiset<Key, std::less<>, KeyContainer>;
@@ -74,15 +74,23 @@ void test_one() {
   }
 }
 
-void test() {
+constexpr bool test() {
   test_one<std::vector<int>>();
-  test_one<std::deque<int>>();
+#ifndef __cpp_lib_constexpr_deque
+  if (!TEST_IS_CONSTANT_EVALUATED)
+#endif
+    test_one<std::deque<int>>();
   test_one<MinSequenceContainer<int>>();
   test_one<std::vector<int, min_allocator<int>>>();
+
+  return true;
 }
 
 int main(int, char**) {
   test();
+#if TEST_STD_VER >= 26
+  static_assert(test());
+#endif
 
   return 0;
 }
diff --git a/libcxx/test/std/containers/container.adaptors/flat.multiset/flat.multiset.operations/equal_range_transparent.pass.cpp b/libcxx/test/std/containers/container.adaptors/flat.multiset/flat.multiset.operations/equal_range_transparent.pass.cpp
index ae16ec1127f31..65bff7a095dc6 100644
--- a/libcxx/test/std/containers/container.adaptors/flat.multiset/flat.multiset.operations/equal_range_transparent.pass.cpp
+++ b/libcxx/test/std/containers/container.adaptors/flat.multiset/flat.multiset.operations/equal_range_transparent.pass.cpp
@@ -36,7 +36,7 @@ static_assert(!CanEqualRange<NonTransparentSet>);
 static_assert(!CanEqualRange<const NonTransparentSet>);
 
 template <class KeyContainer>
-void test_one() {
+constexpr void test_one() {
   using Key = typename KeyContainer::value_type;
   using M   = std::flat_multiset<Key, TransparentComparator, KeyContainer>;
 
@@ -90,9 +90,12 @@ void test_one() {
   }
 }
 
-void test() {
+constexpr bool test() {
   test_one<std::vector<std::string>>();
-  test_one<std::deque<std::string>>();
+#ifndef __cpp_lib_constexpr_deque
+  if (!TEST_IS_CONSTANT_EVALUATED)
+#endif
+    test_one<std::deque<std::string>>();
   test_one<MinSequenceContainer<std::string>>();
   test_one<std::vector<std::string, min_allocator<std::string>>>();
 
@@ -113,10 +116,15 @@ void test() {
     assert(first == m.begin() + 1);
     assert(last == m.begin() + 3);
   }
+
+  return true;
 }
 
 int main(int, char**) {
   test();
+#if TEST_STD_VER >= 26
+  static_assert(test());
+#endif
 
   return 0;
 }
diff --git a/libcxx/test/std/containers/container.adaptors/flat.multiset/flat.multiset.operations/find.pass.cpp b/libcxx/test/std/containers/container.adaptors/flat.multiset/flat.multiset.operations/find.pass.cpp
index 49386a6f77fae..bc9a439eecbb9 100644
--- a/libcxx/test/std/containers/container.adaptors/flat.multiset/flat.multiset.operations/find.pass.cpp
+++ b/libcxx/test/std/containers/container.adaptors/flat.multiset/flat.multiset.operations/find.pass.cpp
@@ -25,7 +25,7 @@
 #include "min_allocator.h"
 
 template <class KeyContainer>
-void test_one() {
+constexpr void test_one() {
   using Key = typename KeyContainer::value_type;
   using M   = std::flat_multiset<Key, std::less<>, KeyContainer>;
   {
@@ -50,15 +50,23 @@ void test_one() {
   }
 }
 
-void test() {
+constexpr bool test() {
   test_one<std::vector<int>>();
-  test_one<std::deque<int>>();
+#ifndef __cpp_lib_constexpr_deque
+  if (!TEST_IS_CONSTANT_EVALUATED)
+#endif
+    test_one<std::deque<int>>();
   test_one<MinSequenceContainer<int>>();
   test_one<std::vector<int, min_allocator<int>>>();
+
+  return true;
 }
 
 int main(int, char**) {
   test();
+#if TEST_STD_VER >= 26
+  static_assert(test());
+#endif
 
   return 0;
 }
diff --git a/libcxx/test/std/containers/container.adaptors/flat.multiset/flat.multiset.operations/find_transparent.pass.cpp b/libcxx/test/std/containers/container.adaptors/flat.multiset/flat.multiset.operations/find_transparent.pass.cpp
index 9d0b75c7b52bc..4c9c403464634 100644
--- a/libcxx/test/std/containers/container.adaptors/flat.multiset/flat.multiset.operations/find_transparent.pass.cpp
+++ b/libcxx/test/std/containers/container.adaptors/flat.multiset/flat.multiset.operations/find_transparent.pass.cpp
@@ -36,7 +36,7 @@ static_assert(!CanFind<NonTransparentSet>);
 static_assert(!CanFind<const NonTransparentSet>);
 
 template <class KeyContainer>
-void test_one() {
+constexpr void test_one() {
   using Key = typename KeyContainer::value_type;
   using M   = std::flat_multiset<Key, TransparentComparator, KeyContainer>;
 
@@ -77,9 +77,12 @@ void test_one() {
   }
 }
 
-void test() {
+constexpr bool test() {
   test_one<std::vector<std::string>>();
-  test_one<std::deque<std::string>>();
+#ifndef __cpp_lib_constexpr_deque
+  if (!TEST_IS_CONSTANT_EVALUATED)
+#endif
+    test_one<std::deque<std::string>>();
   test_one<MinSequenceContainer<std::string>>();
   test_one<std::vector<std::string, min_allocator<std::string>>>();
 
@@ -101,10 +104,15 @@ void test() {
     auto it2 = m.find("charlie");
     assert(it2 == m.end());
   }
+
+  return true;
 }
 
 int main(int, char**) {
   test();
+#if TEST_STD_VER >= 26
+  static_assert(test());
+#endif
 
   return 0;
 }
diff --git a/libcxx/test/std/containers/container.adaptors/flat.multiset/flat.multiset.operations/lower_bound.pass.cpp b/libcxx/test/std/containers/container.adaptors/flat.multiset/flat.multiset.operations/lower_bound.pass.cpp
index ba41b822fda74..07f053316ad32 100644
--- a/libcxx/test/std/containers/container.adaptors/flat.multiset/flat.multiset.operations/lower_bound.pass.cpp
+++ b/libcxx/test/std/containers/container.adaptors/flat.multiset/flat.multiset.operations/lower_bound.pass.cpp
@@ -24,7 +24,7 @@
 #include "min_allocator.h"
 
 template <class KeyContainer>
-void test_one() {
+constexpr void test_one() {
   using Key = typename KeyContainer::value_type;
   {
     using M = std::flat_multiset<Key, std::less<>, KeyContainer>;
@@ -66,15 +66,23 @@ void test_one() {
   }
 }
 
-void test() {
+constexpr bool test() {
   test_one<std::vector<int>>();
-  test_one<std::deque<int>>();
+#ifndef __cpp_lib_constexpr_deque
+  if (!TEST_IS_CONSTANT_EVALUATED)
+#endif
+    test_one<std::deque<int>>();
   test_one<MinSequenceContainer<int>>();
   test_one<std::vector<int, min_allocator<int>>>();
+
+  return true;
 }
 
 int main(int, char**) {
   test();
+#if TEST_STD_VER >= 26
+  static_assert(test());
+#endif
 
   return 0;
 }
diff --git a/libcxx/test/std/containers/container.adaptors/flat.multiset/flat.multiset.operations/lower_bound_transparent.pass.cpp b/libcxx/test/std/containers/container.adaptors/flat.multiset/flat.multiset.operations/lower_bound_transparent.pass.cpp
index c03fb27a7c27e..e674c85ab30e6 100644
--- a/libcxx/test/std/containers/container.adaptors/flat.multiset/flat.multiset.operations/lower_bound_transparent.pass.cpp
+++ b/libcxx/test/std/containers/container.adaptors/flat.multiset/flat.multiset.operations/lower_bound_transparent.pass.cpp
@@ -36,7 +36,7 @@ static_assert(!CanLowerBound<NonTransparentSet>);
 static_assert(!CanLowerBound<const NonTransparentSet>);
 
 template <class KeyContainer>
-void test_one() {
+constexpr void test_one() {
   using Key = typename KeyContainer::value_type;
   using M   = std::flat_multiset<Key, TransparentComparator, KeyContainer>;
 
@@ -83,9 +83,12 @@ void test_one() {
   }
 }
 
-void test() {
+constexpr bool test() {
   test_one<std::vector<std::string>>();
-  test_one<std::deque<std::string>>();
+#ifndef __cpp_lib_constexpr_deque
+  if (!TEST_IS_CONSTANT_EVALUATED)
+#endif
+    test_one<std::deque<std::string>>();
   test_one<MinSequenceContainer<std::string>>();
   test_one<std::vector<std::string, min_allocator<std::string>>>();
 
@@ -107,10 +110,15 @@ void test() {
     auto it2 = m.lower_bound("charlie");
     assert(it2 == m.begin() + 3);
   }
+
+  return true;
 }
 
 int main(int, char**) {
   test();
+#if TEST_STD_VER >= 26
+  static_assert(test());
+#endif
 
   return 0;
 }
diff --git a/libcxx/test/std/containers/container.adaptors/flat.multiset/flat.multiset.operations/upper_bound.pass.cpp b/libcxx/test/std/containers/container.adaptors/flat.multiset/flat.multiset.operations/upper_bound.pass.cpp
index 7828f0500c8b9..d4d19926571d7 100644
--- a/libcxx/test/std/containers/container.adaptors/flat.multiset/flat.multiset.operations/upper_bound.pass.cpp
+++ b/libcxx/test/std/containers/container.adaptors/flat.multiset/flat.multiset.operations/upper_bound.pass.cpp
@@ -24,7 +24,7 @@
 #include "min_allocator.h"
 
 template <class KeyContainer>
-void test_one() {
+constexpr void test_one() {
   using Key = typename KeyContainer::value_type;
   {
     using M = std::flat_multiset<Key, std::less<>, KeyContainer>;
@@ -67,15 +67,23 @@ void test_one() {
   }
 }
 
-void test() {
+constexpr bool test() {
   test_one<std::vector<int>>();
-  test_one<std::deque<int>>();
+#ifndef __cpp_lib_constexpr_deque
+  if (!TEST_IS_CONSTANT_EVALUATED)
+#endif
+    test_one<std::deque<int>>();
   test_one<MinSequenceContainer<int>>();
   test_one<std::vector<int, min_allocator<int>>>();
+
+  return true;
 }
 
 int main(int, char**) {
   test();
+#if TEST_STD_VER >= 26
+  static_assert(test());
+#endif
 
   return 0;
 }
diff --git a/libcxx/test/std/containers/container.adaptors/flat.multiset/flat.multiset.operations/upper_bound_transparent.pass.cpp b/libcxx/test/std/containers/container.adaptors/flat.multiset/flat.multiset.operations/upper_bound_transparent.pass.cpp
index de517fd7e520a..75140a780cceb 100644
--- a/libcxx/test/std/containers/container.adaptors/flat.multiset/flat.multiset.operations/upper_bound_transparent.pass.cpp
+++ b/libcxx/test/std/containers/container.adaptors/flat.multiset/flat.multiset.operations/upper_bound_transparent.pass.cpp
@@ -36,7 +36,7 @@ static_assert(!CanUpperBound<NonTransparentSet>);
 static_assert(!CanUpperBound<const NonTransparentSet>);
 
 template <class KeyContainer>
-void test_one() {
+constexpr void test_one() {
   using Key = typename KeyContainer::value_type;
   using M   = std::flat_multiset<Key, TransparentComparator, KeyContainer>;
 
@@ -83,9 +83,12 @@ void test_one() {
   }
 }
 
-void test() {
+constexpr bool test() {
   test_one<std::vector<std::string>>();
-  test_one<std::deque<std::string>>();
+#ifndef __cpp_lib_constexpr_deque
+  if (!TEST_IS_CONSTANT_EVALUATED)
+#endif
+    test_one<std::deque<std::string>>();
   test_one<MinSequenceContainer<std::string>>();
   test_one<std::vector<std::string, min_allocator<std::string>>>();
 
@@ -105,10 +108,15 @@ void test() {
     auto it = m.upper_bound("beta");
     assert(it == m.begin() + 3);
   }
+
+  return true;
 }
 
 int main(int, char**) {
   test();
+#if TEST_STD_VER >= 26
+  static_assert(test());
+#endif
 
   return 0;
 }
diff --git a/libcxx/test/std/containers/container.adaptors/flat.multiset/helpers.h b/libcxx/test/std/containers/container.adaptors/flat.multiset/helpers.h
index e7ed8a091d3be..82f917756e92c 100644
--- a/libcxx/test/std/containers/container.adaptors/flat.multiset/helpers.h
+++ b/libcxx/test/std/containers/container.adaptors/flat.multiset/helpers.h
@@ -20,7 +20,7 @@
 #include "test_macros.h"
 
 template <class... Args>
-void check_invariant(const std::flat_multiset<Args...>& m) {
+constexpr void check_invariant(const std::flat_multiset<Args...>& m) {
   assert(std::is_sorted(m.begin(), m.end(), m.key_comp()));
 }
 
diff --git a/libcxx/test/std/containers/container.adaptors/flat.multiset/op_compare.pass.cpp b/libcxx/test/std/containers/container.adaptors/flat.multiset/op_compare.pass.cpp
index 94f0f2b34abcc..606cdfc3ba7d2 100644
--- a/libcxx/test/std/containers/container.adaptors/flat.multiset/op_compare.pass.cpp
+++ b/libcxx/test/std/containers/container.adaptors/flat.multiset/op_compare.pass.cpp
@@ -31,7 +31,7 @@
 #include "test_container_comparisons.h"
 
 template <class KeyContainer>
-void test_one() {
+constexpr void test_one() {
   using Key = typename KeyContainer::value_type;
 
   {
@@ -64,9 +64,12 @@ void test_one() {
   }
 }
 
-void test() {
+constexpr bool test() {
   test_one<std::vector<int>>();
-  test_one<std::deque<int>>();
+#ifndef __cpp_lib_constexpr_deque
+  if (!TEST_IS_CONSTANT_EVALUATED)
+#endif
+    test_one<std::deque<int>>();
   test_one<MinSequenceContainer<int>>();
   test_one<std::vector<int, min_allocator<int>>>();
 
@@ -81,7 +84,7 @@ void test() {
   {
     // Comparisons use value_type's native operators, not the comparator
     struct StrongComp {
-      bool operator()(double a, double b) const { return std::strong_order(a, b) < 0; }
+      constexpr bool operator()(double a, double b) const { return std::strong_order(a, b) < 0; }
     };
     using C = std::flat_multiset<double, StrongComp>;
     C s1    = {1};
@@ -96,10 +99,15 @@ void test() {
     assert(s1 != s2);
     assert((s1 <=> s2) == std::partial_ordering::unordered);
   }
+
+  return true;
 }
 
 int main(int, char**) {
   test();
+#if TEST_STD_VER >= 26
+  static_assert(test());
+#endif
 
   return 0;
 }
diff --git a/libcxx/test/std/language.support/support.limits/support.limits.general/flat_map.version.compile.pass.cpp b/libcxx/test/std/language.support/support.limits/support.limits.general/flat_map.version.compile.pass.cpp
index 9c06eee27e0c8..26c8e1bc7d66f 100644
--- a/libcxx/test/std/language.support/support.limits/support.limits.general/flat_map.version.compile.pass.cpp
+++ b/libcxx/test/std/language.support/support.limits/support.limits.general/flat_map.version.compile.pass.cpp
@@ -20,30 +20,50 @@
 
 #if TEST_STD_VER < 14
 
+#  ifdef __cpp_lib_constexpr_flat_map
+#    error "__cpp_lib_constexpr_flat_map should not be defined before c++26"
+#  endif
+
 #  ifdef __cpp_lib_flat_map
 #    error "__cpp_lib_flat_map should not be defined before c++23"
 #  endif
 
 #elif TEST_STD_VER == 14
 
+#  ifdef __cpp_lib_constexpr_flat_map
+#    error "__cpp_lib_constexpr_flat_map should not be defined before c++26"
+#  endif
+
 #  ifdef __cpp_lib_flat_map
 #    error "__cpp_lib_flat_map should not be defined before c++23"
 #  endif
 
 #elif TEST_STD_VER == 17
 
+#  ifdef __cpp_lib_constexpr_flat_map
+#    error "__cpp_lib_constexpr_flat_map should not be defined before c++26"
+#  endif
+
 #  ifdef __cpp_lib_flat_map
 #    error "__cpp_lib_flat_map should not be defined before c++23"
 #  endif
 
 #elif TEST_STD_VER == 20
 
+#  ifdef __cpp_lib_constexpr_flat_map
+#    error "__cpp_lib_constexpr_flat_map should not be defined before c++26"
+#  endif
+
 #  ifdef __cpp_lib_flat_map
 #    error "__cpp_lib_flat_map should not be defined before c++23"
 #  endif
 
 #elif TEST_STD_VER == 23
 
+#  ifdef __cpp_lib_constexpr_flat_map
+#    error "__cpp_lib_constexpr_flat_map should not be defined before c++26"
+#  endif
+
 #  ifndef __cpp_lib_flat_map
 #    error "__cpp_lib_flat_map should be defined in c++23"
 #  endif
@@ -53,6 +73,13 @@
 
 #elif TEST_STD_VER > 23
 
+#  ifndef __cpp_lib_constexpr_flat_map
+#    error "__cpp_lib_constexpr_flat_map should be defined in c++26"
+#  endif
+#  if __cpp_lib_constexpr_flat_map != 202502L
+#    error "__cpp_lib_constexpr_flat_map should have the value 202502L in c++26"
+#  endif
+
 #  ifndef __cpp_lib_flat_map
 #    error "__cpp_lib_flat_map should be defined in c++26"
 #  endif
diff --git a/libcxx/test/std/language.support/support.limits/support.limits.general/flat_set.version.compile.pass.cpp b/libcxx/test/std/language.support/support.limits/support.limits.general/flat_set.version.compile.pass.cpp
index 5985bdc2d7d4f..b29da9fdbe649 100644
--- a/libcxx/test/std/language.support/support.limits/support.limits.general/flat_set.version.compile.pass.cpp
+++ b/libcxx/test/std/language.support/support.limits/support.limits.general/flat_set.version.compile.pass.cpp
@@ -20,30 +20,50 @@
 
 #if TEST_STD_VER < 14
 
+#  ifdef __cpp_lib_constexpr_flat_set
+#    error "__cpp_lib_constexpr_flat_set should not be defined before c++26"
+#  endif
+
 #  ifdef __cpp_lib_flat_set
 #    error "__cpp_lib_flat_set should not be defined before c++23"
 #  endif
 
 #elif TEST_STD_VER == 14
 
+#  ifdef __cpp_lib_constexpr_flat_set
+#    error "__cpp_lib_constexpr_flat_set should not be defined before c++26"
+#  endif
+
 #  ifdef __cpp_lib_flat_set
 #    error "__cpp_lib_flat_set should not be defined before c++23"
 #  endif
 
 #elif TEST_STD_VER == 17
 
+#  ifdef __cpp_lib_constexpr_flat_set
+#    error "__cpp_lib_constexpr_flat_set should not be defined before c++26"
+#  endif
+
 #  ifdef __cpp_lib_flat_set
 #    error "__cpp_lib_flat_set should not be defined before c++23"
 #  endif
 
 #elif TEST_STD_VER == 20
 
+#  ifdef __cpp_lib_constexpr_flat_set
+#    error "__cpp_lib_constexpr_flat_set should not be defined before c++26"
+#  endif
+
 #  ifdef __cpp_lib_flat_set
 #    error "__cpp_lib_flat_set should not be defined before c++23"
 #  endif
 
 #elif TEST_STD_VER == 23
 
+#  ifdef __cpp_lib_constexpr_flat_set
+#    error "__cpp_lib_constexpr_flat_set should not be defined before c++26"
+#  endif
+
 #  ifndef __cpp_lib_flat_set
 #    error "__cpp_lib_flat_set should be defined in c++23"
 #  endif
@@ -53,6 +73,13 @@
 
 #elif TEST_STD_VER > 23
 
+#  ifndef __cpp_lib_constexpr_flat_set
+#    error "__cpp_lib_constexpr_flat_set should be defined in c++26"
+#  endif
+#  if __cpp_lib_constexpr_flat_set != 202502L
+#    error "__cpp_lib_constexpr_flat_set should have the value 202502L in c++26"
+#  endif
+
 #  ifndef __cpp_lib_flat_set
 #    error "__cpp_lib_flat_set should be defined in c++26"
 #  endif
diff --git a/libcxx/test/std/language.support/support.limits/support.limits.general/version.version.compile.pass.cpp b/libcxx/test/std/language.support/support.limits/support.limits.general/version.version.compile.pass.cpp
index cde2f258b7732..2c88562165f34 100644
--- a/libcxx/test/std/language.support/support.limits/support.limits.general/version.version.compile.pass.cpp
+++ b/libcxx/test/std/language.support/support.limits/support.limits.general/version.version.compile.pass.cpp
@@ -204,6 +204,14 @@
 #    error "__cpp_lib_constexpr_dynamic_alloc should not be defined before c++20"
 #  endif
 
+#  ifdef __cpp_lib_constexpr_flat_map
+#    error "__cpp_lib_constexpr_flat_map should not be defined before c++26"
+#  endif
+
+#  ifdef __cpp_lib_constexpr_flat_set
+#    error "__cpp_lib_constexpr_flat_set should not be defined before c++26"
+#  endif
+
 #  ifdef __cpp_lib_constexpr_forward_list
 #    error "__cpp_lib_constexpr_forward_list should not be defined before c++26"
 #  endif
@@ -1108,6 +1116,14 @@
 #    error "__cpp_lib_constexpr_dynamic_alloc should not be defined before c++20"
 #  endif
 
+#  ifdef __cpp_lib_constexpr_flat_map
+#    error "__cpp_lib_constexpr_flat_map should not be defined before c++26"
+#  endif
+
+#  ifdef __cpp_lib_constexpr_flat_set
+#    error "__cpp_lib_constexpr_flat_set should not be defined before c++26"
+#  endif
+
 #  ifdef __cpp_lib_constexpr_forward_list
 #    error "__cpp_lib_constexpr_forward_list should not be defined before c++26"
 #  endif
@@ -2114,6 +2130,14 @@
 #    error "__cpp_lib_constexpr_dynamic_alloc should not be defined before c++20"
 #  endif
 
+#  ifdef __cpp_lib_constexpr_flat_map
+#    error "__cpp_lib_constexpr_flat_map should not be defined before c++26"
+#  endif
+
+#  ifdef __cpp_lib_constexpr_flat_set
+#    error "__cpp_lib_constexpr_flat_set should not be defined before c++26"
+#  endif
+
 #  ifdef __cpp_lib_constexpr_forward_list
 #    error "__cpp_lib_constexpr_forward_list should not be defined before c++26"
 #  endif
@@ -3366,6 +3390,14 @@
 #    error "__cpp_lib_constexpr_dynamic_alloc should have the value 201907L in c++20"
 #  endif
 
+#  ifdef __cpp_lib_constexpr_flat_map
+#    error "__cpp_lib_constexpr_flat_map should not be defined before c++26"
+#  endif
+
+#  ifdef __cpp_lib_constexpr_flat_set
+#    error "__cpp_lib_constexpr_flat_set should not be defined before c++26"
+#  endif
+
 #  ifdef __cpp_lib_constexpr_forward_list
 #    error "__cpp_lib_constexpr_forward_list should not be defined before c++26"
 #  endif
@@ -4840,6 +4872,14 @@
 #    error "__cpp_lib_constexpr_dynamic_alloc should have the value 201907L in c++23"
 #  endif
 
+#  ifdef __cpp_lib_constexpr_flat_map
+#    error "__cpp_lib_constexpr_flat_map should not be defined before c++26"
+#  endif
+
+#  ifdef __cpp_lib_constexpr_flat_set
+#    error "__cpp_lib_constexpr_flat_set should not be defined before c++26"
+#  endif
+
 #  ifdef __cpp_lib_constexpr_forward_list
 #    error "__cpp_lib_constexpr_forward_list should not be defined before c++26"
 #  endif
@@ -6527,6 +6567,20 @@
 #    error "__cpp_lib_constexpr_dynamic_alloc should have the value 201907L in c++26"
 #  endif
 
+#  ifndef __cpp_lib_constexpr_flat_map
+#    error "__cpp_lib_constexpr_flat_map should be defined in c++26"
+#  endif
+#  if __cpp_lib_constexpr_flat_map != 202502L
+#    error "__cpp_lib_constexpr_flat_map should have the value 202502L in c++26"
+#  endif
+
+#  ifndef __cpp_lib_constexpr_flat_set
+#    error "__cpp_lib_constexpr_flat_set should be defined in c++26"
+#  endif
+#  if __cpp_lib_constexpr_flat_set != 202502L
+#    error "__cpp_lib_constexpr_flat_set should have the value 202502L in c++26"
+#  endif
+
 #  ifndef __cpp_lib_constexpr_forward_list
 #    error "__cpp_lib_constexpr_forward_list should be defined in c++26"
 #  endif
diff --git a/libcxx/utils/generate_feature_test_macro_components.py b/libcxx/utils/generate_feature_test_macro_components.py
index c1e579c775746..f3b2e968fdb5e 100644
--- a/libcxx/utils/generate_feature_test_macro_components.py
+++ b/libcxx/utils/generate_feature_test_macro_components.py
@@ -367,6 +367,16 @@ def add_version_header(tc):
             "values": {"c++20": 201907},
             "headers": ["memory"],
         },
+        {
+            "name": "__cpp_lib_constexpr_flat_map",
+            "values": {"c++26": 202502},
+            "headers": ["flat_map"],
+        },
+        {
+            "name": "__cpp_lib_constexpr_flat_set",
+            "values": {"c++26": 202502},
+            "headers": ["flat_set"],
+        },
         {
             "name": "__cpp_lib_constexpr_forward_list",
             "values": {"c++26": 202502},