[libc++] Make <map> std::map constexpr as part of P3372R3

libcxx/docs/FeatureTestMacroTable.rst

@@ -428,6 +428,8 @@ Status
     ---------------------------------------------------------- -----------------
     ``__cpp_lib_constexpr_list``                               ``202502L``
     ---------------------------------------------------------- -----------------
+    ``__cpp_lib_constexpr_map``                                ``202502L``
+    ---------------------------------------------------------- -----------------
     ``__cpp_lib_constexpr_new``                                ``202406L``
     ---------------------------------------------------------- -----------------
     ``__cpp_lib_constexpr_queue``                              ``202502L``

libcxx/docs/ReleaseNotes/22.rst

@@ -59,6 +59,7 @@ Improvements and New Features
   by up to 2.5x
 - The performance of ``map::insert_or_assign`` has been improved by up to 2x
 - ``ofstream::write`` has been optimized to pass through large strings to system calls directly instead of copying them
+- P3372R3: ``constexpr map``(`Github <https://github.com/llvm/llvm-project/issues/128660>`__) (The paper is partially implemented. ``constexpr map`` is implemented in this release)
   in chunks into a buffer.
 - Multiple internal types have been refactored to use ``[[no_unique_address]]``, resulting in faster compile times and
   reduced debug information.

libcxx/include/__iterator/erase_if_container.h

@@ -22,7 +22,8 @@ _LIBCPP_PUSH_MACROS
 _LIBCPP_BEGIN_NAMESPACE_STD
 
 template <class _Container, class _Predicate>
-_LIBCPP_HIDE_FROM_ABI typename _Container::size_type __libcpp_erase_if_container(_Container& __c, _Predicate& __pred) {
+_LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX26 typename _Container::size_type
+__libcpp_erase_if_container(_Container& __c, _Predicate& __pred) {
   typename _Container::size_type __old_size = __c.size();
 
   const typename _Container::iterator __last = __c.end();

libcxx/include/__node_handle

@@ -99,12 +99,12 @@ private:
   __node_pointer_type __ptr_ = nullptr;
   optional<allocator_type> __alloc_;
 
-  _LIBCPP_HIDE_FROM_ABI void __release_ptr() {
+  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX26 void __release_ptr() {
     __ptr_   = nullptr;
     __alloc_ = std::nullopt;
   }
 
-  _LIBCPP_HIDE_FROM_ABI void __destroy_node_pointer() {
+  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX26 void __destroy_node_pointer() {
     if (__ptr_ != nullptr) {
       typedef typename __allocator_traits_rebind< allocator_type, _NodeType>::type __node_alloc_type;
       __node_alloc_type __alloc(*__alloc_);
@@ -113,19 +113,20 @@ private:
     }
   }
 
-  _LIBCPP_HIDE_FROM_ABI __basic_node_handle(__node_pointer_type __ptr, allocator_type const& __alloc)
+  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX26
+  __basic_node_handle(__node_pointer_type __ptr, allocator_type const& __alloc)
       : __ptr_(__ptr), __alloc_(__alloc) {}
 
 public:
-  _LIBCPP_HIDE_FROM_ABI __basic_node_handle() = default;
+  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX26 __basic_node_handle() = default;
 
-  _LIBCPP_HIDE_FROM_ABI __basic_node_handle(__basic_node_handle&& __other) noexcept
+  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX26 __basic_node_handle(__basic_node_handle&& __other) noexcept
       : __ptr_(__other.__ptr_), __alloc_(std::move(__other.__alloc_)) {
     __other.__ptr_   = nullptr;
     __other.__alloc_ = std::nullopt;
   }
 
-  _LIBCPP_HIDE_FROM_ABI __basic_node_handle& operator=(__basic_node_handle&& __other) {
+  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX26 __basic_node_handle& operator=(__basic_node_handle&& __other) {
     _LIBCPP_ASSERT_COMPATIBLE_ALLOCATOR(
         __alloc_ == std::nullopt || __alloc_traits::propagate_on_container_move_assignment::value ||
             __alloc_ == __other.__alloc_,
@@ -144,13 +145,13 @@ public:
     return *this;
   }
 
-  _LIBCPP_HIDE_FROM_ABI allocator_type get_allocator() const { return *__alloc_; }
+  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX26 allocator_type get_allocator() const { return *__alloc_; }
 
-  _LIBCPP_HIDE_FROM_ABI explicit operator bool() const { return __ptr_ != nullptr; }
+  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX26 explicit operator bool() const { return __ptr_ != nullptr; }
 
-  [[nodiscard]] _LIBCPP_HIDE_FROM_ABI bool empty() const { return __ptr_ == nullptr; }
+  [[nodiscard]] _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX26 bool empty() const { return __ptr_ == nullptr; }
 
-  _LIBCPP_HIDE_FROM_ABI void swap(__basic_node_handle& __other) noexcept(
+  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX26 void swap(__basic_node_handle& __other) noexcept(
       __alloc_traits::propagate_on_container_swap::value || __alloc_traits::is_always_equal::value) {
     using std::swap;
     swap(__ptr_, __other.__ptr_);
@@ -159,12 +160,12 @@ public:
       swap(__alloc_, __other.__alloc_);
   }
 
-  _LIBCPP_HIDE_FROM_ABI friend void
+  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX26 friend void
   swap(__basic_node_handle& __a, __basic_node_handle& __b) noexcept(noexcept(__a.swap(__b))) {
     __a.swap(__b);
   }
 
-  _LIBCPP_HIDE_FROM_ABI ~__basic_node_handle() { __destroy_node_pointer(); }
+  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX26 ~__basic_node_handle() { __destroy_node_pointer(); }
 };
 
 template <class _NodeType, class _Derived>
@@ -179,6 +180,16 @@ struct __map_node_handle_specifics {
   using key_type    = __remove_const_t<typename _NodeType::__node_value_type::first_type>;
   using mapped_type = typename _NodeType::__node_value_type::second_type;
 
+  /*
+  As this method this->key() returns a non-const reference
+  to the `key_type` which is const inside `node_type`.
+  This means that we can't make this member method constexpr until CWG2514 is resolved.
+  This disallows making the member function constexpr.
+
+  More here:
+  * https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2025/p3372r3.html#node-handle-key
+  * https://cplusplus.github.io/CWG/issues/2514.html
+  */
   _LIBCPP_HIDE_FROM_ABI key_type& key() const {
     return const_cast<key_type&>(static_cast<_Derived const*>(this)->__ptr_->__get_value().first);
   }