[libc++] Make forward_list constexpr as part of P3372R3 (llvm#129435)

Fixes llvm#128658

Author: winner245

libcxx/docs/FeatureTestMacroTable.rst

@@ -420,6 +420,8 @@ Status
     ---------------------------------------------------------- -----------------
     ``__cpp_lib_constexpr_algorithms``                         ``202306L``
     ---------------------------------------------------------- -----------------
+    ``__cpp_lib_constexpr_forward_list``                       ``202502L``
+    ---------------------------------------------------------- -----------------
     ``__cpp_lib_constexpr_new``                                ``202406L``
     ---------------------------------------------------------- -----------------
     ``__cpp_lib_constexpr_queue``                              ``202502L``

libcxx/include/__memory/allocation_guard.h

@@ -49,24 +49,26 @@ struct __allocation_guard {
   using _Size _LIBCPP_NODEBUG    = typename allocator_traits<_Alloc>::size_type;
 
   template <class _AllocT> // we perform the allocator conversion inside the constructor
-  _LIBCPP_HIDE_FROM_ABI explicit __allocation_guard(_AllocT __alloc, _Size __n)
+  _LIBCPP_CONSTEXPR_SINCE_CXX26 _LIBCPP_HIDE_FROM_ABI explicit __allocation_guard(_AllocT __alloc, _Size __n)
       : __alloc_(std::move(__alloc)),
         __n_(__n),
         __ptr_(allocator_traits<_Alloc>::allocate(__alloc_, __n_)) // initialization order is important
   {}
 
-  _LIBCPP_HIDE_FROM_ABI ~__allocation_guard() _NOEXCEPT { __destroy(); }
+  _LIBCPP_CONSTEXPR_SINCE_CXX26 _LIBCPP_HIDE_FROM_ABI ~__allocation_guard() _NOEXCEPT { __destroy(); }
 
-  _LIBCPP_HIDE_FROM_ABI __allocation_guard(const __allocation_guard&) = delete;
-  _LIBCPP_HIDE_FROM_ABI __allocation_guard(__allocation_guard&& __other) _NOEXCEPT
+  __allocation_guard(const __allocation_guard&)                    = delete;
+  __allocation_guard& operator=(const __allocation_guard& __other) = delete;
+
+  _LIBCPP_CONSTEXPR_SINCE_CXX26 _LIBCPP_HIDE_FROM_ABI __allocation_guard(__allocation_guard&& __other) _NOEXCEPT
       : __alloc_(std::move(__other.__alloc_)),
         __n_(__other.__n_),
         __ptr_(__other.__ptr_) {
     __other.__ptr_ = nullptr;
   }
 
-  _LIBCPP_HIDE_FROM_ABI __allocation_guard& operator=(const __allocation_guard& __other) = delete;
-  _LIBCPP_HIDE_FROM_ABI __allocation_guard& operator=(__allocation_guard&& __other) _NOEXCEPT {
+  _LIBCPP_CONSTEXPR_SINCE_CXX26 _LIBCPP_HIDE_FROM_ABI __allocation_guard&
+  operator=(__allocation_guard&& __other) _NOEXCEPT {
     if (std::addressof(__other) != this) {
       __destroy();
 
@@ -79,17 +81,17 @@ struct __allocation_guard {
     return *this;
   }
 
-  _LIBCPP_HIDE_FROM_ABI _Pointer
+  _LIBCPP_CONSTEXPR_SINCE_CXX26 _LIBCPP_HIDE_FROM_ABI _Pointer
   __release_ptr() _NOEXCEPT { // not called __release() because it's a keyword in objective-c++
     _Pointer __tmp = __ptr_;
     __ptr_         = nullptr;
     return __tmp;
   }
 
-  _LIBCPP_HIDE_FROM_ABI _Pointer __get() const _NOEXCEPT { return __ptr_; }
+  _LIBCPP_CONSTEXPR_SINCE_CXX26 _LIBCPP_HIDE_FROM_ABI _Pointer __get() const _NOEXCEPT { return __ptr_; }
 
 private:
-  _LIBCPP_HIDE_FROM_ABI void __destroy() _NOEXCEPT {
+  _LIBCPP_CONSTEXPR_SINCE_CXX26 _LIBCPP_HIDE_FROM_ABI void __destroy() _NOEXCEPT {
     if (__ptr_ != nullptr) {
       allocator_traits<_Alloc>::deallocate(__alloc_, __ptr_, __n_);
     }

libcxx/include/__memory/pointer_traits.h

@@ -245,8 +245,8 @@ inline _LIBCPP_HIDE_FROM_ABI constexpr auto to_address(_Tp* __p) noexcept {
 }
 
 template <class _Pointer>
-inline _LIBCPP_HIDE_FROM_ABI constexpr auto
-to_address(const _Pointer& __p) noexcept -> decltype(std::__to_address(__p)) {
+inline _LIBCPP_HIDE_FROM_ABI constexpr auto to_address(const _Pointer& __p) noexcept
+    -> decltype(std::__to_address(__p)) {
   return std::__to_address(__p);
 }
 #endif
@@ -302,6 +302,18 @@ concept __resettable_smart_pointer_with_args = requires(_Smart __s, _Pointer __p
 
 #endif
 
+// This function ensures safe conversions between fancy pointers at compile-time, where we avoid casts from/to
+// `__void_pointer` by obtaining the underlying raw pointer from the fancy pointer using `std::to_address`,
+// then dereferencing it to retrieve the pointed-to object, and finally constructing the target fancy pointer
+// to that object using the `std::pointer_traits<>::pinter_to` function.
+template <class _PtrTo, class _PtrFrom>
+_LIBCPP_CONSTEXPR_SINCE_CXX20 _LIBCPP_HIDE_FROM_ABI _PtrTo __static_fancy_pointer_cast(const _PtrFrom& __p) {
+  using __ptr_traits   = pointer_traits<_PtrTo>;
+  using __element_type = typename __ptr_traits::element_type;
+  return __p ? __ptr_traits::pointer_to(*static_cast<__element_type*>(std::addressof(*__p)))
+             : static_cast<_PtrTo>(nullptr);
+}
+
 _LIBCPP_END_NAMESPACE_STD
 
 _LIBCPP_POP_MACROS