Add __static_fancy_pointer_cast

Author: winner245

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 +302,14 @@ concept __resettable_smart_pointer_with_args = requires(_Smart __s, _Pointer __p
 
 #endif
 
+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

libcxx/include/forward_list

@@ -300,8 +300,7 @@ struct __forward_node_traits {
     return __p;
   }
   _LIBCPP_CONSTEXPR_SINCE_CXX26 _LIBCPP_HIDE_FROM_ABI static __begin_node_pointer __as_iter_node(__node_pointer __p) {
-    return __p ? pointer_traits<__begin_node_pointer>::pointer_to(*static_cast<__begin_node*>(std::addressof(*__p)))
-               : static_cast<__begin_node_pointer>(nullptr);
+    return __static_fancy_pointer_cast<__begin_node_pointer>(__p);
   }
 };
 
@@ -316,9 +315,7 @@ struct __forward_begin_node {
   _LIBCPP_CONSTEXPR_SINCE_CXX26 _LIBCPP_HIDE_FROM_ABI explicit __forward_begin_node(pointer __n) : __next_(__n) {}
 
   _LIBCPP_CONSTEXPR_SINCE_CXX26 _LIBCPP_HIDE_FROM_ABI __begin_node_pointer __next_as_begin() const {
-    return __next_ ? pointer_traits<__begin_node_pointer>::pointer_to(
-                         *static_cast<__forward_begin_node*>(std::addressof(*__next_)))
-                   : static_cast<__begin_node_pointer>(nullptr);
+    return __static_fancy_pointer_cast<__begin_node_pointer>(__next_);
   }
 };
 
@@ -374,8 +371,7 @@ class _LIBCPP_TEMPLATE_VIS __forward_list_iterator {
 
   _LIBCPP_CONSTEXPR_SINCE_CXX26 _LIBCPP_HIDE_FROM_ABI __begin_node_pointer __get_begin() const { return __ptr_; }
   _LIBCPP_CONSTEXPR_SINCE_CXX26 _LIBCPP_HIDE_FROM_ABI __node_pointer __get_unsafe_node_pointer() const {
-    return __ptr_ ? pointer_traits<__node_pointer>::pointer_to(*static_cast<__node_type*>(std::addressof(*__ptr_)))
-                  : static_cast<__node_pointer>(nullptr);
+    return __static_fancy_pointer_cast<__node_pointer>(__ptr_);
   }
 
   _LIBCPP_CONSTEXPR_SINCE_CXX26 _LIBCPP_HIDE_FROM_ABI explicit __forward_list_iterator(nullptr_t) _NOEXCEPT
@@ -445,8 +441,7 @@ class _LIBCPP_TEMPLATE_VIS __forward_list_const_iterator {
 
   _LIBCPP_CONSTEXPR_SINCE_CXX26 _LIBCPP_HIDE_FROM_ABI __begin_node_pointer __get_begin() const { return __ptr_; }
   _LIBCPP_CONSTEXPR_SINCE_CXX26 _LIBCPP_HIDE_FROM_ABI __node_pointer __get_unsafe_node_pointer() const {
-    return __ptr_ ? pointer_traits<__node_pointer>::pointer_to(*static_cast<__node_type*>(std::addressof(*__ptr_)))
-                  : static_cast<__node_pointer>(nullptr);
+    return __static_fancy_pointer_cast<__node_pointer>(__ptr_);
   }
 
   _LIBCPP_CONSTEXPR_SINCE_CXX26 _LIBCPP_HIDE_FROM_ABI explicit __forward_list_const_iterator(nullptr_t) _NOEXCEPT