use allocator as template arg and allocator_traits, tests passed in cpp26

Author: vinay-deshmukh

libcxx/include/__tree

@@ -582,10 +582,16 @@ class _LIBCPP_STANDALONE_DEBUG __tree_node : public __tree_node_base<_VoidPtr> {
 public:
   using __node_value_type _LIBCPP_NODEBUG = __get_node_value_type_t<_Tp>;
 
+  union {
   __node_value_type __value_;
+  };
 
   _LIBCPP_HIDE_FROM_ABI __node_value_type& __get_value() { return __value_; }
 
+  template <class _Alloc, class... _Args>
+  explicit __tree_node(_Alloc& __na, _Args&&... __args) {
+    allocator_traits<_Alloc>::construct(__na, std::addressof(__value_), std::forward<_Args>(__args)...);
+  }
   ~__tree_node()                             = delete;
   __tree_node(__tree_node const&)            = delete;
   __tree_node& operator=(__tree_node const&) = delete;
@@ -1808,7 +1814,7 @@ typename __tree<_Tp, _Compare, _Allocator>::__node_holder
 __tree<_Tp, _Compare, _Allocator>::__construct_node(_Args&&... __args) {
   __node_allocator& __na = __node_alloc();
   __node_holder __h(__node_traits::allocate(__na, 1), _Dp(__na));
-  __node_traits::construct(__na, std::addressof(__h->__value_), std::forward<_Args>(__args)...);
+  std::__construct_at(std::addressof(*__h), __na, std::forward<_Args>(__args)...);
   __h.get_deleter().__value_constructed = true;
   return __h;
 }