Allow runtime decision for __assign_value

Author: vinay-deshmukh

libcxx/include/__tree

@@ -1360,14 +1360,35 @@ private:
   }
   _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX26 void __move_assign_alloc(__tree&, false_type) _NOEXCEPT {}
 
+
+  template <class _From, class _ValueT = _Tp, __enable_if_t<__is_tree_value_type_v<_ValueT>, int> = 0>
+  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX26 void
+  __assign_value__sfinae(true_type, __get_node_value_type_t<value_type>& __lhs, _From&& __rhs) {
+
+      __node_allocator& __na = __node_alloc();
+      __node_traits::destroy(__na, std::addressof(__lhs));
+
+      using __node_value_type = __get_node_value_type_t<value_type>;
+      __node_value_type __tmp(__rhs.first, __rhs.second);
+
+      __node_traits::construct(__na, std::addressof(__lhs), std::move(__tmp));
+
+  }
+
+  template <class _From, class _ValueT = _Tp, __enable_if_t<__is_tree_value_type_v<_ValueT>, int> = 0>
+  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX26 void
+  __assign_value__sfinae(false_type, __get_node_value_type_t<value_type>& , _From&& ) {
+// This method body should never be run. It only exists to allow for compilation. See note in __assign_value for more information
+  }
+
   template <class _From, class _ValueT = _Tp, __enable_if_t<__is_tree_value_type_v<_ValueT>, int> = 0>
   _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX26 void
   __assign_value(__get_node_value_type_t<value_type>& __lhs, _From&& __rhs) {
     using __key_type = __remove_const_t<typename value_type::first_type>;
 
 #if _LIBCPP_STD_VER >= 26
 
-    if constexpr (std::is_copy_constructible_v<decltype(__rhs.first)>) {
+    if (std::is_constant_evaluated() && std::is_copy_constructible_v<decltype(__rhs.first)>) {
       // we use copy, and not "move" as the constraint
       // because we can NOT move from `const key_type`, which is how `value_type` is defined
       // atleast for map
@@ -1378,13 +1399,17 @@ private:
       // we get around this by deleting __lhs and creating a new node in-place
       // to avoid const_cast __lhs.first
 
-      __node_allocator& __na = __node_alloc();
-      __node_traits::destroy(__na, std::addressof(__lhs));
+      // We create a sfinae wrapper method here, because if the body of the true_type overload for __assign_value__sfinae() gets template instantiated within __assign_value,
+      // the code will fail to compile where the value is not copy_constructible for runtime execution as well; unless we use `if constexpr`.
+      // Given the copy-constructible code path will be a performance regression, we want to restrict it to only execute during constant evaluation
+      //, we need to delay the template instantiation
 
-      using __node_value_type = __get_node_value_type_t<value_type>;
-      __node_value_type __tmp(__rhs.first, __rhs.second);
+      __assign_value__sfinae(
+        std::integral_constant<bool, std::is_copy_constructible_v<decltype(__rhs.first)>>(),
+        std::forward<decltype(__lhs)>(__lhs),
+        std::forward<decltype(__rhs)>(__rhs)
+      );
 
-      __node_traits::construct(__na, std::addressof(__lhs), std::move(__tmp));
 
     } else
 #endif