[libcxx-commits] [libcxx] [libc++] Introduce __forward_as (PR #118168)

[Nikolas Klauser via libcxx-commits]

https://github.com/philnik777 updated https://github.com/llvm/llvm-project/pull/118168

>From 0e85573c36f0efb98d94be2b49c7aa5e58b4d38b Mon Sep 17 00:00:00 2001
From: Nikolas Klauser <nikolasklauser at berlin.de>
Date: Sat, 30 Nov 2024 12:31:53 +0100
Subject: [PATCH] [libc++] Introduce __forward_as

---
 libcxx/include/__utility/forward_like.h | 17 +++++++++++++++++
 libcxx/include/variant                  |  6 ++----
 2 files changed, 19 insertions(+), 4 deletions(-)

diff --git a/libcxx/include/__utility/forward_like.h b/libcxx/include/__utility/forward_like.h
index 0206ce23a56681..67bdf6d054ea8b 100644
--- a/libcxx/include/__utility/forward_like.h
+++ b/libcxx/include/__utility/forward_like.h
@@ -12,6 +12,7 @@
 
 #include <__config>
 #include <__type_traits/conditional.h>
+#include <__type_traits/is_base_of.h>
 #include <__type_traits/is_const.h>
 #include <__type_traits/is_reference.h>
 #include <__type_traits/remove_reference.h>
@@ -39,6 +40,22 @@ forward_like(_LIBCPP_LIFETIMEBOUND _Up&& __ux) noexcept -> _ForwardLike<_Tp, _Up
   return static_cast<_ForwardLike<_Tp, _Up>>(__ux);
 }
 
+// This function is used for `deducing this` cases where you want to make sure the operation is performed on the class
+// itself and not on a derived class. For example
+//   struct S {
+//     template <class Self>
+//     void func(Self&& self) {
+//       // This will always call `do_something` of S instead of any class derived from S.
+//       std::__forward_as<Self, S>(self).do_something();
+//     }
+//   };
+template <class _Tp, class _As, class _Up>
+[[nodiscard]] _LIBCPP_HIDE_FROM_ABI constexpr _ForwardLike<_Tp, _As>
+__forward_as(_LIBCPP_LIFETIMEBOUND _Up&& __val) noexcept {
+  static_assert(is_base_of_v<_As, remove_reference_t<_Up>>);
+  return static_cast<_ForwardLike<_Tp, _As>>(__val);
+}
+
 #endif // _LIBCPP_STD_VER >= 23
 
 _LIBCPP_END_NAMESPACE_STD
diff --git a/libcxx/include/variant b/libcxx/include/variant
index f604527cd22569..7e44ec54141c23 100644
--- a/libcxx/include/variant
+++ b/libcxx/include/variant
@@ -1309,14 +1309,12 @@ public:
 
   template <__variant_visit_barrier_tag = __variant_visit_barrier_tag{}, class _Self, class _Visitor>
   _LIBCPP_HIDE_FROM_ABI constexpr decltype(auto) visit(this _Self&& __self, _Visitor&& __visitor) {
-    using _VariantT = _OverrideRef<_Self&&, _CopyConst<remove_reference_t<_Self>, variant>>;
-    return std::visit(std::forward<_Visitor>(__visitor), (_VariantT)__self);
+    return std::visit(std::forward<_Visitor>(__visitor), std::__forward_as<_Self, variant>(__self));
   }
 
   template <class _Rp, class _Self, class _Visitor>
   _LIBCPP_HIDE_FROM_ABI constexpr _Rp visit(this _Self&& __self, _Visitor&& __visitor) {
-    using _VariantT = _OverrideRef<_Self&&, _CopyConst<remove_reference_t<_Self>, variant>>;
-    return std::visit<_Rp>(std::forward<_Visitor>(__visitor), (_VariantT)__self);
+    return std::visit<_Rp>(std::forward<_Visitor>(__visitor), std::__forward_as<_Self, variant>(__self));
   }
 #  endif