[libc++] Add [[clang::lifetimebound]] to std::identity

Author: philnik777

libcxx/include/__functional/identity.h

@@ -44,7 +44,7 @@ struct __is_identity<reference_wrapper<const __identity> > : true_type {};
 
 struct identity {
   template <class _Tp>
-  [[nodiscard]] _LIBCPP_HIDE_FROM_ABI constexpr _Tp&& operator()(_Tp&& __t) const noexcept {
+  [[nodiscard]] _LIBCPP_HIDE_FROM_ABI constexpr _Tp&& operator()(_LIBCPP_LIFETIMEBOUND _Tp&& __t) const noexcept {
     return std::forward<_Tp>(__t);
   }
 

libcxx/test/libcxx/utilities/function.objects/lifetimebound.verify.cpp

@@ -0,0 +1,18 @@
+//===----------------------------------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+// UNSUPPORTED: c++03, c++11, c++14, c++17
+// ADDITIONAL_COMPILE_FLAGS: -Wno-pessimizing-move -Wno-unused-variable
+
+#include <functional>
+
+#include "test_macros.h"
+
+void func() {
+  auto&& v1 = std::identity()(1); // expected-warning {{temporary bound to local reference 'v1' will be destroyed at the end of the full-expression}}
+}