[libc++] Add nodiscard attribute to std::make_unique/std::make_shared

libcxx/include/__memory/shared_ptr.h

@@ -740,7 +740,7 @@ _LIBCPP_HIDE_FROM_ABI shared_ptr<_Tp> allocate_shared(const _Alloc& __a, _Args&&
 }
 
 template <class _Tp, class... _Args, __enable_if_t<!is_array<_Tp>::value, int> = 0>
-_LIBCPP_HIDE_FROM_ABI shared_ptr<_Tp> make_shared(_Args&&... __args) {
+[[__nodiscard__]] _LIBCPP_HIDE_FROM_ABI shared_ptr<_Tp> make_shared(_Args&&... __args) {
   return std::allocate_shared<_Tp>(allocator<__remove_cv_t<_Tp> >(), std::forward<_Args>(__args)...);
 }
 
@@ -754,7 +754,7 @@ _LIBCPP_HIDE_FROM_ABI shared_ptr<_Tp> allocate_shared_for_overwrite(const _Alloc
 }
 
 template <class _Tp, __enable_if_t<!is_array<_Tp>::value, int> = 0>
-_LIBCPP_HIDE_FROM_ABI shared_ptr<_Tp> make_shared_for_overwrite() {
+[[nodiscard]] _LIBCPP_HIDE_FROM_ABI shared_ptr<_Tp> make_shared_for_overwrite() {
   return std::allocate_shared_for_overwrite<_Tp>(allocator<__remove_cv_t<_Tp>>());
 }
 
@@ -963,17 +963,17 @@ _LIBCPP_HIDE_FROM_ABI shared_ptr<_Tp> allocate_shared_for_overwrite(const _Alloc
 }
 
 template <class _Tp, __enable_if_t<is_bounded_array<_Tp>::value, int> = 0>
-_LIBCPP_HIDE_FROM_ABI shared_ptr<_Tp> make_shared() {
+[[nodiscard]] _LIBCPP_HIDE_FROM_ABI shared_ptr<_Tp> make_shared() {
   return std::__allocate_shared_bounded_array<_Tp>(allocator<_Tp>());
 }
 
 template <class _Tp, __enable_if_t<is_bounded_array<_Tp>::value, int> = 0>
-_LIBCPP_HIDE_FROM_ABI shared_ptr<_Tp> make_shared(const remove_extent_t<_Tp>& __u) {
+[[nodiscard]] _LIBCPP_HIDE_FROM_ABI shared_ptr<_Tp> make_shared(const remove_extent_t<_Tp>& __u) {
   return std::__allocate_shared_bounded_array<_Tp>(allocator<_Tp>(), __u);
 }
 
 template <class _Tp, __enable_if_t<is_bounded_array<_Tp>::value, int> = 0>
-_LIBCPP_HIDE_FROM_ABI shared_ptr<_Tp> make_shared_for_overwrite() {
+[[nodiscard]] _LIBCPP_HIDE_FROM_ABI shared_ptr<_Tp> make_shared_for_overwrite() {
   return std::__allocate_shared_bounded_array<_Tp>(allocator<__for_overwrite_tag>());
 }
 
@@ -996,17 +996,17 @@ _LIBCPP_HIDE_FROM_ABI shared_ptr<_Tp> allocate_shared_for_overwrite(const _Alloc
 }
 
 template <class _Tp, __enable_if_t<is_unbounded_array<_Tp>::value, int> = 0>
-_LIBCPP_HIDE_FROM_ABI shared_ptr<_Tp> make_shared(size_t __n) {
+[[nodiscard]] _LIBCPP_HIDE_FROM_ABI shared_ptr<_Tp> make_shared(size_t __n) {
   return std::__allocate_shared_unbounded_array<_Tp>(allocator<_Tp>(), __n);
 }
 
 template <class _Tp, __enable_if_t<is_unbounded_array<_Tp>::value, int> = 0>
-_LIBCPP_HIDE_FROM_ABI shared_ptr<_Tp> make_shared(size_t __n, const remove_extent_t<_Tp>& __u) {
+[[nodiscard]] _LIBCPP_HIDE_FROM_ABI shared_ptr<_Tp> make_shared(size_t __n, const remove_extent_t<_Tp>& __u) {
   return std::__allocate_shared_unbounded_array<_Tp>(allocator<_Tp>(), __n, __u);
 }
 
 template <class _Tp, __enable_if_t<is_unbounded_array<_Tp>::value, int> = 0>
-_LIBCPP_HIDE_FROM_ABI shared_ptr<_Tp> make_shared_for_overwrite(size_t __n) {
+[[nodiscard]] _LIBCPP_HIDE_FROM_ABI shared_ptr<_Tp> make_shared_for_overwrite(size_t __n) {
   return std::__allocate_shared_unbounded_array<_Tp>(allocator<__for_overwrite_tag>(), __n);
 }
 

libcxx/include/__memory/unique_ptr.h

@@ -755,12 +755,12 @@ operator<=>(const unique_ptr<_T1, _D1>& __x, nullptr_t) {
 #if _LIBCPP_STD_VER >= 14
 
 template <class _Tp, class... _Args, enable_if_t<!is_array<_Tp>::value, int> = 0>
-inline _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX23 unique_ptr<_Tp> make_unique(_Args&&... __args) {
+[[__nodiscard__]] inline _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX23 unique_ptr<_Tp> make_unique(_Args&&... __args) {
   return unique_ptr<_Tp>(new _Tp(std::forward<_Args>(__args)...));
 }
 
 template <class _Tp, enable_if_t<__is_unbounded_array_v<_Tp>, int> = 0>
-inline _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX23 unique_ptr<_Tp> make_unique(size_t __n) {
+[[__nodiscard__]] inline _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX23 unique_ptr<_Tp> make_unique(size_t __n) {
   typedef __remove_extent_t<_Tp> _Up;
   return unique_ptr<_Tp>(__private_constructor_tag(), new _Up[__n](), __n);
 }
@@ -773,12 +773,12 @@ void make_unique(_Args&&...) = delete;
 #if _LIBCPP_STD_VER >= 20
 
 template <class _Tp, enable_if_t<!is_array_v<_Tp>, int> = 0>
-_LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX23 unique_ptr<_Tp> make_unique_for_overwrite() {
+[[nodiscard]] _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX23 unique_ptr<_Tp> make_unique_for_overwrite() {
   return unique_ptr<_Tp>(new _Tp);
 }
 
 template <class _Tp, enable_if_t<is_unbounded_array_v<_Tp>, int> = 0>
-_LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX23 unique_ptr<_Tp> make_unique_for_overwrite(size_t __n) {
+[[nodiscard]] _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX23 unique_ptr<_Tp> make_unique_for_overwrite(size_t __n) {
   return unique_ptr<_Tp>(__private_constructor_tag(), new __remove_extent_t<_Tp>[__n], __n);
 }
 

libcxx/test/libcxx/memory/nodiscard.verify.cpp

@@ -0,0 +1,33 @@
+//===----------------------------------------------------------------------===//
+//
+// 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
+//
+//===----------------------------------------------------------------------===//
+
+// check that functions are marked [[nodiscard]] as an extension in C++17
+
+// [[nodiscard]] std::make_unique(Args&&...);
+// [[nodiscard]] std::make_shared(Args&&...);
+//
+// [[nodiscard]] std::make_unique(size_t);
+// [[nodiscard]] std::make_shared(size_t);
+//
+// [[nodiscard]] std::make_unique_for_overwrite();
+// [[nodiscard]] std::make_shared_for_overwrite();
+
+#include <memory>
+
+void f() {
+
+    std::make_unique<int>(1); // expected-warning {{ignoring return value of function}}
+    std::make_shared<int>(1); // expected-warning {{ignoring return value of function}}
+
+    std::make_unique<int[]>(5); // expected-warning {{ignoring return value of function}}
+    std::make_shared<int[]>(5); // expected-warning {{ignoring return value of function}}
+
+    std::make_unique_for_overwrite<int>(); // expected-warning {{ignoring return value of function}}
+    std::make_shared_for_overwrite<int>(); // expected-warning {{ignoring return value of function}}
+
+}