[libc++] Mark std::unique_ptr::release() as nodiscard

libcxx/include/__memory/unique_ptr.h

@@ -278,7 +278,7 @@ class _LIBCPP_UNIQUE_PTR_TRIVIAL_ABI _LIBCPP_TEMPLATE_VIS unique_ptr {
     return __ptr_ != nullptr;
   }
 
-  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX23 pointer release() _NOEXCEPT {
+  [[__nodiscard__]] _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX23 pointer release() _NOEXCEPT {
     pointer __t = __ptr_;
     __ptr_      = pointer();
     return __t;

libcxx/include/deque

@@ -2187,7 +2187,7 @@ void deque<_Tp, _Allocator>::__add_back_capacity() {
     typedef __allocator_destructor<_Allocator> _Dp;
     unique_ptr<pointer, _Dp> __hold(__alloc_traits::allocate(__a, __block_size), _Dp(__a, __block_size));
     __buf.push_back(__hold.get());
-    __hold.release();
+    (void)__hold.release();
 
     for (__map_pointer __i = __map_.end(); __i != __map_.begin();)
       __buf.push_front(*--__i);

libcxx/include/locale

@@ -2459,7 +2459,7 @@ _LIBCPP_HIDE_FROM_ABI void __double_or_nothing(unique_ptr<_Tp, void (*)(void*)>&
   if (__t == 0)
     __throw_bad_alloc();
   if (__owns)
-    __b.release();
+    (void)__b.release();
   __b = unique_ptr<_Tp, void (*)(void*)>(__t, free);
   __new_cap /= sizeof(_Tp);
   __n = __b.get() + __n_off;

libcxx/test/std/utilities/smartptr/unique.ptr/unique.ptr.class/unique.ptr.modifiers/release.nodiscard.verify.cpp

@@ -0,0 +1,21 @@
+//===----------------------------------------------------------------------===//
+//
+// 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
+//
+//===----------------------------------------------------------------------===//
+
+// REQUIRES: stdlib=libc++
+
+// <memory>
+//
+// unique_ptr
+//
+// constexpr pointer release() noexcept; // nodiscard as an extension
+
+#include <memory>
+
+void f(std::unique_ptr<int> p) {
+  p.release(); // expected-warning {{ignoring return value of function declared with 'nodiscard' attribute}}
+}

libcxx/test/std/utilities/smartptr/unique.ptr/unique.ptr.class/unique.ptr.modifiers/release.pass.cpp

@@ -7,10 +7,10 @@
 //===----------------------------------------------------------------------===//
 
 // <memory>
-
+//
 // unique_ptr
-
-// test release
+//
+// constexpr pointer release() noexcept;
 
 #include <memory>
 #include <cassert>