[libc++][any] Applied [[nodiscard]]

`[[nodiscard]]` should be applied to functions where discarding the return value is most likely a correctness issue.
- https://libcxx.llvm.org/CodingGuidelines.html#apply-nodiscard-where-relevant

Author: H-G-Hristov

libcxx/include/any

@@ -262,10 +262,10 @@ public:
   _LIBCPP_HIDE_FROM_ABI void swap(any& __rhs) _NOEXCEPT;
 
   // 6.3.4 any observers
-  _LIBCPP_HIDE_FROM_ABI bool has_value() const _NOEXCEPT { return __h_ != nullptr; }
+  [[nodiscard]] _LIBCPP_HIDE_FROM_ABI bool has_value() const _NOEXCEPT { return __h_ != nullptr; }
 
 #    if _LIBCPP_HAS_RTTI
-  _LIBCPP_HIDE_FROM_ABI const type_info& type() const _NOEXCEPT {
+  [[nodiscard]] _LIBCPP_HIDE_FROM_ABI const type_info& type() const _NOEXCEPT {
     if (__h_) {
       return *static_cast<type_info const*>(this->__call(_Action::_TypeInfo));
     } else {
@@ -492,17 +492,17 @@ inline _LIBCPP_HIDE_FROM_ABI void any::swap(any& __rhs) _NOEXCEPT {
 inline _LIBCPP_HIDE_FROM_ABI void swap(any& __lhs, any& __rhs) _NOEXCEPT { __lhs.swap(__rhs); }
 
 template <class _Tp, class... _Args>
-inline _LIBCPP_HIDE_FROM_ABI any make_any(_Args&&... __args) {
+[[nodiscard]] inline _LIBCPP_HIDE_FROM_ABI any make_any(_Args&&... __args) {
   return any(in_place_type<_Tp>, std::forward<_Args>(__args)...);
 }
 
 template <class _Tp, class _Up, class... _Args>
-inline _LIBCPP_HIDE_FROM_ABI any make_any(initializer_list<_Up> __il, _Args&&... __args) {
+[[nodiscard]] inline _LIBCPP_HIDE_FROM_ABI any make_any(initializer_list<_Up> __il, _Args&&... __args) {
   return any(in_place_type<_Tp>, __il, std::forward<_Args>(__args)...);
 }
 
 template <class _ValueType>
-inline _LIBCPP_HIDE_FROM_ABI _ValueType any_cast(any const& __v) {
+[[nodiscard]] inline _LIBCPP_HIDE_FROM_ABI _ValueType any_cast(any const& __v) {
   using _RawValueType = __remove_cvref_t<_ValueType>;
   static_assert(is_constructible<_ValueType, _RawValueType const&>::value,
                 "ValueType is required to be a const lvalue reference "
@@ -514,7 +514,7 @@ inline _LIBCPP_HIDE_FROM_ABI _ValueType any_cast(any const& __v) {
 }
 
 template <class _ValueType>
-inline _LIBCPP_HIDE_FROM_ABI _ValueType any_cast(any& __v) {
+[[nodiscard]] inline _LIBCPP_HIDE_FROM_ABI _ValueType any_cast(any& __v) {
   using _RawValueType = __remove_cvref_t<_ValueType>;
   static_assert(is_constructible<_ValueType, _RawValueType&>::value,
                 "ValueType is required to be an lvalue reference "
@@ -526,7 +526,7 @@ inline _LIBCPP_HIDE_FROM_ABI _ValueType any_cast(any& __v) {
 }
 
 template <class _ValueType>
-inline _LIBCPP_HIDE_FROM_ABI _ValueType any_cast(any&& __v) {
+[[nodiscard]] inline _LIBCPP_HIDE_FROM_ABI _ValueType any_cast(any&& __v) {
   using _RawValueType = __remove_cvref_t<_ValueType>;
   static_assert(is_constructible<_ValueType, _RawValueType>::value,
                 "ValueType is required to be an rvalue reference "
@@ -538,7 +538,7 @@ inline _LIBCPP_HIDE_FROM_ABI _ValueType any_cast(any&& __v) {
 }
 
 template <class _ValueType>
-inline _LIBCPP_HIDE_FROM_ABI add_pointer_t<add_const_t<_ValueType>> any_cast(any const* __any) _NOEXCEPT {
+[[nodiscard]] inline _LIBCPP_HIDE_FROM_ABI add_pointer_t<add_const_t<_ValueType>> any_cast(any const* __any) _NOEXCEPT {
   static_assert(!is_void_v<_ValueType>, "_ValueType may not be void.");
   static_assert(!is_reference<_ValueType>::value, "_ValueType may not be a reference.");
   return std::any_cast<_ValueType>(const_cast<any*>(__any));
@@ -555,7 +555,7 @@ inline _LIBCPP_HIDE_FROM_ABI _RetType __pointer_or_func_cast(void*, /*IsFunction
 }
 
 template <class _ValueType>
-_LIBCPP_HIDE_FROM_ABI add_pointer_t<_ValueType> any_cast(any* __any) _NOEXCEPT {
+[[nodiscard]] _LIBCPP_HIDE_FROM_ABI add_pointer_t<_ValueType> any_cast(any* __any) _NOEXCEPT {
   using __any_imp::_Action;
   static_assert(!is_void_v<_ValueType>, "_ValueType may not be void.");
   static_assert(!is_reference<_ValueType>::value, "_ValueType may not be a reference.");

libcxx/test/libcxx/utilities/any/nodiscard.verify.cpp

@@ -0,0 +1,45 @@
+//===----------------------------------------------------------------------===//
+//
+// 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: std-at-least-c++17
+
+// Check that functions are marked [[nodiscard]]
+
+#include <any>
+#include <utility>
+#include <vector>
+
+#include "test_macros.h"
+
+void test() {
+  std::any a{94};
+
+  // expected-warning@+1 {{ignoring return value of function declared with 'nodiscard' attribute}}
+  a.has_value();
+#if !defined(TEST_HAS_NO_RTTI)
+  // expected-warning@+1 {{ignoring return value of function declared with 'nodiscard' attribute}}
+  a.type();
+#endif
+
+  // expected-warning@+1 {{ignoring return value of function declared with 'nodiscard' attribute}}
+  std::make_any<int>(82);
+  // expected-warning@+1 {{ignoring return value of function declared with 'nodiscard' attribute}}
+  std::make_any<std::vector<int>>({94, 82, 50});
+
+  // expected-warning@+1 {{ignoring return value of function declared with 'nodiscard' attribute}}
+  std::any_cast<const int&>(std::as_const(a));
+  // expected-warning@+1 {{ignoring return value of function declared with 'nodiscard' attribute}}
+  std::any_cast<int&>(a);
+  // expected-warning@+1 {{ignoring return value of function declared with 'nodiscard' attribute}}
+  std::any_cast<int&&>(std::move(a));
+
+  // expected-warning@+1 {{ignoring return value of function declared with 'nodiscard' attribute}}
+  std::any_cast<int*>(&a);
+  // expected-warning@+1 {{ignoring return value of function declared with 'nodiscard' attribute}}
+  std::any_cast<const int*>(&std::as_const(a));
+}