[libcxx-commits] [libcxx] [libc++] Applied `[[nodiscard]]` to Language Support (partially) (PR #169611)
Hristo Hristov via libcxx-commits
libcxx-commits at lists.llvm.org
Tue Nov 25 23:13:35 PST 2025
https://github.com/H-G-Hristov updated https://github.com/llvm/llvm-project/pull/169611
>From 253c4d86045866045f1b5eb5c7b1fc2e95bb308b Mon Sep 17 00:00:00 2001
From: Hristo Hristov <hghristov.rmm at gmail.com>
Date: Wed, 26 Nov 2025 08:19:57 +0200
Subject: [PATCH] [libc++] Applied `[[nodiscard]]` to Language Support
(partially)
https://wg21.link/#support
`[[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
The following was implemented in this patch:
- [x] `<compare>`
- [x] `<corotine>`
- [x] `<initializer_list>`
- [x] Integer comparisons
---
libcxx/include/__compare/is_eq.h | 12 +--
libcxx/include/__coroutine/coroutine_handle.h | 18 ++--
.../__coroutine/noop_coroutine_handle.h | 10 +-
libcxx/include/__utility/cmp.h | 14 +--
libcxx/include/initializer_list | 12 ++-
.../diagnostics/utility.nodiscard.verify.cpp | 28 ++++--
.../language.support/nodiscard.verify.cpp | 94 +++++++++++++++++++
7 files changed, 153 insertions(+), 35 deletions(-)
create mode 100644 libcxx/test/libcxx/language.support/nodiscard.verify.cpp
diff --git a/libcxx/include/__compare/is_eq.h b/libcxx/include/__compare/is_eq.h
index 9a82df1ebe88b..ee4d11bc7c792 100644
--- a/libcxx/include/__compare/is_eq.h
+++ b/libcxx/include/__compare/is_eq.h
@@ -20,12 +20,12 @@ _LIBCPP_BEGIN_NAMESPACE_STD
#if _LIBCPP_STD_VER >= 20
-_LIBCPP_HIDE_FROM_ABI inline constexpr bool is_eq(partial_ordering __c) noexcept { return __c == 0; }
-_LIBCPP_HIDE_FROM_ABI inline constexpr bool is_neq(partial_ordering __c) noexcept { return __c != 0; }
-_LIBCPP_HIDE_FROM_ABI inline constexpr bool is_lt(partial_ordering __c) noexcept { return __c < 0; }
-_LIBCPP_HIDE_FROM_ABI inline constexpr bool is_lteq(partial_ordering __c) noexcept { return __c <= 0; }
-_LIBCPP_HIDE_FROM_ABI inline constexpr bool is_gt(partial_ordering __c) noexcept { return __c > 0; }
-_LIBCPP_HIDE_FROM_ABI inline constexpr bool is_gteq(partial_ordering __c) noexcept { return __c >= 0; }
+[[nodiscard]] _LIBCPP_HIDE_FROM_ABI inline constexpr bool is_eq(partial_ordering __c) noexcept { return __c == 0; }
+[[nodiscard]] _LIBCPP_HIDE_FROM_ABI inline constexpr bool is_neq(partial_ordering __c) noexcept { return __c != 0; }
+[[nodiscard]] _LIBCPP_HIDE_FROM_ABI inline constexpr bool is_lt(partial_ordering __c) noexcept { return __c < 0; }
+[[nodiscard]] _LIBCPP_HIDE_FROM_ABI inline constexpr bool is_lteq(partial_ordering __c) noexcept { return __c <= 0; }
+[[nodiscard]] _LIBCPP_HIDE_FROM_ABI inline constexpr bool is_gt(partial_ordering __c) noexcept { return __c > 0; }
+[[nodiscard]] _LIBCPP_HIDE_FROM_ABI inline constexpr bool is_gteq(partial_ordering __c) noexcept { return __c >= 0; }
#endif // _LIBCPP_STD_VER >= 20
diff --git a/libcxx/include/__coroutine/coroutine_handle.h b/libcxx/include/__coroutine/coroutine_handle.h
index b7add258510eb..b26a650748832 100644
--- a/libcxx/include/__coroutine/coroutine_handle.h
+++ b/libcxx/include/__coroutine/coroutine_handle.h
@@ -44,9 +44,9 @@ struct coroutine_handle<void> {
}
// [coroutine.handle.export.import], export/import
- _LIBCPP_HIDE_FROM_ABI constexpr void* address() const noexcept { return __handle_; }
+ [[nodiscard]] _LIBCPP_HIDE_FROM_ABI constexpr void* address() const noexcept { return __handle_; }
- _LIBCPP_HIDE_FROM_ABI static constexpr coroutine_handle from_address(void* __addr) noexcept {
+ [[nodiscard]] _LIBCPP_HIDE_FROM_ABI static constexpr coroutine_handle from_address(void* __addr) noexcept {
coroutine_handle __tmp;
__tmp.__handle_ = __addr;
return __tmp;
@@ -55,7 +55,7 @@ struct coroutine_handle<void> {
// [coroutine.handle.observers], observers
_LIBCPP_HIDE_FROM_ABI constexpr explicit operator bool() const noexcept { return __handle_ != nullptr; }
- _LIBCPP_HIDE_FROM_ABI bool done() const {
+ [[nodiscard]] _LIBCPP_HIDE_FROM_ABI bool done() const {
_LIBCPP_ASSERT_VALID_EXTERNAL_API_CALL(__is_suspended(), "done() can be called only on suspended coroutines");
return __builtin_coro_done(__handle_);
}
@@ -100,7 +100,7 @@ struct coroutine_handle {
_LIBCPP_HIDE_FROM_ABI constexpr coroutine_handle(nullptr_t) noexcept {}
- _LIBCPP_HIDE_FROM_ABI static coroutine_handle from_promise(_Promise& __promise) {
+ [[nodiscard]] _LIBCPP_HIDE_FROM_ABI static coroutine_handle from_promise(_Promise& __promise) {
using _RawPromise = __remove_cv_t<_Promise>;
coroutine_handle __tmp;
__tmp.__handle_ =
@@ -114,9 +114,9 @@ struct coroutine_handle {
}
// [coroutine.handle.export.import], export/import
- _LIBCPP_HIDE_FROM_ABI constexpr void* address() const noexcept { return __handle_; }
+ [[nodiscard]] _LIBCPP_HIDE_FROM_ABI constexpr void* address() const noexcept { return __handle_; }
- _LIBCPP_HIDE_FROM_ABI static constexpr coroutine_handle from_address(void* __addr) noexcept {
+ [[nodiscard]] _LIBCPP_HIDE_FROM_ABI static constexpr coroutine_handle from_address(void* __addr) noexcept {
coroutine_handle __tmp;
__tmp.__handle_ = __addr;
return __tmp;
@@ -130,7 +130,7 @@ struct coroutine_handle {
// [coroutine.handle.observers], observers
_LIBCPP_HIDE_FROM_ABI constexpr explicit operator bool() const noexcept { return __handle_ != nullptr; }
- _LIBCPP_HIDE_FROM_ABI bool done() const {
+ [[nodiscard]] _LIBCPP_HIDE_FROM_ABI bool done() const {
_LIBCPP_ASSERT_VALID_EXTERNAL_API_CALL(__is_suspended(), "done() can be called only on suspended coroutines");
return __builtin_coro_done(__handle_);
}
@@ -150,7 +150,7 @@ struct coroutine_handle {
}
// [coroutine.handle.promise], promise access
- _LIBCPP_HIDE_FROM_ABI _Promise& promise() const {
+ [[nodiscard]] _LIBCPP_HIDE_FROM_ABI _Promise& promise() const {
return *static_cast<_Promise*>(__builtin_coro_promise(this->__handle_, alignof(_Promise), false));
}
@@ -165,7 +165,7 @@ struct coroutine_handle {
// [coroutine.handle.hash]
template <class _Tp>
struct hash<coroutine_handle<_Tp>> {
- _LIBCPP_HIDE_FROM_ABI size_t operator()(const coroutine_handle<_Tp>& __v) const noexcept {
+ [[nodiscard]] _LIBCPP_HIDE_FROM_ABI size_t operator()(const coroutine_handle<_Tp>& __v) const noexcept {
return hash<void*>()(__v.address());
}
};
diff --git a/libcxx/include/__coroutine/noop_coroutine_handle.h b/libcxx/include/__coroutine/noop_coroutine_handle.h
index 692398a8a8431..b9c54d3b42bef 100644
--- a/libcxx/include/__coroutine/noop_coroutine_handle.h
+++ b/libcxx/include/__coroutine/noop_coroutine_handle.h
@@ -35,7 +35,7 @@ struct coroutine_handle<noop_coroutine_promise> {
// [coroutine.handle.noop.observers], observers
_LIBCPP_HIDE_FROM_ABI constexpr explicit operator bool() const noexcept { return true; }
- _LIBCPP_HIDE_FROM_ABI constexpr bool done() const noexcept { return false; }
+ [[nodiscard]] _LIBCPP_HIDE_FROM_ABI constexpr bool done() const noexcept { return false; }
// [coroutine.handle.noop.resumption], resumption
_LIBCPP_HIDE_FROM_ABI constexpr void operator()() const noexcept {}
@@ -43,13 +43,13 @@ struct coroutine_handle<noop_coroutine_promise> {
_LIBCPP_HIDE_FROM_ABI constexpr void destroy() const noexcept {}
// [coroutine.handle.noop.promise], promise access
- _LIBCPP_HIDE_FROM_ABI noop_coroutine_promise& promise() const noexcept {
+ [[nodiscard]] _LIBCPP_HIDE_FROM_ABI noop_coroutine_promise& promise() const noexcept {
return *static_cast<noop_coroutine_promise*>(
__builtin_coro_promise(this->__handle_, alignof(noop_coroutine_promise), false));
}
// [coroutine.handle.noop.address], address
- _LIBCPP_HIDE_FROM_ABI constexpr void* address() const noexcept { return __handle_; }
+ [[nodiscard]] _LIBCPP_HIDE_FROM_ABI constexpr void* address() const noexcept { return __handle_; }
private:
_LIBCPP_HIDE_FROM_ABI friend coroutine_handle<noop_coroutine_promise> noop_coroutine() noexcept;
@@ -86,7 +86,9 @@ inline noop_coroutine_handle::__noop_coroutine_frame_ty_ noop_coroutine_handle::
# endif
// [coroutine.noop.coroutine]
-inline _LIBCPP_HIDE_FROM_ABI noop_coroutine_handle noop_coroutine() noexcept { return noop_coroutine_handle(); }
+[[nodiscard]] inline _LIBCPP_HIDE_FROM_ABI noop_coroutine_handle noop_coroutine() noexcept {
+ return noop_coroutine_handle();
+}
_LIBCPP_END_NAMESPACE_STD
diff --git a/libcxx/include/__utility/cmp.h b/libcxx/include/__utility/cmp.h
index 68864e23e0397..7cfe640ceb423 100644
--- a/libcxx/include/__utility/cmp.h
+++ b/libcxx/include/__utility/cmp.h
@@ -31,7 +31,7 @@ concept __comparison_can_promote_to =
sizeof(_Tp) < sizeof(_Ip) || (sizeof(_Tp) == sizeof(_Ip) && __signed_integer<_Tp>);
template <__signed_or_unsigned_integer _Tp, __signed_or_unsigned_integer _Up>
-_LIBCPP_HIDE_FROM_ABI constexpr bool cmp_equal(_Tp __t, _Up __u) noexcept {
+[[nodiscard]] _LIBCPP_HIDE_FROM_ABI constexpr bool cmp_equal(_Tp __t, _Up __u) noexcept {
if constexpr (is_signed_v<_Tp> == is_signed_v<_Up>)
return __t == __u;
else if constexpr (__comparison_can_promote_to<_Tp, int> && __comparison_can_promote_to<_Up, int>)
@@ -45,12 +45,12 @@ _LIBCPP_HIDE_FROM_ABI constexpr bool cmp_equal(_Tp __t, _Up __u) noexcept {
}
template <__signed_or_unsigned_integer _Tp, __signed_or_unsigned_integer _Up>
-_LIBCPP_HIDE_FROM_ABI constexpr bool cmp_not_equal(_Tp __t, _Up __u) noexcept {
+[[nodiscard]] _LIBCPP_HIDE_FROM_ABI constexpr bool cmp_not_equal(_Tp __t, _Up __u) noexcept {
return !std::cmp_equal(__t, __u);
}
template <__signed_or_unsigned_integer _Tp, __signed_or_unsigned_integer _Up>
-_LIBCPP_HIDE_FROM_ABI constexpr bool cmp_less(_Tp __t, _Up __u) noexcept {
+[[nodiscard]] _LIBCPP_HIDE_FROM_ABI constexpr bool cmp_less(_Tp __t, _Up __u) noexcept {
if constexpr (is_signed_v<_Tp> == is_signed_v<_Up>)
return __t < __u;
else if constexpr (__comparison_can_promote_to<_Tp, int> && __comparison_can_promote_to<_Up, int>)
@@ -64,22 +64,22 @@ _LIBCPP_HIDE_FROM_ABI constexpr bool cmp_less(_Tp __t, _Up __u) noexcept {
}
template <__signed_or_unsigned_integer _Tp, __signed_or_unsigned_integer _Up>
-_LIBCPP_HIDE_FROM_ABI constexpr bool cmp_greater(_Tp __t, _Up __u) noexcept {
+[[nodiscard]] _LIBCPP_HIDE_FROM_ABI constexpr bool cmp_greater(_Tp __t, _Up __u) noexcept {
return std::cmp_less(__u, __t);
}
template <__signed_or_unsigned_integer _Tp, __signed_or_unsigned_integer _Up>
-_LIBCPP_HIDE_FROM_ABI constexpr bool cmp_less_equal(_Tp __t, _Up __u) noexcept {
+[[nodiscard]] _LIBCPP_HIDE_FROM_ABI constexpr bool cmp_less_equal(_Tp __t, _Up __u) noexcept {
return !std::cmp_greater(__t, __u);
}
template <__signed_or_unsigned_integer _Tp, __signed_or_unsigned_integer _Up>
-_LIBCPP_HIDE_FROM_ABI constexpr bool cmp_greater_equal(_Tp __t, _Up __u) noexcept {
+[[nodiscard]] _LIBCPP_HIDE_FROM_ABI constexpr bool cmp_greater_equal(_Tp __t, _Up __u) noexcept {
return !std::cmp_less(__t, __u);
}
template <__signed_or_unsigned_integer _Tp, __signed_or_unsigned_integer _Up>
-_LIBCPP_HIDE_FROM_ABI constexpr bool in_range(_Up __u) noexcept {
+[[nodiscard]] _LIBCPP_HIDE_FROM_ABI constexpr bool in_range(_Up __u) noexcept {
return std::cmp_less_equal(__u, numeric_limits<_Tp>::max()) &&
std::cmp_greater_equal(__u, numeric_limits<_Tp>::min());
}
diff --git a/libcxx/include/initializer_list b/libcxx/include/initializer_list
index 00e0d4ea4a2df..44cd45668388b 100644
--- a/libcxx/include/initializer_list
+++ b/libcxx/include/initializer_list
@@ -78,11 +78,17 @@ public:
_LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX14 initializer_list() _NOEXCEPT : __begin_(nullptr), __size_(0) {}
- _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX14 size_t size() const _NOEXCEPT { return __size_; }
+ [[__nodiscard__]] _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX14 size_t size() const _NOEXCEPT {
+ return __size_;
+ }
- _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX14 const _Ep* begin() const _NOEXCEPT { return __begin_; }
+ [[__nodiscard__]] _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX14 const _Ep* begin() const _NOEXCEPT {
+ return __begin_;
+ }
- _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX14 const _Ep* end() const _NOEXCEPT { return __begin_ + __size_; }
+ [[__nodiscard__]] _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX14 const _Ep* end() const _NOEXCEPT {
+ return __begin_ + __size_;
+ }
};
template <class _Ep>
diff --git a/libcxx/test/libcxx/diagnostics/utility.nodiscard.verify.cpp b/libcxx/test/libcxx/diagnostics/utility.nodiscard.verify.cpp
index 524be96736bad..2f5b3ba0fc642 100644
--- a/libcxx/test/libcxx/diagnostics/utility.nodiscard.verify.cpp
+++ b/libcxx/test/libcxx/diagnostics/utility.nodiscard.verify.cpp
@@ -10,8 +10,6 @@
// check that <utility> functions are marked [[nodiscard]]
-// clang-format off
-
#include <utility>
#include "test_macros.h"
@@ -19,15 +17,33 @@
void test() {
int i = 0;
- std::forward<int>(i); // expected-warning {{ignoring return value of function declared with 'nodiscard' attribute}}
- std::forward<int>(1); // expected-warning {{ignoring return value of function declared with 'nodiscard' attribute}}
- std::move(i); // expected-warning {{ignoring return value of function declared with 'nodiscard' attribute}}
- std::move_if_noexcept(i); // expected-warning {{ignoring return value of function declared with 'nodiscard' attribute}}
+ std::forward<int>(i); // expected-warning {{ignoring return value of function declared with 'nodiscard' attribute}}
+ std::forward<int>(1); // expected-warning {{ignoring return value of function declared with 'nodiscard' attribute}}
+ std::move(i); // expected-warning {{ignoring return value of function declared with 'nodiscard' attribute}}
+ // expected-warning at +1 {{ignoring return value of function declared with 'nodiscard' attribute}}
+ std::move_if_noexcept(i);
#if TEST_STD_VER >= 17
std::as_const(i); // expected-warning {{ignoring return value of function declared with 'nodiscard' attribute}}
#endif
+#if TEST_STD_VER >= 20
+ // expected-warning at +1 {{ignoring return value of function declared with 'nodiscard' attribute}}
+ std::cmp_equal(94, 82);
+ // expected-warning at +1 {{ignoring return value of function declared with 'nodiscard' attribute}}
+ std::cmp_not_equal(94, 82);
+ // expected-warning at +1 {{ignoring return value of function declared with 'nodiscard' attribute}}
+ std::cmp_less(94, 82);
+ // expected-warning at +1 {{ignoring return value of function declared with 'nodiscard' attribute}}
+ std::cmp_greater(94, 82);
+ // expected-warning at +1 {{ignoring return value of function declared with 'nodiscard' attribute}}
+ std::cmp_less_equal(94, 82);
+ // expected-warning at +1 {{ignoring return value of function declared with 'nodiscard' attribute}}
+ std::cmp_greater_equal(94, 82);
+ // expected-warning at +1 {{ignoring return value of function declared with 'nodiscard' attribute}}
+ std::in_range<long>(49);
+#endif
+
#if TEST_STD_VER >= 23
enum E { Apple, Orange } e = Apple;
std::to_underlying(e); // expected-warning {{ignoring return value of function declared with 'nodiscard' attribute}}
diff --git a/libcxx/test/libcxx/language.support/nodiscard.verify.cpp b/libcxx/test/libcxx/language.support/nodiscard.verify.cpp
new file mode 100644
index 0000000000000..fb0676dc7cadb
--- /dev/null
+++ b/libcxx/test/libcxx/language.support/nodiscard.verify.cpp
@@ -0,0 +1,94 @@
+//===----------------------------------------------------------------------===//
+//
+// 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
+
+// check that <utility> functions are marked [[nodiscard]]
+
+#include <compare>
+#include <coroutine>
+#include <functional>
+#include <hash>
+#include <initializer_list>
+
+#include "test_macros.h"
+
+void test() {
+#if TEST_STD_VER >= 20
+ { // <compare>
+ int x = 94;
+ int y = 82;
+ auto oRes = x <=> y;
+
+ std::is_eq(oRes); // expected-warning {{ignoring return value of function declared with 'nodiscard' attribute}}
+ std::is_neq(oRes); // expected-warning {{ignoring return value of function declared with 'nodiscard' attribute}}
+ std::is_lt(oRes); // expected-warning {{ignoring return value of function declared with 'nodiscard' attribute}}
+ std::is_lteq(oRes); // expected-warning {{ignoring return value of function declared with 'nodiscard' attribute}}
+ std::is_gt(oRes); // expected-warning {{ignoring return value of function declared with 'nodiscard' attribute}}
+ std::is_gteq(oRes); // expected-warning {{ignoring return value of function declared with 'nodiscard' attribute}}
+ }
+#endif
+
+#if TEST_STD_VER >= 20
+ { // <coroutine>
+ struct EmptyPromise {
+ } promise;
+
+ {
+ std::coroutine_handle<void> cr{};
+
+ // expected-warning at +1 {{ignoring return value of function declared with 'nodiscard' attribute}}
+ cr.address();
+ // expected-warning at +1 {{ignoring return value of function declared with 'nodiscard' attribute}}
+ std::coroutine_handle<void>::from_address(&promise);
+ // expected-warning at +1 {{ignoring return value of function declared with 'nodiscard' attribute}}
+ cr.done();
+
+ std::hash<std::coroutine_handle<void>> hash;
+ hash(cr); // expected-warning {{ignoring return value of function declared with 'nodiscard' attribute}}
+ }
+ {
+ std::coroutine_handle<EmptyPromise> cr;
+
+ // expected-warning at +1 {{ignoring return value of function declared with 'nodiscard' attribute}}
+ std::coroutine_handle<EmptyPromise>::from_promise(promise);
+ // expected-warning at +1 {{ignoring return value of function declared with 'nodiscard' attribute}}
+ cr.address();
+ // expected-warning at +1 {{ignoring return value of function declared with 'nodiscard' attribute}}
+ std::coroutine_handle<EmptyPromise>::from_address(&promise);
+ // expected-warning at +1 {{ignoring return value of function declared with 'nodiscard' attribute}}
+ cr.done();
+ // expected-warning at +1 {{ignoring return value of function declared with 'nodiscard' attribute}}
+ cr.promise();
+ }
+ {
+ std::coroutine_handle<std::noop_coroutine_promise> cr = std::noop_coroutine();
+
+ // expected-warning at +1 {{ignoring return value of function declared with 'nodiscard' attribute}}
+ cr.done();
+ // expected-warning at +1 {{ignoring return value of function declared with 'nodiscard' attribute}}
+ cr.promise();
+ // expected-warning at +1 {{ignoring return value of function declared with 'nodiscard' attribute}}
+ cr.address();
+
+ // expected-warning at +1 {{ignoring return value of function declared with 'nodiscard' attribute}}
+ std::noop_coroutine();
+ }
+ }
+#endif
+
+#if TEST_STD_VER >= 20
+ { // <initializer_list>
+ std::initializer_list<int> il{94, 82, 49};
+
+ il.size(); // expected-warning {{ignoring return value of function declared with 'nodiscard' attribute}}
+ il.begin(); // expected-warning {{ignoring return value of function declared with 'nodiscard' attribute}}
+ il.end(); // expected-warning {{ignoring return value of function declared with 'nodiscard' attribute}}
+ }
+#endif
+}
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