[libc++] Mark std::expected as nodiscard #139651
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@llvm/pr-subscribers-clang @llvm/pr-subscribers-libcxx Author: None (halbi2) ChangesFixes #130656 @hulxv seems no longer to be active, this patch version fixes the errors in the test suite from #130820. Patch is 39.58 KiB, truncated to 20.00 KiB below, full version: https://github.com/llvm/llvm-project/pull/139651.diff 8 Files Affected:
diff --git a/libcxx/include/__expected/expected.h b/libcxx/include/__expected/expected.h
index 0f446b870723b..1ecf46c879bd6 100644
--- a/libcxx/include/__expected/expected.h
+++ b/libcxx/include/__expected/expected.h
@@ -448,7 +448,7 @@ class __expected_base {
};
template <class _Tp, class _Err>
-class expected : private __expected_base<_Tp, _Err> {
+class [[nodiscard]] expected : private __expected_base<_Tp, _Err> {
static_assert(!is_reference_v<_Tp> && !is_function_v<_Tp> && !is_same_v<remove_cv_t<_Tp>, in_place_t> &&
!is_same_v<remove_cv_t<_Tp>, unexpect_t> && !__is_std_unexpected<remove_cv_t<_Tp>>::value &&
__valid_std_unexpected<_Err>::value,
@@ -1377,7 +1377,7 @@ class __expected_void_base {
template <class _Tp, class _Err>
requires is_void_v<_Tp>
-class expected<_Tp, _Err> : private __expected_void_base<_Err> {
+class [[nodiscard]] expected<_Tp, _Err> : private __expected_void_base<_Err> {
static_assert(__valid_std_unexpected<_Err>::value,
"[expected.void.general] A program that instantiates expected<T, E> with a E that is not a "
"valid argument for unexpected<E> is ill-formed");
diff --git a/libcxx/test/libcxx/utilities/expected/expected.expected/and_then.mandates.verify.cpp b/libcxx/test/libcxx/utilities/expected/expected.expected/and_then.mandates.verify.cpp
index c46ab633295c1..7c95fc19cd693 100644
--- a/libcxx/test/libcxx/utilities/expected/expected.expected/and_then.mandates.verify.cpp
+++ b/libcxx/test/libcxx/utilities/expected/expected.expected/and_then.mandates.verify.cpp
@@ -51,7 +51,7 @@ void test() {
// U is not a specialization of std::expected
{
std::expected<int, int> f1(1);
- f1.and_then(lval_return_not_std_expected); // expected-note{{in instantiation of function template specialization 'std::expected<int, int>::and_then<int (&)(int &)>' requested here}}
+ (void)f1.and_then(lval_return_not_std_expected); // expected-note{{in instantiation of function template specialization 'std::expected<int, int>::and_then<int (&)(int &)>' requested here}}
// expected-error-re@*:* {{static assertion failed {{.*}}The result of f(value()) must be a specialization of std::expected}}
// expected-error-re@*:* {{{{.*}}cannot be used prior to '::' because it has no members}}
// expected-error-re@*:* {{no matching constructor for initialization of{{.*}}}}
@@ -60,7 +60,7 @@ void test() {
// !std::is_same_v<U:error_type, E>
{
std::expected<int, int> f1(1);
- f1.and_then(lval_error_type_not_same_as_int); // expected-note{{in instantiation of function template specialization 'std::expected<int, int>::and_then<std::expected<int, NotSameAsInt> (&)(int &)>' requested here}}
+ (void)f1.and_then(lval_error_type_not_same_as_int); // expected-note{{in instantiation of function template specialization 'std::expected<int, int>::and_then<std::expected<int, NotSameAsInt> (&)(int &)>' requested here}}
// expected-error-re@*:* {{static assertion failed {{.*}}The result of f(value()) must have the same error_type as this expected}}
}
}
@@ -70,7 +70,7 @@ void test() {
// U is not a specialization of std::expected
{
const std::expected<int, int> f1(1);
- f1.and_then(clval_return_not_std_expected); // expected-note{{in instantiation of function template specialization 'std::expected<int, int>::and_then<int (&)(const int &)>' requested here}}
+ (void)f1.and_then(clval_return_not_std_expected); // expected-note{{in instantiation of function template specialization 'std::expected<int, int>::and_then<int (&)(const int &)>' requested here}}
// expected-error-re@*:* {{static assertion failed {{.*}}The result of f(value()) must be a specialization of std::expected}}
// expected-error-re@*:* {{{{.*}}cannot be used prior to '::' because it has no members}}
// expected-error-re@*:* {{no matching constructor for initialization of{{.*}}}}
@@ -79,7 +79,7 @@ void test() {
// !std::is_same_v<U:error_type, E>
{
const std::expected<int, int> f1(1);
- f1.and_then(clval_error_type_not_same_as_int); // expected-note{{in instantiation of function template specialization 'std::expected<int, int>::and_then<std::expected<int, NotSameAsInt> (&)(const int &)>' requested here}}
+ (void)f1.and_then(clval_error_type_not_same_as_int); // expected-note{{in instantiation of function template specialization 'std::expected<int, int>::and_then<std::expected<int, NotSameAsInt> (&)(const int &)>' requested here}}
// expected-error-re@*:* {{static assertion failed {{.*}}The result of f(value()) must have the same error_type as this expected}}
}
@@ -90,7 +90,7 @@ void test() {
// U is not a specialization of std::expected
{
std::expected<int, int> f1(1);
- std::move(f1).and_then(rval_return_not_std_expected); // expected-note{{in instantiation of function template specialization 'std::expected<int, int>::and_then<int (&)(int &&)>' requested here}}
+ (void)std::move(f1).and_then(rval_return_not_std_expected); // expected-note{{in instantiation of function template specialization 'std::expected<int, int>::and_then<int (&)(int &&)>' requested here}}
// expected-error-re@*:* {{static assertion failed {{.*}}The result of f(std::move(value())) must be a specialization of std::expected}}
// expected-error-re@*:* {{{{.*}}cannot be used prior to '::' because it has no members}}
// expected-error-re@*:* {{no matching constructor for initialization of{{.*}}}}
@@ -99,7 +99,7 @@ void test() {
// !std::is_same_v<U:error_type, E>
{
std::expected<int, int> f1(1);
- std::move(f1).and_then(rval_error_type_not_same_as_int); // expected-note{{in instantiation of function template specialization 'std::expected<int, int>::and_then<std::expected<int, NotSameAsInt> (&)(int &&)>' requested here}}
+ (void)std::move(f1).and_then(rval_error_type_not_same_as_int); // expected-note{{in instantiation of function template specialization 'std::expected<int, int>::and_then<std::expected<int, NotSameAsInt> (&)(int &&)>' requested here}}
// expected-error-re@*:* {{static assertion failed {{.*}}The result of f(std::move(value())) must have the same error_type as this expected}}
}
}
@@ -109,7 +109,7 @@ void test() {
// U is not a specialization of std::expected
{
const std::expected<int, int> f1(1);
- std::move(f1).and_then(crval_return_not_std_expected); // expected-note{{in instantiation of function template specialization 'std::expected<int, int>::and_then<int (&)(const int &&)>' requested here}}
+ (void)std::move(f1).and_then(crval_return_not_std_expected); // expected-note{{in instantiation of function template specialization 'std::expected<int, int>::and_then<int (&)(const int &&)>' requested here}}
// expected-error-re@*:* {{static assertion failed {{.*}}The result of f(std::move(value())) must be a specialization of std::expected}}
// expected-error-re@*:* {{{{.*}}cannot be used prior to '::' because it has no members}}
// expected-error-re@*:* {{no matching constructor for initialization of{{.*}}}}
@@ -118,7 +118,7 @@ void test() {
// !std::is_same_v<U:error_type, E>
{
const std::expected<int, int> f1(1);
- std::move(f1).and_then(crval_error_type_not_same_as_int); // expected-note{{in instantiation of function template specialization 'std::expected<int, int>::and_then<std::expected<int, NotSameAsInt> (&)(const int &&)>' requested here}}
+ (void)std::move(f1).and_then(crval_error_type_not_same_as_int); // expected-note{{in instantiation of function template specialization 'std::expected<int, int>::and_then<std::expected<int, NotSameAsInt> (&)(const int &&)>' requested here}}
// expected-error-re@*:* {{static assertion failed {{.*}}The result of f(std::move(value())) must have the same error_type as this expected}}
}
}
diff --git a/libcxx/test/libcxx/utilities/expected/expected.expected/nodiscard.verify.cpp b/libcxx/test/libcxx/utilities/expected/expected.expected/nodiscard.verify.cpp
new file mode 100644
index 0000000000000..ea5d51cdf051c
--- /dev/null
+++ b/libcxx/test/libcxx/utilities/expected/expected.expected/nodiscard.verify.cpp
@@ -0,0 +1,23 @@
+//===----------------------------------------------------------------------===//
+//
+// 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++23
+
+// <expected>
+
+// Test that ignoring std::expected generates [[nodiscard]] warnings.
+
+#include <expected>
+
+std::expected<int, int> returns_expected();
+std::expected<void, int> returns_expected_void();
+
+void test() {
+ returns_expected(); // expected-warning {{ignoring return value of type 'expected<int, int>'}}
+ returns_expected_void(); // expected-warning {{ignoring return value of type 'expected<void, int>'}}
+}
diff --git a/libcxx/test/libcxx/utilities/expected/expected.expected/or_else.mandates.verify.cpp b/libcxx/test/libcxx/utilities/expected/expected.expected/or_else.mandates.verify.cpp
index af1fa53307960..8064463977e48 100644
--- a/libcxx/test/libcxx/utilities/expected/expected.expected/or_else.mandates.verify.cpp
+++ b/libcxx/test/libcxx/utilities/expected/expected.expected/or_else.mandates.verify.cpp
@@ -51,7 +51,7 @@ void test() {
// G is not a specialization of std::expected
{
std::expected<int, int> f1(std::unexpected<int>(1));
- f1.or_else(lval_return_not_std_expected); // expected-note{{in instantiation of function template specialization 'std::expected<int, int>::or_else<int (&)(int &)>' requested here}}
+ (void)f1.or_else(lval_return_not_std_expected); // expected-note{{in instantiation of function template specialization 'std::expected<int, int>::or_else<int (&)(int &)>' requested here}}
// expected-error-re@*:* {{static assertion failed {{.*}}The result of f(error()) must be a specialization of std::expected}}
// expected-error-re@*:* {{{{.*}}cannot be used prior to '::' because it has no members}}
// expected-error-re@*:* {{no matching constructor for initialization of{{.*}}}}
@@ -60,7 +60,7 @@ void test() {
// !std::is_same_v<G:value_type, T>
{
std::expected<int, int> f1(std::unexpected<int>(1));
- f1.or_else(lval_error_type_not_same_as_int); // expected-note{{in instantiation of function template specialization 'std::expected<int, int>::or_else<std::expected<NotSameAsInt, int> (&)(int &)>' requested here}}
+ (void)f1.or_else(lval_error_type_not_same_as_int); // expected-note{{in instantiation of function template specialization 'std::expected<int, int>::or_else<std::expected<NotSameAsInt, int> (&)(int &)>' requested here}}
// expected-error-re@*:* {{static assertion failed {{.*}}The result of f(error()) must have the same value_type as this expected}}
}
}
@@ -70,7 +70,7 @@ void test() {
// G is not a specialization of std::expected
{
const std::expected<int, int> f1(std::unexpected<int>(1));
- f1.or_else(clval_return_not_std_expected); // expected-note{{in instantiation of function template specialization 'std::expected<int, int>::or_else<int (&)(const int &)>' requested here}}
+ (void)f1.or_else(clval_return_not_std_expected); // expected-note{{in instantiation of function template specialization 'std::expected<int, int>::or_else<int (&)(const int &)>' requested here}}
// expected-error-re@*:* {{static assertion failed {{.*}}The result of f(error()) must be a specialization of std::expected}}
// expected-error-re@*:* {{{{.*}}cannot be used prior to '::' because it has no members}}
// expected-error-re@*:* {{no matching constructor for initialization of{{.*}}}}
@@ -79,7 +79,7 @@ void test() {
// !std::is_same_v<G:value_type, T>
{
const std::expected<int, int> f1(std::unexpected<int>(1));
- f1.or_else(clval_error_type_not_same_as_int); // expected-note{{in instantiation of function template specialization 'std::expected<int, int>::or_else<std::expected<NotSameAsInt, int> (&)(const int &)>' requested here}}
+ (void)f1.or_else(clval_error_type_not_same_as_int); // expected-note{{in instantiation of function template specialization 'std::expected<int, int>::or_else<std::expected<NotSameAsInt, int> (&)(const int &)>' requested here}}
// expected-error-re@*:* {{static assertion failed {{.*}}The result of f(error()) must have the same value_type as this expected}}
}
}
@@ -89,7 +89,7 @@ void test() {
// G is not a specialization of std::expected
{
std::expected<int, int> f1(std::unexpected<int>(1));
- std::move(f1).or_else(rval_return_not_std_expected); // expected-note{{in instantiation of function template specialization 'std::expected<int, int>::or_else<int (&)(int &&)>' requested here}}
+ (void)std::move(f1).or_else(rval_return_not_std_expected); // expected-note{{in instantiation of function template specialization 'std::expected<int, int>::or_else<int (&)(int &&)>' requested here}}
// expected-error-re@*:* {{static assertion failed {{.*}}The result of f(std::move(error())) must be a specialization of std::expected}}
// expected-error-re@*:* {{{{.*}}cannot be used prior to '::' because it has no members}}
// expected-error-re@*:* {{no matching constructor for initialization of{{.*}}}}
@@ -98,7 +98,7 @@ void test() {
// !std::is_same_v<G:value_type, T>
{
std::expected<int, int> f1(std::unexpected<int>(1));
- std::move(f1).or_else(rval_error_type_not_same_as_int); // expected-note{{in instantiation of function template specialization 'std::expected<int, int>::or_else<std::expected<NotSameAsInt, int> (&)(int &&)>' requested here}}
+ (void)std::move(f1).or_else(rval_error_type_not_same_as_int); // expected-note{{in instantiation of function template specialization 'std::expected<int, int>::or_else<std::expected<NotSameAsInt, int> (&)(int &&)>' requested here}}
// expected-error-re@*:* {{static assertion failed {{.*}}The result of f(std::move(error())) must have the same value_type as this expected}}
}
}
@@ -108,7 +108,7 @@ void test() {
// G is not a specialization of std::expected
{
const std::expected<int, int> f1(std::unexpected<int>(1));
- std::move(f1).or_else(crval_return_not_std_expected); // expected-note{{in instantiation of function template specialization 'std::expected<int, int>::or_else<int (&)(const int &&)>' requested here}}
+ (void)std::move(f1).or_else(crval_return_not_std_expected); // expected-note{{in instantiation of function template specialization 'std::expected<int, int>::or_else<int (&)(const int &&)>' requested here}}
// expected-error-re@*:* {{static assertion failed {{.*}}The result of f(std::move(error())) must be a specialization of std::expected}}
// expected-error-re@*:* {{{{.*}}cannot be used prior to '::' because it has no members}}
// expected-error-re@*:* {{no matching constructor for initialization of{{.*}}}}
@@ -117,7 +117,7 @@ void test() {
// !std::is_same_v<G:value_type, T>
{
const std::expected<int, int> f1(std::unexpected<int>(1));
- std::move(f1).or_else(crval_error_type_not_same_as_int); // expected-note{{in instantiation of function template specialization 'std::expected<int, int>::or_else<std::expected<NotSameAsInt, int> (&)(const int &&)>' requested here}}
+ (void)std::move(f1).or_else(crval_error_type_not_same_as_int); // expected-note{{in instantiation of function template specialization 'std::expected<int, int>::or_else<std::expected<NotSameAsInt, int> (&)(const int &&)>' requested here}}
// expected-error-re@*:* {{static assertion failed {{.*}}The result of f(std::move(error())) must have the same value_type as this expected}}
}
}
diff --git a/libcxx/test/libcxx/utilities/expected/expected.expected/transform_error.mandates.verify.cpp b/libcxx/test/libcxx/utilities/expected/expected.expected/transform_error.mandates.verify.cpp
index 61374094b7adf..4c8b4999375af 100644
--- a/libcxx/test/libcxx/utilities/expected/expected.expected/transform_error.mandates.verify.cpp
+++ b/libcxx/test/libcxx/utilities/expected/expected.expected/transform_error.mandates.verify.cpp
@@ -58,12 +58,12 @@ void test() {
// Test & overload
{
std::expected<int, int> e;
- e.transform_error(return_unexpected<int&>); // expected-error-re@*:* {{static assertion failed {{.*}}The result of {{.*}} must be a valid template argument for unexpected}}
+ (void)e.transform_error(return_unexpected<int&>); // expected-error-re@*:* {{static assertion failed {{.*}}The result of {{.*}} must be a valid template argument for unexpected}}
// expected-error-re@*:* 0-1 {{{{(excess elements in struct initializer|no matching constructor for initialization of)}}{{.*}}}}
// expected-error-re@*:* {{static assertion failed {{.*}}[expected.object.general] A program that instantiates the definition of template expected<T, E> for {{.*}} is ill-formed.}}
// expected-error-re@*:* 0-1 {{union member {{.*}} has reference type {{.*}}}}
- e.transform_error(return_no_object<int&>); // expected-error-re@*:* {{static assertion failed {{.*}}The result of {{.*}} must be a valid template argument for unexpected}}
+ (void)e.transform_error(return_no_object<int&>); // expected-error-re@*:* {{static assertion failed {{.*}}The result of {{.*}} must be a valid template argument for unexpected}}
// expected-error-re@*:* 0-1 {{{{(excess elements in struct initializer|no matching constructor for initialization of)}}{{.*}}}}
// expected-error-re@*:* {{static assertion failed {{.*}}[expected.object.general] A program that instantiates the definition of template expected<T, E> for {{.*}} is ill-formed.}}
// expected-warning-re@*:* 0-1 {{union member {{.*}} has reference type {{.*}}, which is a Microsoft extension}}
@@ -72,27 +72,27 @@ void test() {
// Test const& overload
{
const std::expected<int, int> e;
- e.transform_error(return_unexpected<const int &>); // expected-error-re@*:* {{static assertion failed {{.*}}The result of {{.*}} must be a valid template argument for unexpected}}
+ (void)e.transform_error(return_unexpected<const int &>); // expected-error-re@*:* {{static assertion failed {{.*}}The result of {{.*}} must be a valid template argument for unexpected}}
// expected-error-re@*:* 0-2 {{{{(excess elements in struct initializer|no matching constructor for initialization of)}}{{.*}}}}
- e.transform_error(return_no_object<const int &>); // expected-error-re@*:* {{static assertion failed {{.*}}The result of {{.*}} must be a valid template argument for unexpected}}
+ (void)e.transform_error(return_no_object<const int &>); // expected-error-re@*:* {{static assertion failed {{.*}}The result of {{.*}} must be a valid template argument for unexpected}}
// expected-error-re@*:* 0-2 {{{{(excess elements in struct initializer|no matching constructor for initialization of)}}{{.*}}}}
}
// Test && overload
{
std::expected<int, int> e;
- std::move(e).transform_error(return_unexpected<int&&>); // expected-error-re@*:* {{static assertion failed {{.*}}The result of {{.*}} must be a valid template argument for unexpected}}
+ (void)std::move(e).transform_error(return_unexpected<int&&>); // expected-error-re@*:* {{static assertion failed {{.*}}The result of {{.*}} must be a valid template argument for unexpected}}
// expected-error-re@*:* 0-2 {{{{(excess elements in struct initializer|no matching constructor for initialization of)}}{{.*}}}}
- std::move(e).transform_error(return_no_object<int&&>); // expected-error-re@*:* {{static assertion failed {{.*}}The result of {{.*}} must be a valid template argument for unexpected}}
+ (void)std::move(e).transform_error(return_no_object<int&&>); // expected-error-re@*:* {{static assertion failed {{.*}}The result of {{.*}} must be a valid template argument for unexpected}}
// expected-error-re@*:* 0-2 {{{{(excess elements in struct initializer|no matching constructor...
[truncated]
|
|
✅ With the latest revision this PR passed the C/C++ code formatter. |
philnik777
requested changes
May 13, 2025
|
@philnik777 oh, I missed that consensus, I saw the comments from @zmodem and @hulxv about LLVM and Rust and from @strega-nil-re and @StephanTLavavej about Microsoft C++. Now there is a way to disable the warning. @huixie90 what do you think, are you willing to "give it a try?" |
What way? |
The programmer can pass |
That doesn't allow you to discard on a per-function basis. |
llvmbot
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Aug 22, 2025
Fulfills the requirement that @huixie90 stated in llvm#139651 that Clang should have a way to disable [[nodiscard]] on a function by function basis. This will allow us finally to resume fixing llvm#130656.
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Fixes #130656
@hulxv seems no longer to be active, this patch version fixes the errors in the test suite from #130820.