https://gcc.gnu.org/bugzilla/show_bug.cgi?id=116502
Bug ID: 116502
Summary: [15 Regression] -Wunused-result warning is not
supressed if coroutine awaiter returns a reference
Product: gcc
Version: 15.0
Status: UNCONFIRMED
Keywords: c++-coroutines
Severity: normal
Priority: P3
Component: c++
Assignee: unassigned at gcc dot gnu.org
Reporter: daklishch at gmail dot com
CC: arsen at gcc dot gnu.org
Target Milestone: ---
After 2664c1bf83855b79d8c43772e71b87ed4f09c174 (cp+coroutines: teach
convert_to_void to diagnose discarded co_awaits), the following code produces
-Wunused-result warning, while the caller clearly wants to suppress it by
casting the co_await result to void.
```
$ cat ../test.cpp
#include <coroutine>
struct SuspendNever {
bool await_ready() noexcept;
void await_suspend(std::coroutine_handle<>) noexcept;
void await_resume() noexcept;
};
struct Coroutine;
struct PromiseType {
Coroutine get_return_object();
SuspendNever initial_suspend();
SuspendNever final_suspend() noexcept;
void return_void();
};
struct Coroutine {
using promise_type = PromiseType;
};
struct Awaiter {
bool await_ready();
void await_suspend(std::coroutine_handle<>);
[[nodiscard]] int& await_resume();
};
Coroutine foo()
{
(void)co_await Awaiter {};
}
$ ~/gcc-15-trunk/bin/g++ ../test.cpp -c -std=c++20 -fno-exceptions
-Wunused-result -o /dev/null
../test.cpp: In function ‘Coroutine foo()’:
../test.cpp:30:29: warning: ignoring return value of ‘int&
Awaiter::await_resume()’, declared with attribute ‘nodiscard’ [-Wunused-result]
30 | (void)co_await Awaiter {};
| ^
../test.cpp:25:24: note: declared here
25 | [[nodiscard]] int& await_resume();
| ^~~~~~~~~~~~
```
The warning is correctly suppressed unless Awaiter::await_resume returns a
reference.
