[Bug c++/109867] New: -Wswicht-default reports missing default in coroutine

[lukaslang.bugtracker at outlook dot com via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109867

Bug ID: 109867
   Summary: -Wswicht-default reports missing default in coroutine
   Product: gcc
   Version: 14.0
Status: UNCONFIRMED
  Severity: normal
  Priority: P3
 Component: c++
  Assignee: unassigned at gcc dot gnu.org
  Reporter: lukaslang.bugtracker at outlook dot com
  Target Milestone: ---

Consider the following implementation of a simple coroutine
(https://godbolt.org/z/rcevTd5f6):

#include 

struct task
{
struct promise_type
{
std::suspend_always initial_suspend();
std::suspend_always final_suspend() noexcept;
void unhandled_exception();
task get_return_object();
void return_value(int);
};
};

int main()
{
auto t = []() -> task
{
co_return 2;
}();
} 

Compiling this with -std=c++20 -Wswitch-default -Werror results in an error at
the end of the coroutine body:

:20:5: error: switch missing default case [-Werror=switch-default]
   20 | }();

Since I can get neither clang nor msvc to complain, I assume this is a bug, or
am I missing something? If this is indeed a bug, can I work around this without
having to disable the warning?

[Bug c++/110171] New: [[nodiscard]] of await_resume ignored when discarding result of co_await expression

[lukaslang.bugtracker at outlook dot com via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110171

Bug ID: 110171
   Summary: [[nodiscard]] of await_resume ignored when discarding
result of co_await expression
   Product: gcc
   Version: 11.3.0
Status: UNCONFIRMED
  Severity: normal
  Priority: P3
 Component: c++
  Assignee: unassigned at gcc dot gnu.org
  Reporter: lukaslang.bugtracker at outlook dot com
  Target Milestone: ---

(Note: The example & bug is very similar to this here:
https://developercommunity.visualstudio.com/t/nodiscard-not-honored-by-co_await/1500944)

It seems that discarding the `co_await`ed result of an awaitable with
`[[nodiscard]]` `await_resume` doesn't lead to the expected warning
([Godbolt](https://godbolt.org/z/o47EdzMj8)):

```
#include 

struct must_check_result
{
bool await_ready() { return false; }
void await_suspend(std::coroutine_handle<>) {}
[[nodiscard]] bool await_resume() { return {}; }
};

struct task {};

namespace std
{
template
struct coroutine_traits
{
struct promise_type
{
task get_return_object() { return {}; }
suspend_always initial_suspend() noexcept { return {}; }
suspend_always final_suspend() noexcept { return {}; }
void return_void() {}
void unhandled_exception() {}
};
};
}

task example()
{
co_await must_check_result(); // no warning even though await_ready is
marked [[nodiscard]]
must_check_result().await_resume();
}
```