[Bug c++/100127] [coroutines] internal compiler error compiling promise with custom awaiter

[iains at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=100127

--- Comment #4 from Iain Sandoe <iains at gcc dot gnu.org> ---
(In reply to Riccardo Brugo from comment #0)
> Created attachment 50621 [details]

Of course, we should not ICE - but ...

>     struct promise_type {
>         std::optional<value_type> _value = std::nullopt;
> 
>         future get_return_object() {
>             return future{handle_type::from_promise(*this)};
>         }
>         void return_value(value_type val) {
>             _value = static_cast<value_type &&>(val);
>         }
>         auto initial_suspend() noexcept {
>             class awaiter {
>                 std::optional<value_type> & value;
>             public:
>                 explicit awaiter(std::optional<value_type> & val) noexcept :
> value{val} {}
>                 bool await_ready() noexcept { return value.has_value(); }
>                 void await_suspend(handle_type) noexcept { }
>                 value_type & await_resume() noexcept { return *value; }
>             };
> 
>             return awaiter{_value};
>         }

The ICE is caused by the code handling a reference from the co_await - where it
was not expecting any value to be returned from the initial or final
await_resume() functions.

---

How do you expect to use the value returned by value_type & await_resume()
noexcept { return *value; } in this?

see: https://eel.is/c++draft/dcl.fct.def.coroutine#5

essentially the value(s) of the initial and final co_await expressions are
discarded.  So of course, you can have any side-effect you like from the
evaluation of the expressions - but there is no way for you to see the result
of the await_resume().

Ergo it might as well be void...

however, let's fix the ICE anyway.