For the awaitable returned by `as_awaitable`, conditionally mark `await_resume` as `[[noreturn]]`

[ericniebler]

Issue by ericniebler
Wednesday Oct 06, 2021 at 16:51 GMT
Originally opened as NVIDIA/stdexec#210

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In R2, the specification of as_awaitable reads:

• Otherwise, if sender_traits<remove_cvref_t<S>>::value_types<Tuple, Variant> would have the form Variant<Tuple<>> or Variant<>, then single-sender-value-type<S> is an alias for type void.

@lewissbaker wonders:

I wonder if the result is Variant<> whether we should specify that the await_resume() method of the returned awaitable will be tagged as [[noreturn]]?