[WIP] mandate that a `unhandled_stopped` be `noexcept`

[ericniebler]

in [exec.as.awaitable], the concept awaitable-sender is true for senders that can be adapted into awaitables. it is specified as:

template<class Sndr, class Promise>
  concept awaitable-sender =
    single-sender<Sndr, env_of_t<Promise>> &&
    sender-to<Sndr,
              typename sender-awaitable<Sndr, Promise>::awaitable-receiver> &&    // see below
    requires (Promise& p) {
      { p.unhandled_stopped() } -> convertible_to<coroutine_handle<>>;
    };

this says a sender can only be awaitable if the promise type provides an unhandled_stopped() member function that returns a coroutine_handle.

it does not require or mandate that unhandled_stopped() be noexcept.

since it is likely to be only called from a receiver's set_stopped member function (which is required to be noexcept), there is nothing to be done with an exception thrown from unhandled_stopped() except terminate.

[ericniebler]

on the other hand, unhandled_exception was originally required to be noexcept, but we talked Gor out of that. many uses of coroutines are synchronous (eg, std::generator), so throwing is safe. a noexcept unhandled_exception forces the exception to be saved as an exception_ptr and only later thrown. that resulted in significantly worse codegen for std::generator.

could there be a similar argument for unhandled_stopped()?