Is it permissible for a coroutine's promise_type to not have return_void or return_value, as long as the coroutine body does not conclude abruptly?

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Yes, it is permissible for the promisetype of a coroutine to not have returnvoid or returnvalue, as long as the coroutine body does not conclude abruptly. In this case, the compiler will assume that the coroutine returns void, and it will not generate any code for the return statement. However, it is recommended to always have either returnvoid or returnvalue in the promisetype, as it makes the intent of the coroutine more clear and avoids any confusion or unintended behavior.