(From submission #574. See also coroutines-ts#17 and coroutines-ts#30.)

Subclause 9.5.4 [dcl.fct.def.coroutine] paragraph 14 specifies:

If the evaluation of the expression promise.unhandled_exception() exits via an exception, the coroutine is considered suspended at the final suspend point and the exception propagates to the caller or resumer.

However, implementations destroy the promise object, the parameter copies, and the coroutine state before propagating the exception, which is consistent with the as-if code presented in 9.5.4 [dcl.fct.def.coroutine] paragraph 5.