Oh, choosing a good error message is hard :(

I've a few comments.  Sorry, this thread is growing rather rapidly, but please help me to pick one of them:


> From the point of view of the error message, the reason I changed my suggestion was because I liked your idea of optimising it for the "only using await" case and trusting that the folks delving into the lower level plumbing of calling methods manually can figure it out. [...]

We either have to use a sub-optimal error message tailored for 'await' expression users, or we can choose a longer error message.  I'm not very fond of this option, but maybe it's because I can't come up with something long and concise at the same time.  Maybe something along the lines of: "cannot resume terminated coroutine (was it awaited on before?)"


> That is, "Cannot resume terminated coroutine" is always accurate, but relies on the reader knowing that "await" both resumes a coroutine and waits for it to terminate.

Not sure I like the "terminated" word here.  The coroutine was either awaited before, which means that the coroutine object is now exhausted, or it was manually closed at some point.  To me "terminated" is closer to the latter.


> "Coroutine was previously awaited on" may be wrong about how the coroutine was originally terminated, but at least hints that the error may be related to awaiting the coroutine.

Agree.


> "Cannot resume previously awaited coroutine" would be inaccurate under the same circumstances.

Thinking about "resume"...  If a user sees this message pointing to an "await" expression, it might confuse them, since "await coro" does not just resume "coro".  It inits and consumes "coro" until it raises a StopIteration.


"Cannot await previously awaited coroutine" would only be entirely accurate for "double await" errors, but doesn't rely on the reader making any assumptions at all in that case.

Agree.  But I don't like that we have two "await" words in one short message.


> (The bulk of the problem here is that my brain is happy to accept "awaited" as a novel adjective modifying "coroutine", but balks at "awaited" as a verb or "awaited on" as a verb phrase. I'm extrapolating from that to guess that other folks would find the verb form similarly jarring)

I trust your brain here, you're a native speaker ;)  If you think that "Cannot await previously awaited coroutine" is the best option here, let's stick to it.