hmm, fn select_all_or_error is currently implemented as

    fn select_all_or_error(&mut self, infcx: &InferCtxt<'tcx>) -> Vec<E> {
        let errors = self.select_where_possible(infcx);
        if !errors.is_empty() {
            return errors;
        }

        self.collect_remaining_errors(infcx)
    }

it feels a bit footgunny to not add the overflowing obligations if there are other errors. I think changing the if errors.is_empty() branch to also add the remaining errors would be better 🤔

or no, in case select_where_possible returns an error, we want to ignore overflow and ambiguity errors. I think we also want to ignore these ambiguity errors here, so maybe change this to

            let errors = fulfill_cx.select_all_or_error(self.infcx);
            if errors.is_empty() {
                Ok(self.infcx.resolve_vars_if_possible(value))
            } else {
                // In case normalization resulted in an error, we want to
                // drain any still ambiguous goal to avoid triggering the
                // assertion on the next iteration.
                let _ = fulfill_cx.collect_remaining_errors(self.infcx));
                Err(errors)
            }

alternatively we could drop the remaining obligations in the !errors.is_empty() branch in select_all_or_error itself? 🤔

alternatively we could drop the remaining obligations in the !errors.is_empty() branch in select_all_or_error itself? 🤔

Hm, feels like a much larger and more subtle change. I'd rather keep it located to deep normalization.

or no, in case select_where_possible returns an error, we want to ignore overflow and ambiguity errors. I think we also want to ignore these ambiguity errors here, so maybe change this to [...]

Functionally equivalent to what I have right now, but I guess alright.

Functionally equivalent to what I have right now, but I guess alright.

Correction -- not functionally equivalent, but doesn't matter for UI tests. I originally had let _ = fulfill_cx.collect_remaining_errors(self.infcx)) on the good path of deep norm too, but it felt spooky. I guess it's fine though.