What about adding a third alternative here to handle the error message for a starred expr after the colon? Something along the lines of:

| expression ':' b='*' bitwise_or { RAISE_SYNTAX_ERROR_KNOWN_LOCATION(a, "cannot use a starred expression in a dictionary value") }

The above rule might need some changing as well to accommodate the case where this happens in the first kvpair.

The problem is that we cannot easily do it in the first pair because that can happen in a set. Only when we know it must be a dictionary (because we have seen the first valid pair) then we know that these things are incorrect.

In this case though, because we're parsing the ':' token, can't we be optimistic and assume that they're trying to create a dictionary? We might have some false positives for this alternative, but in most cases the error report would be correct, right?

In this case though, because we're parsing the ':' token, can't we be optimistic and assume that they're trying to create a dictionary?

Yep yep, I am referring to this part of your message:

to accommodate the case where this happens in the first kvpair.

Ah, yes, the joy of seeing you own error:

    self.check_intersection(alt, other_alt)
  File "/Users/pgalindo3/github/python/master/Tools/peg_generator/pegen/validator.py", line 44, in check_intersection
    raise ValidationError(
pegen.validator.ValidationError: In invalid_kvpair there is an alternative that will never be visited:

I just ordered them incorrectly 😆

I did the exact same thing, while I was testing if my recommendation works! 🚀

We might have some false positives for this alternative, but in most cases the error report would be correct, right?

I think I know what you mean here, let me try to get it to work also on the first alternative

Ok, done in ce1a129

That was exactly what I meant. Nice. Sorry if that wasn't clear enough.