That syntax is illegal. Are you saying it should be legal? Certainly we could improve the diagnostic to suggest the correct syntax.

Comment by Vatsal Manot (JIRA)

Hi @belkadan, thanks for letting me know. At first, I was unsure about the nature of this issue, but I'll update the text now that that's cleared up. And yes, I'm saying that it should be legal.

This is labeled "Parser" "StarterBug." Should the parser be modified such that it allows this syntax? Or is that something that would need to be discussed on swift-evolution?

If this requires a swift-evolution proposal, I suggest closing the issue until we can be certain it's a change that the Swift team is willing to make.

The right syntax here is associatedtype AssociatedTypeA: A, B
Because this is type-inheritance-clause, and that only allows comma-separated type-identifier­.

I believe we have already improved the diagnostics for this case:

    associatedtype AB: protocol<A, B> // error: protocol composition is neither allowed nor needed here
                                      // (with fix-it replacing 'protocol<A, B>' with 'A, B')

But, using the new protocol composition syntax A & B lacks the fix-it.

@rintaro, sounds like the TODO you linked to deserves its own report, and this report should be closed as "invalid" or "wontfix". Assigning to @belkadan to close or decide otherwise. 🙂

On master we now allow both 'associatedtype T : A, B' and 'associatedtype T : A & B'.