Lifting the "Self or associated type" constraint on existentials

[dabrahams (Dave Abrahams)]

@Joe_Groff

If we were going to introduce a new syntax for existentials, I think it'd be much less confusing if we did so uniformly, deprecating the existing syntax after a late-enough language version, than to have two syntaxes where one only works some of the time. That seems to me like it'd only make things more confusing, personally.

I "loved" this comment when I saw it originally, but now I'm having second thoughts, because an idea I buried in an aside earlier is growing on me. Suppose:

1. Self-conforming-protocol existentials can be spelled with just the protocol name
2. Other existentials always need a where clause, which may be “empty:” Collection where _.

I think that might be enough to solve my problem with implicitly-unavailable APIs, and it wouldn't punish many uses of generalized existentials, which will have a where clause anyway. Finally, if we decide this was all a mistake, we can always lift restriction #2.

I realize this doesn't give us an immediate way to fix the technical problems @Joe_Groff is trying to address with this pitch, but it is an eventual future I think I could support. If we can agree that it's a good idea, maybe we can find a reasonable evolution path to get there that includes an immediate fix for the technical problems.

Thoughts?