Lifting the "Self or associated type" constraint on existentials

[dabrahams (Dave Abrahams)]

IMO we should not “simply lift” this constraint without also addressing the concerns I've raised about “partial” protocol types:

Introduce Any<P> as a better way of writing existentials

I've tried to say this before, but hopefully this post is clearer. I definitely support reconsidering the syntax of existential types, but I would not like to make any incremental changes to existentials without holistically considering where we'll end up. In particular, there are at least three possibilities we should consider that will affect our syntax choices: “Partial” protocol types, i.e. existentials whose protocol declares requirements not available on an instance of the existential…

I think the remedies I propose in that post would address all those concerns well enough to allow me to support going forward with this, but I'd want to have those remedies approved first.