"Existentials" and "opaque types", but for concrete generic types?

[Slava_Pestov]

@mpangburn

• If we took the existing rules for existential and opaque types, and applied them to concrete generic types with rules like "given X<T>, treat any X as an existential as though X were a protocol with associatedtype T", would we encounter any new soundness holes?
• Are there terms of art for the concepts described here specific to generic types, other than the terms we use for the analogous features of protocols? (If a higher-kinded type is "a generic type without its type arguments bound yet", these are like "a generic with its type arguments bound, but we're not sure with what".)
• Do these concepts exist in any other programming languages?

You can think of an any P as packaging a value of type Self with the generic signature <Self where Self: P>, so let’s pretend we have this syntax:

any <Self where Self: P> Self

What you’re asking for is an existential where the head type is something other than Self. For example:

any <T where …> X<T>