[SR-9501] Subclass existential with predefined associated type witness should be supported as existential

[AnthonyLatsis]

Previous ID	SR-9501
Radar	None
Original Reporter	@AnthonyLatsis
Type	Bug
Status	Resolved
Resolution	Duplicate

Environment

Xcode 10.1 (10B61) & master

Additional Detail from JIRA

Votes	0
Component/s	Compiler
Labels	Bug
Assignee	None
Priority	Medium

md5: 075e0f26634a1fe1f91a9305b41c2dac

duplicates:

• [SR-4758] SE-0142: derived protocol that restricts associatedtype should be allowed as a variable type #47335

Issue Description:

This is a variation of SR-4758 that I believe should be faced separately. Here, the associated type is known to be concrete through a type alias in the class component of a subclass existential (rather than through where clauses on protocol declarations that directly constraint an assoc. type).

class Class {
  typealias Assoc = Int
}
protocol Proto {
    associatedtype Assoc
}

func foo(arg: Class & Proto) {} // protocol 'Proto' can only be used as a generic constraint because it has Self or associated type requirements.

[belkadan]

This doesn't work today, but it probably should:

class Class {
  typealias Assoc = Int
}
protocol Proto {
  associatedtype Assoc
}
class Sub: Class, Proto {
  typealias Assoc = String // shadows Class.Assoc
}