[SR-9260] Allow protocol types with fully specified associated types in all type positions (function arguments etc.)

[swift-ci]

Previous ID	SR-9260
Radar	rdar://problem/56835024
Original Reporter	schwarja (JIRA User)
Type	New Feature
Status	Resolved
Resolution	Duplicate

Environment

Xcode Version 10.1 (10B61)

Additional Detail from JIRA

Votes	0
Component/s	Compiler
Labels	New Feature
Assignee	None
Priority	Medium

md5: 6965b9b1f82b44fc4710d6389a544fac

duplicates:

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

Issue Description:

I created a following snippet:

protocol ViewModel {
    associatedtype Input
    associatedtype Output
    func transform(input: Input) -> Output
}
protocol Injectable {
    associatedtype VM
    var viewModel: VM? { get set }
}
struct RootInput {}
struct RootOutput {}
protocol RootViewModelProtocol: ViewModel where Input == RootInput, Output == RootOutput {}
 
class RootVC: Injectable {
    var viewModel: RootViewModelProtocol?
}

Observed: The snippet is not compiled. An error is detected on this line:

[belkadan]

I think we have a dup of this but I can't find it at the moment.

[belkadan]

@swift-ci create

[typesanitizer]

Minimal example:

protocol P {
  associated type AT
}
protocol PSub: P where AT == () {}
class C {
  func f(x: PSub) {} // error: Protocol 'PSub' can only be used as a generic constraint because it has Self or associated type requirements
}

I think this falls under a feature request "in case where all the associated types are known to be concrete types, it oughta’ compile"; reclassifying as "New Feature".