[SR-5357] Type-constrained protocol cannot be used as a generic argument for similarly constrained generic parameter

[jarrodldavis]

Previous ID	SR-5357
Radar	rdar://problem/33095936
Original Reporter	@jarrodldavis
Type	Bug
Status	Resolved
Resolution	Duplicate

Environment

macOS Sierra 10.12.5 (16F73), Xcode 9.0 beta 2 (9M137d)

Additional Detail from JIRA

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

md5: 503457675ee4c46224ec7dac3e0dc725

duplicates:

• SR-55 non-@objc protocol existentials do not conform to their own protocol type

Issue Description:

Given a generic type X<Y> where Y is constrained to some type Z and a protocol A that constrains Self to Z, a variable or constant declaration of X<A> should be considered valid. Currently, as of Xcode 9 beta 2, the compiler gives a warning similar to "'X' requires that 'A' inherit from 'Z'" even though that is already true.

// Imagine these sheet classes and the bed struct in a separate module somewhere

class Sheet {
    func pullOver() {
        print("sleep time")
    }
}

class ComfySheet : Sheet {
    public static let veryComfy: ComfySheet = {
        return ComfySheet()
    }()

    func comfort() {
        print("comfortable")
    }
}

class BouncySheet : Sheet {
    public static let veryBouncy: BouncySheet = {
        return BouncySheet()
    }()

    func bounce() {
        print("bounce")
    }
}

class AwfulSheet : Sheet {
    override func pullOver() {
        print("ew")
    }
}

struct Bed<SheetType> where SheetType : Sheet {
    public let size: Double
    public let sheets: SheetType
}

// This protocol and the corresponding extensions are editable

protocol EnjoymentMeasurable where Self : Sheet {
    func enjoy()
}

extension ComfySheet : EnjoymentMeasurable {
    func enjoy() {
        print("8/10")
    }
}

extension BouncySheet : EnjoymentMeasurable {
    func enjoy() {
        print("11/10")
    }
}

func doSomething() {
    let bed: Bed<EnjoymentMeasurable> // error here: "'Bed' requires that 'EnjoymentMeasurable' inherit from 'Sheet'"
    bed = Bed(size: 4.0, sheets: ComfySheet.veryComfy)
    bed.sheets.enjoy()
}

[belkadan]

This is arguably correct because EnjoymentMeasurable could have static requirements, and then Bed wouldn't be able to call those methods because the protocol type doesn't implement them. But it is a little frustrating when the protocol does not have static requirements.

@slavapestov, @jckarter, how are we tracking these?

[jckarter]

SR-55 is the canonical "protocol types should conform to their protocol" issue.