[SR-8261] Generics don't identify Protocol with class requirement properly

[swift-ci]

Previous ID	SR-8261
Radar	None
Original Reporter	drumnkyle (JIRA User)
Type	Bug
Status	Resolved
Resolution	Duplicate

Environment

Xcode 9.4.1 production on Swift 4.1.

Additional Detail from JIRA

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

md5: 58c4e0a8cae1656677208c2eb3fd58f4

duplicates:

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

Issue Description:

When you make a generic that requires that the generic type inherit from AnyObject (be a class), it is not satisfied if you use a protocol that is constrained to only class type. Here is example code.

class WeakReference<T: AnyObject> {
    private(set) weak var value: T?

    init(_ value: T?) {
        self.value = value
    }
}

public protocol TempoSetter: class {
    var currentTempo: Int { get set }
}

class ABC {
    var tempoSetters: Set<WeakReference<TempoSetter>> = []
    init() {}
}

Expected🙁

This compiles

Actual:

error: 'WeakReference' requires that 'TempoSetter' be a class type
    var tempoSetters: Set<WeakReference<TempoSetter>> = []

[belkadan]

This is unfortunately somewhat correct behavior, since a protocol value has to carry both the class reference and a pointer to the information about how the class implements the protocol. See SR-55 and its dups for more details.