[SR-10053] Protocols extending class types are not recognized as class types

[swift-ci]

Previous ID	SR-10053
Radar	rdar://problem/48702830
Original Reporter	jbfit (JIRA User)
Type	Bug
Status	Resolved
Resolution	Duplicate

Attachment: Download

Environment

Swift compiler 4.2-RELEASE

Additional Detail from JIRA

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

md5: 7098f9a0a7e244cda268053f1aa1d117

duplicates:

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

Issue Description:

class Alpha<T> where T: AnyObject {
    private let value: T
    init(_ value: T) {
        self.value = value
    }
}

protocol Beta: AnyObject {}

class Gamma: Beta {}

print(Alpha<Beta>(Gamma()))

Expected:

• The above code should compile: The requirement on generic `T` is that is that it extends from `AnyObject`. Since protocol `Beta` also extends from `AnyObject` it holds that any class `Foo` that implements protocol `Beta` will also extend from `AnyObject`

Actual:

• Code does not compile. Error message is error: "'Alpha' requires that 'Beta' be a class type"

Note: `class` and `AnyObject` are type-aliased to the same thing, so changing to `Beta: class` produces the same error

Also, this works as expected:

class Alpha<T> where T: AnyObject {
    private let value: T
    init(_ value: T) {
        self.value = value
    }
}
class Beta {}
class Gamma: Beta {}
print(Alpha<AnyObject>(Gamma())) 

[belkadan]

This is unfortunately correct behavior, because a protocol value isn't just a reference to the object; it also tracks information about how the type conforms to the protocol. See SR-55 and its dups for more discussion.