[SR-8637] 'Protocol composition type' with generic types/inheritance

[swift-ci]

Previous ID	SR-8637
Radar	None
Original Reporter	Aaron Bosnjak (JIRA User)
Type	Bug
Status	Resolved
Resolution	Duplicate

Additional Detail from JIRA

Votes	0
Component/s
Labels	Bug
Assignee	None
Priority	Medium

md5: 199e398a3105e390dde3f33e16a96d64

duplicates:

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

Issue Description:

I've been running into some problems with trying to use a 'protocol composition type' (e.g. MyType & MyProtocol, I'll refer to it as a PCT for brevity) with generics, both by trying to use it as a generic constraint and as the parameter to a generic type. This code sample more succinctly shows my problem in less technical words:

protocol MyProtocol: AnyObject { }
class MyClass { }
class MySubclass: MyClass, MyProtocol { }
class MyGenericClass<T> {
    func foo(with o: T) { }
}
extension MyGenericClass where T: MyClass {
    func bar(with o: T) { }
}
// this extension compiles. My understanding, though, is that it's treating the
// PCT as a protocol to conform to rather than a class to inherit from
extension MyGenericClass where T: MyClass & MyProtocol {
    func baz(with o: T) { }
}

// works fine; 'MySubclass' fills the requirement for 'MyClass & MyProtocol'
// in this context
var myClass: (MyClass & MyProtocol)?
myClass = MySubclass()

let i = MyGenericClass<MyClass>()
i.foo(with: MySubclass())
i.bar(with: MySubclass())

let j = MyGenericClass<MyClass & MyProtocol>()
j.foo(with: MySubclass())
j.bar(with: MySubclass()) // error
j.baz(with: MySubclass()) // error

If I try to add a constraint on MyGenericClass.T that it must be an AnyObject, I get this error when I declare j: ''MyGenericClass' requires that 'MyClass & MyProtocol' be a class type". I expected that MySubclass would fill the type requirements on the last two lines, but it doesn't - is this intended, or just an overlooked detail due to the way PCTs are implemented?

Thanks!

[belkadan]

It's correct behavior; the representation of a composition type is different than the representation of "a type that has these two constraints". You could use == in your constraint instead of :, but I suspect that's not what you want either.