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[SR-8637] 'Protocol composition type' with generic types/inheritance #51152

swift-ci opened this issue Aug 24, 2018 · 1 comment

[SR-8637] 'Protocol composition type' with generic types/inheritance #51152

swift-ci opened this issue Aug 24, 2018 · 1 comment


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@swift-ci swift-ci commented Aug 24, 2018

Previous ID SR-8637
Radar None
Original Reporter Aaron Bosnjak (JIRA User)
Type Bug
Status Resolved
Resolution Duplicate
Additional Detail from JIRA
Votes 0
Labels Bug
Assignee None
Priority Medium

md5: 199e398a3105e390dde3f33e16a96d64


  • 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>() MySubclass()) MySubclass())

let j = MyGenericClass<MyClass & MyProtocol>() MySubclass()) 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?


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@belkadan belkadan commented Aug 30, 2018

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.

@swift-ci swift-ci transferred this issue from apple/swift-issues Apr 25, 2022
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