Does swift inheritance from generic class prevent protocol conformance via extension?

Question

@objc public protocol P1 {
  func p1foo()
}

@objc public protocol P2 {
  func p2foo()
}

class A<T>: NSObject, P1 {
  func p1foo() { }
}

class B: A<Int> {

}

extension B: P2 {
  func p2foo() { }
}

-> Type 'B' does not conform to protocol 'P2'.

Why? Removing the generic solves this error, but I don't understand - B should be a fully-specific type at this point...

matt · Accepted Answer · 2016-09-02 17:02:50Z

3

It's the combination of the @objc and the generic. Objective-C knows nothing of Swift generics, so your notion that B should adopt an @objc protocol causes the compiler to throw a wobbly.

You've already detected this from one direction; and you can equally see it from the other. You can make your code compile by deleting the @objc attributes. Or you can make it compile by removing the generic. You can't have both at once.

answered Sep 2, 2016 at 17:02

matt

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