[swift-evolution] [Discussion] Allowing extending existentials

David Hart david at hartbit.com
Wed Feb 22 00:53:37 CST 2017

Hello list,

Found out yesterday that you can’t extend all existentials in Swift:

protocol P1 {}
extension P1 {}
// works as expected

protocol P2 {}
extension P1 & P2 {}
// error: non-nominal type 'P1 & P2' cannot be extended

extension Any {}
// error: non-nominal type 'Any' cannot be extended

extension AnyObject {}
// error: 'AnyObject' protocol cannot be extended

I’d like to write a proposal to lift some of those restrictions. But the question is: which should be lifted? P1 & P2 seems like an obvious case. But what about Any and AnyObject? Is there a design reason that we shouldn’t allow it?

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