Open Radar

 
Summary:
If protocol A has associated type requirements it can not be used in a heterogeneous collection.

protocol ProtocolA {
	typealias : Element
}

var n : [ProtocolA]  // Protocol `ProtocolA` can only be used as a generic constraint because it has Self of associated type requirements

Allow the type requirement to be  resolved  in an inheriting protocol: 

protocol ProtocolB : ProtocolA {
	Element = ConcreteType
}

var n : [ProtocolB] // okay to use ProtocolB in a heterogeneous collection.


I am using the heterogeneous collection as one example, but it allows ProtocolB to be used as a  type (without generics). This would be very useful in many situations.

If we can provide default implementations for method and property requirements in extensions, why can’t we also provide type reference requirements? 

Notes:
Please see attached markup.