[swift-evolution] Allow protocol vars to match derived types

[Mark Anders]

Consider the following (you can paste it in a Playground to see the error):

class Node { }

class Containable : Node{}


protocol Refers {

    var to : Node {get}

}


class Link : Refers {

    var to : Node

    init(n : Node) {

        to = n

    }

}


class Contains : Refers {

    var to : Containable

    init(c : Containable) {

        to = c

    }

}

This currently does not work because it seems that to adopt a protocol, the
type of protocol var must match exactly.

It would be great if objects could be said to adopt a protocol if the type
of the var is the type or a derived type.
This would allow me to treat the structure in a type safe way (i.e. only a
Containable can have a Contains relationship),
while me to have a set of Refers and iterate through each Node.

Is there a reason why the type must match exactly?  Or could protocols be
enhanced to to allow matching
derived types, similar to assignment and func parameter rules?


Mark
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