Why can't a get-only property requirement in a protocol be satisfied by a property which conforms?

Question

Why does the following code produce an error?

protocol ProtocolA {
    var someProperty: ProtocolB { get }
}

protocol ProtocolB {}
class ConformsToB: Protoc         


        

Answer 1

In addition to Harmish's great response, if you want to keep using the same property name on both SomeClass and ProtocolA, you can do

protocol ProtocolB {}

protocol ProtocolA {
    var _someProperty_protocolA: ProtocolB { get }
}

extension ProtocolA {
    var someProperty: ProtocolB {
        return _someProperty_protocolA
    }
}

class ConformsToB: ProtocolB {}

class SomeClass: ProtocolA {


    // the *actual* implementation of someProperty.
    var _someProperty: ConformsToB

    var someProperty: ConformsToB {
      // You can't expose someProperty directly as
      // (SomeClass() as ProtocolA).someProperty would
      // point to the getter in ProtocolA and loop
      return _someProperty
    }

    // dummy property to satisfy protocol conformance.
    var _someProperty_protocolA: ProtocolB {
        return someProperty
    }

    init(someProperty: ConformsToB) {
        self.someProperty = someProperty
    }
}

let foo = SomeClass(someProperty: ConformsToB())
// foo.someProperty is a ConformsToB
// (foo as ProtocolA).someProperty is a ProtocolB

This can be useful when you are conforming to another protocol ProtocolA2 that would originally also have constraint on someProperty as well, or when you want to hide your hack around swift limitations.

I'm now curious to know why Swift is not doing this for me directly.