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@JimRoepcke
Last active June 29, 2016 04:24
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Please help me understand why the Swift 2.2 compiler in Xcode 7.3.1 says the class does not conform to the protocol
//: Playground - noun: a place where people can play
protocol Thingy: class {}
protocol ThingTaking: class {
associatedtype ThingType: Thingy
func some(thing: ThingType)
}
protocol MyThingy: Thingy {}
class WhyDoesThisNotConformToThingTaking: ThingTaking {
func some(thing: MyThingy) {
}
}
/*
Playground execution failed: Weird.playground:7:7: error: type 'WhyDoesThisNotConformToThingTaking' does not conform to protocol 'ThingTaking'
class WhyDoesThisNotConformToThingTaking: ThingTaking {
^
Weird.playground:6:20: note: unable to infer associated type 'ThingType' for protocol 'ThingTaking'
associatedtype ThingType: Thingy
^
Weird.playground:9:10: note: inferred type 'MyThingy' (by matching requirement 'some') is invalid: does not conform to 'Thingy'
func some(thing: MyThingy) {
^
*/
@CraigSiemens
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Not really a solution, more of an observation but if MyThingy is a class it works.

Also, if WhyDoesThisNotConformToThingTaking is setup to be a generic class it works.

class WhyDoesThisNotConformToThingTaking<T: Thingy>: ThingTaking {    
    func some(thing: T) {

    }
}

I think associatedtype has to evaluate to a type that isn't a protocol.

@zxhfirefox
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zxhfirefox commented Jun 29, 2016

I think, because compiler can not determined that when you actually call with some variable of WhyDoesThisNotConformToThingTaking like:
let x = WhyDoesThisNotConformToThingTaking (); x.some(y)
in above, y' s type can not be checked at compile time if some accept a protocol type.
when you add something like
class Thing : MyThingy {}
and change
func some(thing: MyThingy) { }
to
func some(thing: Thing) {}

it works.

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