Please help me understand why the Swift 2.2 compiler in Xcode 7.3.1 says the class does not conform to the protocol

Weird.swift

//: 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) {
 ^
*/

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.

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.