Associatedtype depends on protocol

Question

I struggle with associatedtype to make it work as I want. So ask for clarification and explanation why example below doesn't work. Bellow example is executable in playground.

I have this structures Foo and Bar where both implement DocumentTemplate.

public protocol DocumentTemplate {
}

public struct Foo: DocumentTemplate {
    public var _id: String!
}

public struct Bar: DocumentTemplate {
    public var _name: String!
}

I have generic protocol DocumentCollectable require type T inherit from DocumentTemplate, and two implementations of UniversalCollectable depends on DocumentTemplate and FooCollectable depends on Foo.

public protocol DocumentCollectable {
    //type T must implement protocol DocumentTemplate
    //error workaround remove ': DocumentTemplate'
    associatedtype T: DocumentTemplate 

    func collect(_ d: T)
}

public class UniversalCollectable: DocumentCollectable {
    public typealias T = DocumentTemplate //error

    public func collect(_ d: T) {
        print("universal done")
    }
}

public class FooCollectable: DocumentCollectable {
    public typealias T = Foo

    public func collect(_ d: T) {
        print("foo done")
    }
}

When I execute this:

let u = UniversalCollectable()
u.collect(Foo())
u.collect(Bar())

let f = FooCollectable()
f.collect(Foo())

I'll get error message

error: type 'UniversalCollectable' does not conform to protocol 'DocumentCollectable'

But if I make change to associatedtype T not inherit from DocumentTemplate, then everything run smooth. But I need type T to inherit from DocumentTemplate.