a.swift

protocol Theme {
	associatedtype Kind
}

struct Theme1: Theme {
	typealias Kind = String
	let kind: Kind
}

let theme1 = Theme1(kind: "abc")

struct Theme2: Theme {
	typealias Kind = Int
	let kind: Kind
}

let theme2 = Theme2(kind: 999) // OK

// --------------------------

func current1() -> Theme1 {
	return theme1
}
print(current1()) // OK

// --------------------------

func current2<T>() -> T {
	return theme2 as! T
}
print(current2()) // OK

// --------------------------

func current3<T: Theme>() -> T {
	return theme2 as! T
}
print(current3() as Theme2) // OK

// --------------------------

func current4<T: Theme>() -> T {
	return theme2 // of type `Theme2`
}
print(current4()) // error: generic parameter 'T' could not be inferred

// --------------------------

func current5<T>() -> T where T: Theme, T.Kind == String {
	return theme2 // of type `Theme2`
}
print(current5(Theme2.self)) // error: generic parameter 'T' could not be inferred