struct S<T> {
    var v: (A, B, C)
}
extension S where T == Int {
    typealias A = Int
}
extension S where T == Bool {
    typealias B = Bool
}
extension S where T == Float {
    typealias C = Float
}
print(S<String>.A.self) // Int
print(S<String>.B.self) // Bool
print(S<String>.C.self) // Float
print(S<Double>.A.self) // Int
print(S<Double>.B.self) // Bool
print(S<Double>.C.self) // Float

protocol P {
    associatedtype Q
    var v: (D, E, F) { get }
}
extension P where Q == Int {
    typealias D = Int
}
extension P where Q == Bool {
    typealias E = Bool
}
extension P where Q == Float {
    typealias F = Float
}
struct R<T> : P {
    typealias Q = T
    var v: (D, E, F)
}
print(R<String>.D.self) // Int
print(R<String>.E.self) // Bool
print(R<String>.F.self) // Float
print(R<Double>.D.self) // Int
print(R<Double>.E.self) // Bool
print(R<Double>.F.self) // Float

// typealias misinterprets where clause of extension

struct S<A> {
    var hmm: (A, B, C)
}
extension S where A == Bool {
    typealias B = A
    static func printB() { print(B.self) }
}
extension S where A == Float {
    typealias C = A
    static func printC() { print(C.self) }
}
let a = S(hmm: ("strange", true, Float(42.1)))
dump(a)
// Will print:
▿ TestXc9b3.S<Swift.String>
  ▿ hmm: (3 elements)
    - .0: "strange"
    - .1: true
    - .2: 42.0999985