protocol Pattern {
  associatedtype In
  associatedtype Out
  func match(_: In) -> Out?
}
...
let myValue: T = // ... something of type T
let pattern: MyTPattern<(U,U)> = // ... something that matches a T to (U, U)
let otherPattern: OtherTPattern<V> = // ... something that matches a T to V
switch myValue {
  case (let a: Int, let b: Int) <- pattern: // ... tries `pattern`, then tries default U->Int pattern
  case let pair <- pattern: // ... tries `pattern`, pair has type (U,U)
  case let d: Double <- otherPattern: // ... tries `otherPattern`, then tries default V->Double pattern
  case let num <- otherPattern: // ... tries `otherPattern`, num has type V
}