Switch expression results cannot be assigned to `self`.

[JessyCatterwaul]

Compiles:

func ƒ<Value>(value: Value) -> Value {
  switch value {
  case let value: value
  }
}

Does not compile:

extension Bool { // Type doesn't matter, though `Never` will crash the compiler.
  init(value: Self) {
    self = switch value {
    case let value: value // This will compile with a warning, but assign the argument, not the bound value.
    case let v: v // Cannot find 'v' in scope
    }
  }
}

Wrapping in a closure is a solution:

extension Bool {
  init(value: Self) {
    self = { switch value {
    case let value: value
    } } ()
  }
}

[hamishknight]

Duplicate of #66412, filed #66490 to track the Never crash