Issue Description:

We can match a optional value against a optional and non-optional expression, but the compiler does not detect “equivalent” cases:

let x: Int? = 1

switch x {
case 1: print("one")
case .some(1): print("some one") // This is never executed.
default: print("default")
}

case .some(1): is never executed, but the compiler does not emit a warning. It would be an improvement if the compiler would emit a “... is already handled by previous pattern ...” warning.

Tested with Xcode 9.3.1, with both the default toolchain and the Swift 4.2 snapshot from May 20, 2018.

Related discussion in the Swift forum: Matching optionals in a switch statement