Issue Description:

Reproduce:

This snippet reproduces the issue:

protocol MyProto {
   var title: String? { get }
}

let tuple = (x: 100, y: 100)

// Results in a warning as expected: Cast always fails
let castedTupleToInt = tuple as? Int

// No warning, although cast will always fail
let castedTuple = tuple as? MyProto

The way I see it, it should be pretty easy for the compiler to detect that casting a tuple to a protocol always fails, since there’s currently no way for a tuple to conform to a protocol.

Motivation:

We came across the bug when we refactored a method to return a tuple instead of a protocol type, and thought the compiler would make us aware of all the call sites that need to adopt the the refactored return type. However, wherever we casted the return type, no warning was raised and the code just silently returned early because the cast always fails at runtime (as it should, of course).

Previous Discussion:

There was a brief discussion on the swift-users mailing list: "Casting tuples to protocols always fails (and the compiler should know)"