Sema: `if case _ as` casting pattern doesn't work despite being implemented already

[simanerush]

Steps To Reproduce
For example, this code snippet

protocol T {}
struct A: T {}

func test_switch<T>(arr: T) {
  if case _ as A = arr {}
}

should work fine. However, it gives the following error:

error: cannot convert value of type 'T' to specified type 'A'
  if case _ as A = arr {}
                   ^~~
                       as! A

and emits non-relevant diagnostics.

Expected behavior
Even though this works by binding the result of the cast to a constant (if case let t as A = arr {}), the case specified above should also work.

Environment (please fill out the following information)

• OS: macOS 12.5.1
• Swift version: 5.8-dev

[CodaFi]

The workaround pattern here is if case is A = arr

[simanerush]

@CodaFi So a version of this with the as pattern is not intended to be working in the first place?

[AnthonyLatsis]

Robert is simply pointing out that case is A = foo, which is semantically equivalent, can be used to work around the issue with case _ as A = foo; both are expected to work.

[hamishknight]

This was fixed by #64280, and should therefore be fixed in Swift 5.9 with Xcode 15 beta. Adding a test case in #68035.