[SR-7664] Inconsistent optional casting behaviour with generics

[hamishknight]

Previous ID	SR-7664
Radar	rdar://problem/40171034
Original Reporter	@hamishknight
Type	Bug
Status	Resolved
Resolution	Done

Environment

Swift version 4.2-dev (LLVM d9cd6a5c49, Clang ed8876508e, Swift c363dad)
Target: x86_64-apple-darwin17.5.0

Additional Detail from JIRA

Votes	0
Component/s	Compiler
Labels	Bug
Assignee	None
Priority	Medium

md5: 241ea289c2d537b07e80e0f58cf55573

is duplicated by:

• SR-8037 Method generic type inference is broken in Swift 4.2

Issue Description:

This cast succeeds:

func foo(_ t: Int??) -> Int? {
  return t as? Int
}
let x = foo(Int??(3))
print(x as Any) // Optional(3)

but it fails when we use a generic placeholder satisfied by Int rather than Int directly. This cast fails:

func bar<U>(_ t: Int??, _: U.Type) -> U? {
  return t as? U
}

let y = bar(Int??(3), Int.self)
print(y as Any) // nil

[belkadan]

@rudkx, you were looking at some of this, right?

[belkadan]

@swift-ci create

[rudkx]

I was looking at something a little different than this that did not involve generics.

This seems like it must be an issue with the conditional casting support in the runtime.

[mikeash]

Fixed in this PR: #16772