[SR-6441] Optionals used as a Generic types fail casting nil

[swift-ci]

Previous ID	SR-6441
Radar	rdar://problem/35700189
Original Reporter	Jan Mazurczak (JIRA User)
Type	Bug

Environment

Xcode version 9.1 (9B55) Swift 4 Tested in Swift Playgrounds on iOS too

Additional Detail from JIRA

| | | |------------------|-----------------| |Votes | 0 | |Component/s | Standard Library | |Labels | Bug | |Assignee | None | |Priority | Medium | md5: 8bf4f977f4fc88a46b367d7a04a9bb0f

Issue Description:

struct A<Message> {
    func received(message: Any?) {
        if let message = message as? Message {
            print("success")
        } else {
            print("fail")
        }
    }
}

let a = A<Int?>()
a.received(message: nil)

In above example I didn't find a way to recognise message as generic type when that type is an optional.

Expected:

When generic type Message is Optional casting nil as? Message should always succeed with nil result.

Actual result:

Casting fails.

[belkadan]

cc @jckarter

[jckarter]

@swift-ci create