[SR-9837] Optional<T>.none -> Optional<U>.none casting regression in generic context

[hamishknight]

Previous ID	SR-9837
Radar	rdar://problem/47743688
Original Reporter	@hamishknight
Type	Bug
Status	Resolved
Resolution	Done

Environment

Apple Swift version 5.0-dev (LLVM eb302e257f, Clang a113643bc4, Swift 646034c)
Target: x86_64-apple-darwin18.2.0

Additional Detail from JIRA

Votes	0
Component/s	Compiler
Labels	Bug, 5.0Regression, Runtime
Assignee	@hamishknight
Priority	Medium

md5: ec7bbde0897708940075adc9253e1aca

Issue Description:

In Swift 4.2 the following cast correctly succeeds:

func test<T>(_ type: T.Type) -> T? {
  return Any?.none as? T
}

print(test(Bool?.self)) // Optional(nil)

However, as discovered on the forums, in Swift 5 it incorrectly fails and prints nil instead. This does appear to be fixed on master, but we should add a test case.

[belkadan]

cc @jckarter, @slavapestov

[jckarter]

@mikeash took a look, and it looks like this is fixed in the most recent Swift 5 snapshots.