duplicates:

• SR-4248 Runtime exception casting an Any? nil to a generic optional

Issue Description:

The following code produces different results when compiled with swift 5 compiler with -swift-version 4.2, and when compiled with either a swift 4.2 compiler or a swift 5 compiler(not using -swift-version).

Basically when in source code compatibility mode the casting from T to Any?(Optional) fails when T resolves to Optional.

   func testfunc() -> T {

    let anynil: Any? = nil
    if let castedanynil = anynil as? T {
        print("casting successful")
    }
    else {
        print("casting failed")
    }
    fatalError()

    }

    let lala: Any? = testfunc()

Prints "casting failed" when build with swift 5 compiler with input sources version 4.2 e.g .swift -swift-version 4.2 test.swift
Prints "casting successful" in any other case
for example when complied with Apple Swift version 4.2.1 (swift-4.2.1-RELEASE), also prints "casting successful"