I'm intrigued by the idea of supporting both API versions in corelibs-xctest, but I'm thinking that it really isn't very feasible. Wouldn't that mean that the entire framework would need to be buildable by both the Swift 2.2 and 3 compilers, because binary distribution isn't possible? And not only corelibs-xctest, but also corelibs-foundation?

I'm wondering if I'm misunderstanding the approach that you are suggesting here @modocache?

I was wondering whether we could use the compile-time Swift version checks from Swift 2.2+ to write something like this:

#if swift(>=3.0)
public func expectation(withDescription description: String, file: StaticString = #file, line: UInt = #line) -> XCTestExpectation {
#else
public func expectationWithDescription(description: String, file: StaticString = #file, line: UInt = #line) -> XCTestExpectation {
#endif
    // ...implementation code.
}

But now that you mention it, I think you're right: I guess this isn't feasible. It isn't just a matter of the APIs--as you pointed out, it's also a matter of being able to compile the corelibs-xctest code on two versions of Swift. That's too much of a burden to seriously consider.

Thanks for pointing that out! Closing this.

I do like this line of thinking though, and I'm optimistic that once we have the major breaking changes of Swift 3 behind us that it will get easier to maintain broader version support in libraries!