This looks like another case of disallowed IUOs, like in SR-7273.

Was able to reproduce this locally, but it failed with the 4.0 configurations in addition to 3.2.

FAIL: ReactiveCocoa, 4.0, 72dd20, ReactiveCocoa-watchOS, generic/platform=watchOS
FAIL: ReactiveCocoa, 4.0, 72dd20, ReactiveCocoa-tvOS, generic/platform=tvOS
FAIL: ReactiveCocoa, 4.0, 72dd20, ReactiveCocoa-iOS, generic/platform=iOS
FAIL: ReactiveCocoa, 4.0, 72dd20, ReactiveCocoa-macOS, generic/platform=macOS
FAIL: ReactiveCocoa, 3.2, 72dd20, ReactiveCocoa-watchOS, generic/platform=watchOS
FAIL: ReactiveCocoa, 3.2, 72dd20, ReactiveCocoa-tvOS, generic/platform=tvOS
FAIL: ReactiveCocoa, 3.2, 72dd20, ReactiveCocoa-iOS, generic/platform=iOS
FAIL: ReactiveCocoa, 3.2, 72dd20, ReactiveCocoa-macOS, generic/platform=macOS

I've reached out to the ReactiveCocoa maintainer for an updated hash.

Comment by Anders Ha (JIRA)

@clackary It seems to be a legit regression. There is no IUO involved here at least from a language user POV (according to Xcode Quick Tips).

`method` is `Method?`. `method_getImplementation` is `(Method) -> IMP`. `_rac_objc_msgForward` is `IMP`. So the resulting line should have produced `IMP`.

The interesting bit is that it compiles fine if I explicitly declare the result type to be `IMP`. So it seems more like a type inference issue to me.

@swift-ci create

When compiling with -swift-version 3, IUOs actually do come into play here because the API notes for method_getImplementation (and class_getInstanceMethod) say that they take IUOs and return IUOs.

import Foundation

func test(m: Method?, i: IMP) {
  let r = m.map(method_getImplementation) ?? i
  let _: IMP = r
}

This is a simplified example that demonstrates the problem.

I'll take a closer look at this soon, but what it looks like is happening is that for some reason the type checker is now selecting the ?? that returns T? rather than the one that returns T.

I've attached a reduced test case that more closely represents the original code.

For now I would suggest working around the issue when compiling with compilers built from swift-4.1-branch by explicitly typing 'impl' as 'IMP'.

Compilers built with swift-4.2-branch and master appear to compile this without error.