Comment by Manuel "StuFF mc" Carrasco Molina (JIRA)
Many things are weird here. First of all rdar://41080489 says the bug is "fixed", but it's not. I'm surprised there are 2 "sources", and it's not closed here. Also, it's not closed anyways because their suggested fix might help (I didn't even bother checking) but then how to explain that this fix isn't needed in Xcode 9.4.1 for the project to compile? Would it be a bug in Xcode 9.4.1 and Xcode 10 would be right?
That bug is closed as "Behaves Correctly", and it is. Did ADC not pass the explanation along to you?
…well, here it is, anyway.
The [Objective-C] code is not available for use as DEFINES_MODULE=NO, thus no module definition is created and being passed into the Swift compiler. There was a bug in the legacy build system that would create this module. We have a issue (41779737) to track providing a better error message in this circumstance, but this is the correct behavior.
(Oh, and the reason this bug didn't get closed is because it turned out to be an Xcode change, not a Swift change, and the Xcode people don't usually go look at the Swift JIRA.)