[4.0] IRGen: EmptyBoxType's representation cannot be nil because of a conflict with extra inhabitant assumption in indirect enums

[aschwaighofer]

Use a singleton empty heap box representation instead of nil for EmptyBoxTypes. The nil representation that we used is not compatible with assumption we make about extra inhabitant bits we make in enums.

• Explanation: Due to using an incompatible representation of empty box types and single reference enum payloads we can’t distinguish between the first empty payload case and the payload case because both map to 0.

enum Payload {
case c
}

indirect enum Test {
case a
case b(Payload)
}

Test.b(.c) // creates .a

• Scope of Issue: This has not been working since at least Xcode 8.3.f

• Risk: Low. The code path is the same for the rest of the compiler for handling reference counted payloads. This change therefore should not expose other bugs.

• Testing: Existing and added Swift CI tests

[aschwaighofer]

@jckarter Can you review this for the swift-4.0-branch?

[aschwaighofer]

@swift-ci Please test

[jckarter]

This seems fine for 4.0. For master, is there any reason not to export the empty box object symbol and open-code references instead of going through a runtime entry point? swift_getEmptyBox might be a better name since it's not allocating anything. Should swift_allocBox return the empty box when given an empty type?