[SR-12513] Crazy Cross Module Conformance

[dabrahams]

Previous ID	SR-12513
Radar	rdar://problem/62201632
Original Reporter	@dabrahams
Type	Bug

Attachment: Download

Additional Detail from JIRA

Votes	0
Component/s	Compiler
Labels	Bug
Assignee	None
Priority	Medium

md5: bff8be030eae13e3fbbaaf365faacf69

Issue Description:

The attached project (also https://github.com/dabrahams/CrossModuleConformance) demonstrates that Swift's semantics around conformances and multiple modules are nondeterministic, if not insane. I've never found any documentation of how this is supposed to work, and the explanations I've heard from people have always been vague at best. Somebody needs to write down what is supposed to be happening, and then make the compiler do that!

[dabrahams]

Actually I updated the project because there are so many ways Swift is insane about this stuff that I can't even count. See the README.

[beccadax]

@swift-ci create

[woolsweater]

FWIW, I was looking at this on a machine where Swift 5.1.3 (from Xcode 11.3.1) was installed and all the tests pass.

[dabrahams]

Just tested with Swift 5.2.4 and all the insanity appears to be gone except for the availability of idA without importing A in Test3, which is not about conformances, but which persists.

[dabrahams]

The other remaining bit of insanity is that the behavior of conformances across modules is still not documented!