[cxx-interop] Adding swift_name attributes to virtual methods overrides

[susmonteiro]

The design for overriding virtual methods that may be renamed with a swift_name attribute is as follows:

• only the base method can be renamed with a swift_name attribute
• all the overriding methods will inherit their name from the base method, independently of whether they don't have a swift_name attribute or have a different one
• a swift_name attribute on a virtual method override is going to be ignored. We emit a warning and the method will be marked unavailable
• in the case of multiple inheritance:
  • if the deriving class overrides the method, we validate that all the overriding methods share the same name. If not, the method is marked unavailable and cannot be called with any name. The idea here is to preserve virtual dispatch.
  • if the deriving class does not override the method, then it will inherit all the method names from all its base classes and these will all be valid names. The reasoning is that in C++, this would result in ambiguous methods, as there would be no way to pick which method to call. In Swift, this allows the developer to explicitly choose with overriding method to call.

Follow up to #82485

rdar://155032235

[susmonteiro]

@swift-ci please smoke test

[Xazax-hun]

It could be a follow-up PR but I think we should emit a warning when:

• An override has a swift_name different from the swift_name of the base
• There is a swift_name attribute on an override that is ignored

the program will crash if we try to use that name to call the method

This sounds a bit harsh. Is it possible to do something else, e.g., making it unavailable?

[susmonteiro]

@Xazax-hun you think we should emit a warning in those 2 situations even if the method is not being used? This is feasible, but I'm wondering if it would generate too many warnings

This sounds a bit harsh. Is it possible to do something else, e.g., making it unavailable?

Sorry, my description wasn't very explicit, but that's exactly what I'm doing: marking the method unavailable and, if the method is called, then the program won't compile because the method is unavailable

[Xazax-hun]

I wonder if we want to do the opposite and only import the derived method in this case.

[susmonteiro]

Sure! In the case where the override has the same swift_name, we synthesize a new thunk for the method.

However, when the base method has a swift_name but the override doesn't, we still skip the generation of a thunk and clone the one from the base method. I improved the comment explaining why this is correct.

[susmonteiro]

@swift-ci please smoke test

[Xazax-hun]

Mostly looks good to me, I have some nits and questions inline.

[susmonteiro]

@swift-ci please smoke test

[Xazax-hun]

LGTM, thanks!