[Pitch 3] Objective-C implementations in Swift

[beccadax (Becca Royal-Gordon)]

Follow-up on recent posts in the second pitch thread:

@CrystDragon

I'm curious about one point, in the proposal, you said you won't support objc lightweight generics, and I wonder is it possible that OC generics is supported in a limited way?

For example, user can still define a generic class in header files, but when implemented using Swift, Swift sees the header in a way that all generic signatures definitions are dropped, and all generic references are erased to Any.

One of the limitations of @implementation is that it has the same "view" of the imported class as any other Swift code would, so it's not possible to import the code with lightweight generics in other code but without it for @implementation. (Except by playing games with different header content, I suppose, though clang modules intentionally don't make that easy to do.)

Ultimately, I do think @implementation could support lightweight generics, although it might require changing the way all Swift code imports them. It's just an especially complex problem that's only necessary for a very small number of classes, so I chose to subset it out.

@grynspan

Perhaps @implementation(objc) so that the language is explicit, then? Or are you expecting the compiler can always infer the language correctly?

I think the compiler will always be able to infer this correctly. When @implementation is applied to an extension, it can infer it from the type being extended; if @implementation is eventually applied to other declarations, it can infer it from other attributes applied to the declaration (@objc, @_cdecl, @_expose(Cxx)).