Excuse the vague title.
I'm trying to build a package to help me use third-party cloud storage APIs (Firebase Storage for example), adding Combine support, etc. This package does the same thing with CloudKit. Everything compiles fine, but when I import the package module into a separate project of mine, the module is apparently missing some public symbols...
Specifically, the ones wrapped inside of an #if canImport(FirebaseStorage) condition. Since Firebase doesn't support SwiftPM yet, this part of the package behaves as expected in the package project itself; it simply skips compiling that whole bit. I figured that a client project that can import this module would compile it fine.
Aside: What I'm trying to do looks something like optional dependencies. I don't want to have to import Firebase to use this package's other features. I have considered splitting the package into separate sub-packages, each depending upon the particular third-party library I want to use. I might do that anyway. But the problem remains that Firebase doesn't yet support SwiftPM (although I hear they're close).
My issue appears similar to this one. My client project just doesn't seem to see the conditioned symbols, though it can import Firebase and FirebaseStorage just fine! I mean that the generated module header is missing them entirely, preventing my client project from compiling when I use them.
It seems to me that the compile condition never leaves the package's own scope of dependent targets. Is this the case? Or am I missing something obvious? I had always assumed that Swift Packages just import and compile the Swift source files into named modules, but now I think that is not so.
Is there a way to build code into a Swift Package that compiles only when the client can import a third-party module that does not yet support SwiftPM? Or does conditional compilation not work that way?
EDIT: Here is the Swift documentation on conditional compilation, for reference.