How does #if canImport(...) determine what can be imported?

[ole (Ole Begemann)]

Quoting @NeoNacho (How to exclude a local package from an Xcode target? - #5 by NeoNacho):

The way canImport works is if the module exists in your search paths, it'll be true. Typically when using Xcode or SwiftPM, there's just one shared directory for any modules being built, so whether a dependency has been declared won't affect whether canImport will return true. Generally speaking, the compiler doesn't know at all about your declared dependencies from the package manifest today.

@Spencer_Kohan

because it seems the platforms doesn't actually define which platforms the package can be compiled to. I.e. there is actually no way include Linux in the `platforms field.

Yes, platforms defines the minimum deployment target for the listed platforms. Non-mentioned platforms are implicitly included by default (with SwiftPM's default min deployment target, if the concept makes sense for the platform).

As far as I know, there is no way to specify an unsupported platform in a SwiftPM manifest.