(KMP) Ignore some commonTests in a specific platform

In Kotlin Multiplatform Project, it is common to write tests in commonTest, as you can run it on all platforms you configured. However, while you want to make test "common" as much as you can, there might be some test cases you can't run on some specific platform. So, what to do?

A very naive way is to move those "non-common" test cases to platform-specific test directories. It is okay if you only target two platforms, but what if you want 3 or more target platforms? I don't think it's a good practice to have the same test cases in several platform-specific test directories.

So here's the thing. Kotlin Multiplatform Project has 'expect/actual mechanism', with which you can provide platform-specific declarations. With this, you can declare custom @Ignore annotation, which selectively ignores test cases on specific platforms.

Assume you have 2 target platforms; Android and iOS.
You want to create an annotation which tells test runner to ignore the annotated cases/classes on iOS.

First, you need to need the 'expect' declaration in common code.

// commonMain
@Target(AnnotationTarget.CLASS, AnnotationTarget.FUNCTION)
expect annotation class IgnoreIos

Then you need to write corresponding 'actual' declarations in each platform.

// androidMain
@Target(AnnotationTarget.CLASS, AnnotationTarget.FUNCTION)
actual annotation class IgnoreIos
// iosMain
actual typealias IgnoreIos = kotlin.test.Ignore

The key point here is in iOS 'IgnoreIos' is a typealias for kotlin.test.Ignore, but it's just a useless annotation in Android. So it works as kotlin.test.Ignore in iOS but does nothing in Android.