T except one class-CodePudding

Suppose I've a class called Devil

class Devil

and I've method called support

fun <T> support(t : T){

}

I want to restrict this function to only accept classes other than Devil (all classes in the world, but not Devil). Something like

fun <T except Devil> support(t : T){

}

How do I do this in Kotlin? Is this even possible?

CodePudding user response：

This is very likely an XY problem, so please do not apply this solution without considering something else.

One way to achieve this is to declare a non-generic overload of support() with the explicit type Devil, and deprecate that function with ERROR level:

fun <T> support(t: T) {
    // do your stuff
}

@Deprecated("support() cannot be used with type Devil", level = DeprecationLevel.ERROR)
fun support(devil: Devil) {
    error("NOPE.")
}

Note that this would also exclude subtypes of Devil - why is not explicitly stated in your question, but might be what you want.

However, nothing prevents users from working around it by explicitly specifying a supertype:

support<Any>(Devil()) // compiles fine

Similarly, as @gidds pointed out, this approach also doesn't prevent compilation if you pass in a variable with a static type that is not Devil even if it holds an instance of Devil (because the compiler will choose the generic overload in that case):

val hiddenDevil: Any = Devil()
support(hiddenDevil) // compiles fine