Is there a way in Kotlin to ensure that a generic type that inherits a interface still is a interface or to assume that it would be?

Question

I want to declare a function with a generic type which has to inherit from another generic type that should be an interface, and some other class. But Kotlin complains and doesn't let me do this.

class SomeClass<T: SomeInterface> {

    fun <R>someFunktion(task: R) where  R : SomeOtherClass, R : T {}
}

If I just replace the generic type T in the function declaration with SomeInterface, it works.

class SomeClass<T: SomeInterface> {

    fun <R>someFunktion(task: R) where  R : SomeOtherClass, R : SomeInterface {}
}

So it seems the problem is that Kotlin does not know whether T is an interface or a class. If it would be a class it could not work because Kotlin does not support multi inheritance for classes. So does someone know a solution? Thank you already in advance for every effort.

Karsten Gabriel · Accepted Answer · 2022-07-25 19:21:13Z

1

Writing your above code example results in the Kotlin compiler to complain:

Type parameter cannot have any other bounds if it's bounded by another type parameter

It says that you cannot have your variable R be bound by a second type parameter T. However, for whatever reason, the problem can just be suppressed: If you add the annotation

@Suppress("BOUNDS_NOT_ALLOWED_IF_BOUNDED_BY_TYPE_PARAMETER")
fun <R>someFunktion(task: R) where  R : SomeOtherClass, R : T {}

then Kotlin will stop complaining and everything works just fine.

answered Jul 25, 2022 at 19:21

Karsten Gabriel

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