Constrain Kotlin generic to not null in one case

Issue

I have a generic type (T: Any?) that I need constrained in one situation never to be null:

class Repository<T> { // T may be a non-null or nullable type.
  fun getFromFoo(): T {}
  fun getFromBar(): T {} // This is never null. Can I mark it as so?
}


val repository = Repository<String>()
val fromFoo: String = repository.getFromFoo() // Returns a non-null String.
val fromBar: String = repository.getFromBar() // How do I make this work?


val repository = Repository<String?>()
val fromFoo: String? = repository.getFromFoo() // Returns a nullable String.
val fromBar: String = repository.getFromBar() // How do I make this return a non-null String?

While I would ideally refactor these into separate types (like FooRepository and BarRepository), is there any way to get this type constraint functionality?

Solution

What you need is the type T & Any — an intersection of generic T and Any. It represents such subtype of T that can never contain nulls.

Unfortunately the intersection types and this intersection in particular currently are non-denotable in Kotlin, meaning that you can't declare a return type of a function to be non-nullable subtype of T.

Answered By - Ilya