Extension functions for generic classes in Kotlin

Question

What\'s wrong with my extension function below

class Foo {
    fun  Foo.plus(that: Foo): Foo = throw Exception()         


        

Answer 1

Your method plus expects the parameter to have the same generic type parameter T as the receiver. Hence, you can't add a Foo to a Foo.

If you want to be able to add all types of Foo, than you need to declare your extension function like so:

operator fun  Foo.plus(that: Foo): Foo = throw Exception()