How to define an extension function for a companion object of a typealias?

Question

I thought typealiases were the same as the original type, just a different name.

I figure typealiases have the same references as the original type.

typealias Celsius = Double

fun Double.Companion.foo() {} // Works
fun Celsius.Companion.foo() {} // Does not work

Here, Companion is accessible from Double but Celsius gives an unresolved reference error.

Answer 1

No, you can't access to the companion objects via typealias. One possible workaround to create one more typealias for concrete companion:

typealias CelsiusCompanion = Double.Companion

After that you can use it as following:

fun CelsiusCompanion.foo() {}

Answer 2

If you want to define an extension function, it is not possible as hluhovskyi already stated, but things are differently if you just want to invoke functions of a companion object.

There are two ways of accessing functions and properties within a companion object. You can either specify the access explicitely or implicitely. The implicit way works with a typealias the explicit one does not.

Consider this minimal example:

class ClassWithCompanion{
    companion object {
        fun sayHello() {
            println("Hello")
        }
    } 
}

typealias Alias = ClassWithCompanion

fun main(args: Array<String>) {
    ClassWithCompanion.sayHello() // implicit
    ClassWithCompanion.Companion.sayHello() // explicit 

    Alias.sayHello() // implicit (works)
    Alias.Companion.test() // explicit (does not work)
}