I've found that I can only call a receiver function while invoking a parent class' constructor if a ...

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[bbaldino]

I've found that I can only call a receiver function while invoking a parent class' constructor if a companion object is defined. Is this some subtle type system thing? Or a bug?

fun <T : Any> T.double(num: Int): Int = num * 2

abstract class Parent(val num: Int)

// Works
class Child(
    num: Int
) : Parent(double(num)) {
    private companion object
}

// Error: unresolved reference 'double'
class Child2(num: Int) : Parent(double(num))

[diesieben07]

In your "Works" example the receiver is inferred to be the companion object, which is the only possible receiver in scope.
In the 2nd case there is no such receiver (this) cannot be leaked to outside functions here, because the object is not yet fully constructed.

[diesieben07]

Why does that double function even have a receiver? It's not used...

[bbaldino]

Interesting

[bbaldino]

It's a simplified example

[bbaldino]

Thanks for explaining!