Is it possible with KSP to generate a companion object for an already existing class (declared by the user)? I need to add a “static” extension function to classes I do not own, however they might not have a companion object since this is not in my control.

No, you need a compiler plugin to do if you compile the sources, otherwise the "user" have to add it.

no, unfortunately you need the user to add

companion object

themself

+ vote for this issue please 😉: https://youtrack.jetbrains.com/issue/KT-11968/Research-and-prototype-namespace-based-solution-for-statics-and-static-extensions

Just to clarify... this would work correct?

class Potato {
    companion object
}

// Generate w/ KSP
fun Potato.Companion.b() = 2

fun main() {
    println(Potato.b())
}

@Norbi thank you for the clarification, voted 🙂

Not sure if it applies in your case, but you can require an annotation triggering codegen to be applied to the companion object, which therefore guarantees that it is declared and you can add extensions to it

Oh! This applies to me, I should have mentioned I will require users to add an annotation. How does the codegen work?

How does the codegen work?

This is the case when you need a compiler plugin 🙂 (Because KSP can generate only new classes and resource, modifying an existing class is possible only by using compiler plugins.)