Kotlin varargs functional interfaces cannot be called

[mhsmith]

This issue was discovered with Kotlin 1.3.41; maybe it's been fixed in a newer version.

In Electron Cash, I passed Python a reference to a Kotlin function with arguments (String, vararg PyObject). As expected, this was represented as an object with a method invoke(String, PyObject[]).

However, the method isn't marked as varargs in the Java reflection interface, so it can only be called with exactly one array-compatible argument. Passing anything else will invoke the base interface method Function2(Object, Object), which gives the error "is abstract and cannot be called".

I can't find any references to anyone else seeing this in the reflection API, but here's someone who says that a vararg Kotlin method decompiled into a vararg Java method.

The first step would be to look at the generated class file and see whether the method is marked as vararg there.