@tamasvajk the isAnonymClass relation is supposed to be a bijection, meaning a one-to-one ClassInstanceExpr <-> AnonymousType relationship. If we extract the default value in both the $default function and a JvmOverloads function, then you end up with something like

fun f(x: SomeInterface = { a -> a }, y: Int)

becoming

fun f$default(var x: SomeInterface?, var y: Int?, argsMask: Int, ...) {
  if (!(argsMask & 1)) x = { a -> a }
  ...    
}

fun f(y: Int) {
  f({ a -> a }, y)
}

Thus the source-level class representing the anonymous class has been duplicated. We can handle that either by allowing that duplication and extracting two classes, or work around the problem by implementing fun f(y: Int) { f$default(null, y, 3, ...) }, which is the option I've taken here.