The class Infer.Inference context defines this callback function:

void addFreeTypeListener(List<Type> types, FreeTypeListener ftl) {
   freeTypeListeners.put(ftl, freeVarsIn(types));
}

where FreeTypeListener is defined as follows:

interface FreeTypeListener {
   void typesInferred(InferenceContext inferenceContext);
}

This is a functional interfaces, and usages of the callback should be tweaked to use lambdas instead of anonymous inner classes.