_Whose_ destructor, if not the receiving-type? Is there a third "binding" object, because then you have three potentially-unrelated lifetimes.

>> Generally callback registration is not where you really want type erasure anyways.

I'm responding to the article: "Some mechanisms for doing callbacks require a modification to, or derivation of, the caller or callee types. The fact that an object is connected to another object in a particular application often has nothing to do with its type. As we'll see below, mechanisms that are type intrusive can reduce the flexibility and increase the complexity of application code. "