This is going to break ASON. Why is __instanceof being removed? There doesn't seem like a logical reason for this to occur.

It's a change resulting from #2548, where remaining open issues with interfaces have been tackled as well that required a more sophisticated approach to implement instanceof to also support interfaces. It's now compiler generated. In turn, the old mechanism to perform these checks became unused, and as such redundant. Can you explain the use case a little? Perhaps there are ways to support it otherwise.

abstract class BaseClass {}

class MyClass extends BaseClass {}

ASON.deserialize<BaseClass>(buffer); // no longer can validate a serialized MyClass

This breaks the entirety of the as-lunatic package and I would have to ask end users of ASON to do this:

let result = ASON.deserialize<BaseClass>(); // oh no it's an Array
assert(result instanceof BaseClass); // not ergonomic and backwards uncompatible

also support interfaces

Okay. I understand. Sorry for not understanding. I just feel like I have no workarounds for this.

Can ASON.deserialize<T> perhaps do:

function deserialize<T>(...) {
  let ret = ...;
  if (ret instanceof T) ... // AS allows this
}

The assert would panic if it isn't an instanceof T... let me try that.

I guess one question there is what is used as the type of ret so this doesn't become precomputed, hmm.