popFunc doesn't actually ensure that it pops an instruction

[sandersn]

It's probably an artifact from the translation from C#, but popFunc doesn't actually ensure that it pops an instruction. popType<T> uses this.pop() as T which is just a cast (assertion) in Typescript. So I think this error will never occur.

This pattern:

function f<T>(): T {
  return whatever as T;
}

Is not useful in Typescript and is misleading when coming from C#, because it's basically just a cast. I'd recommend replacing popType with this.pop() as T.

[sandersn]

the error's predicate should probably be if (!(i instanceof CatInstruction)) ...