Why are we only checking between base/derived? Basically any value-side reference to a class should be an error before its declaration.

👍

Does this cover the ordering across files when used with -outFile? That is, Derived and Base are in different files, but when compiled into the single output the order can go a wrong way — is that also reported?

Also what about this case, will it be (incorrectly) reported as an error?

function some() {
  class Derived extends Base {
  }

  return new Derived(); 
}

class Base { }

Textually Derived is preceding the Base here, but it won't be cause any problems during execution.

My logic suggests ordering warnings should only apply if Derived and Base are top-level or namespace-level classes.

Yep, this fix uses the function isBlockScopedNameDeclaredBeforeUse which knows how to handle both outFile ordering and "evaluated later" scenarios correctly. It's the same function we use to detect improper ordering / use of let variables so that logic should be pretty well ironed-out by now.