impossible to require null or disallow undefined

index.js

const assert = require('assert');
const mongoose = require('mongoose');

const ThingSchema = new mongoose.Schema({
  // https://stackoverflow.com/a/44336895/429091, doesn’t support
  // subdocuments.
  undefinedDisallowed: {
    type: String,
    required: function () {
      return this.undefinedDisallowed === undefined;
    },
    default: null,
  },
});

const Thing = mongoose.model('Thing', ThingSchema);

const thing = new Thing();
assert.ifError(thing.validateSync());
thing.undefinedDisallowed = undefined;
assert.ok(thing.validateSync());

const SuperDocumentSchema = new mongoose.Schema({
  thing: {
    type: ThingSchema,
    // https://stackoverflow.com/q/35494875/429091
    default: ThingSchema,
  },
});

const SuperDocument = mongoose.model('SuperThing', SuperDocumentSchema);

const superDocument = new SuperDocument();
// We have to mark the value dirty to cause validation to happen.
superDocument.thing.undefinedDisallowed = null;
// Throws because superThing.undefinedDisallowed is undefined even
// though superThing.thing.undefinedDisallowed is null.
assert.ifError(superDocument.validateSync());

output.txt

ohnob@DESKTOP-A4G2C0J MSYS ~/repos/mongoose-validators-hard-to-use
$ node --version
v6.11.1

ohnob@DESKTOP-A4G2C0J MSYS ~/repos/mongoose-validators-hard-to-use
$ node index.js

assert.js:377
assert.ifError = function(err) { if (err) throw err; };
                                          ^
ValidationError: SuperThing validation failed: thing.undefinedDisallowed: Path `undefinedDisallowed` is required.
    at MongooseError.ValidationError (C:\Users\ohnob\repos\mongoose-validators-hard-to-use\node_modules\mongoose\lib\error\validation.js:28:11)
    at model.Document.invalidate (C:\Users\ohnob\repos\mongoose-validators-hard-to-use\node_modules\mongoose\lib\document.js:1611:32)
    at C:\Users\ohnob\repos\mongoose-validators-hard-to-use\node_modules\mongoose\lib\document.js:1557:13
    at Array.forEach (native)
    at model.Document.validateSync (C:\Users\ohnob\repos\mongoose-validators-hard-to-use\node_modules\mongoose\lib\document.js:1539:9)
    at Object.<anonymous> (C:\Users\ohnob\repos\mongoose-validators-hard-to-use\index.js:38:30)
    at Module._compile (module.js:570:32)
    at Object.Module._extensions..js (module.js:579:10)
    at Module.load (module.js:487:32)
    at tryModuleLoad (module.js:446:12)
    at Function.Module._load (module.js:438:3)
    at Module.runMain (module.js:604:10)
    at run (bootstrap_node.js:389:7)
    at startup (bootstrap_node.js:149:9)
    at bootstrap_node.js:504:3