TypeScript does not recognize getters with union types

[wesleycho]

I have a particular setup

class Foo {
  get bar() {
    return 'foobar';
  }
}

class Bar {
  get foo() {
    return 'foobar';
  }
}

class Foobar {
  data: Foo | Bar;

  baz() {
    if (isFooInstance(this.data) {
      return this.data.bar;
    }
  }
}

TypeScript throws an error in this situation saying bar does not exist on Foo | Bar - this is incorrect behavior IMO, especially if I have already screened out what type it is via other mechanisms.

[RyanCavanaugh]

Duplicate of #3812

[wesleycho]

Ah sorry, couldn't find anything mentioning union types in particular when searching for issues.