class A {
    length: 1
    constructor() {
      this.length = 1
    }
}

class B {
    length: 2
    constructor() {
      this.length = 2
    }
}

function getTypedArray(flag: boolean) {
  return flag ? new A() : new B();
}
function getTypedArrayConstructor(flag: boolean) {
  return flag ? A : B;
}
const a = getTypedArray(true);              // A | B
const b = getTypedArrayConstructor(false);  // A constructor | B constructor

if (!(a instanceof b)) {
  console.log(a.length);  // Used to be property 'length' does not exist on type 'never'.
}