Comment by TheRealPomax

Hmm...

  const numbers: Array<number> = [];
  // HegelError: Type "Array<number>" is incompatible with type "Array<number | string>"
  const numbersOrStrings: Array<string | number> = numbers;

The very first example is already an odd choice, given that <number> is type-compatible with <string|number>. Errors should be flagged once we try to call functions on the elements, with calls on number[...] validated based on <number>, and calls on numbersOrStrings[...] validated based on the shared API between <string> and <number> (which is a tiny API surface, but there is one).

  numbersOrStrings[1] = "Hello, TypeError!";

Why is this not an error based on the incorrect assignment we already had? "HegelError: cannot assign to alias numberOfStrings of numbers, numbers is of type <number>".

  // HegelError: Property "toFixed" does not exist in "Number | undefined"
  numbers[1].toFixed(1);

Heck even this error is weird: we declared numbers as an array of <number>, not an array of <number|undefined>. If you're going to be that strict, track list sizes and go "HegelError: Property numbers[1] is undefined, which is type-incompatible with <number>".