Flow can't seem to choose between union types

try flow

declare function gen(): [Error, null] | [null, number];

const [err, number] = gen();

if (!err) {
  (number: number)
}

that last cast throws an error that number can be null even though the if statement should force (err: null) and (number: number)

Any suggestions on how to re-write this would be appreciated!

[rvion]

@markkahn

this works:

declare function gen(): [Error, null] | [null, number];

const [err, number] = gen();

if (number != null) {
	(number: number)
}

@rvion - of course, but I didn't provide the best example trying to slim it down from my actual code. Better is:

declare function gen(): [Error, null] | [null, number];

function foo() {
  const [err, number] = gen();

  if (err) {
    return;
  }

  (number: number)
}

I'd expect it to be able to pick [err, number] = [null, number] after the if statement.