Tuple type is inferred from array even after mutable operation on it

[irnc]

a at the end of following program has tuple type defined as [string, string]

// @flow

const a = ['1', '2'];
const fn = (p: [string, string]) => {
  console.log(p);
};

a.unshift(3);

fn(a);

I would expect a to be inferred as [number, string, string] or Array<string | number>. Seeing tuple of 2 elements was unexpected.

Aforementioned code results in no error, please try it on Try Flow.

On the contrarry fn2(a) as defined below evaluates without errors:

const fn2 = (p: Array<string | number>) => {
  console.log(p);
};

[asolove]

This is very interesting. What's most interesting to me is that the unshift is marked as not allowed if you explicitly add the tuple type to a like this:

const a: [string, string] = ['1', '2'];
const fn = (p: [string, string]) => {
  console.log(p);
};

a.unshift(3); // property 'unshift' not found on $ReadOnlyArray

fn(a);

I played around with lots of other options and it seems like there is no type I can explicitly add to a to cause the behavior seen when leaving the type out.

So here's my guess: because a literal like a could be used as either an array or a tuple, Flow gives it an internal type something like Array<A> | [A, A] and then waits to see how it is used. So both Array<string|number> (for unshift) and using it as a tuple (for fn) should be allowed on their own. But it seems like one of those usages should rule out the other. Perhaps Flow is failing to refine the type of a based on how it is used the first time?