Mixed refined to an array produces a read-only array, which breaks a lot of existing code.

[lxe]

Flow version: 0.98

https://flow.org/try/#0GYVwdgxgLglg9mABMOcAUBDAXIgggJ3wwE8AeAWxgA8BTAEwD4BKRAb0QF8AoL0SWBIgBGGfGjg5KtOi1ZdEiGMERoCRYgDoYAZzUlxTWfIXJUBgNzHuHIA

function foo(a: Array<mixed>) { }

function bar(o: mixed) {
  if (Array.isArray(o)) { // There's no way to dynamically refine to a $ReadOnlyArray
    foo(o);
  }
}

Expected behavior

No Errors

Actual behavior

5:     foo(o);
           ^ Cannot call `foo` with `o` bound to `a` because read-only array type [1] is incompatible with array type [2].
References:
3: function bar(o: mixed) {
                   ^ [1]
1: function foo(a: Array<mixed>) { }
                   ^ [2]

What was the practical rationale behind this change?

[goodmind]

5639532

[goodmind]

@dsainati1

[dsainati1]

This is the same logic behind #7685. Writing to an array refined from mixed is trivially unsound, as you can see in this example:

function bad(array: mixed) {
  if (Array.isArray(array)) {
    const problem: Array<mixed> = array;
    problem[1] = 0;
  }
}

bad((["3"] : Array<string>));

[lxe]

I don't really know what "unsound" means, but if I have a mixed the types of its properties should be understood at both reading (refinements) and writing (assignments).

If I'm doing let a:mixed = {}; a.foo = 'string' it should be fine, but then if later I do a.foo ++ flow should tell me that I'm wrong.

I think you're sacrificing practicality and usefulness of Flow in the name of "soundness"