Array<A|B> should be compatible with Array<A>

[aldendaniels]

This:

declare function func(things: Array<string | number>): void;
declare var value: string[];
func(value);

Gives:

1: declare function func(things: Array<string | number>): void;
                                                ^ number. This type is incompatible with
2: declare var value: string[];
                      ^ string

[aldendaniels]

Perhaps related: #2507

[omerzach]

Arrays are invariant: T being a subtype of U does not mean Array<T> is a subtype of Array<U>. Otherwise you could push a number onto things inside of func and there would be a number in value, which has type Array<string>.

[jesseschalken]

Can't because func(value) could add a number to value.

function func(things: Array<string | number>): void {
  things.push(1);
}
var value: string[] = ['f'];
func(value);
console.log(typeof value[1]); // a number!

[aldendaniels]

@jesseschalken @omerzach Wow, thanks for the quick follow up! Is there a way to make flow allow this if the function doesn't mutate the array - e.g. using a readonly array?

[jesseschalken]

You can use func<T:string|number>(things: Array<T>): void or, of any Iterable will do, func(things: Iterable<string|number>): void (Iterable<T> is covariant in T).

[jesseschalken]

See also #2333

[aldendaniels]

@jesseschalken Sure enough! func(things: $ReadOnlyArray<T>) also appears to work

[aldendaniels]

Closing in favor of #2333