Flow: Array of a type is seen as incompatible with an Array of superset Union

[mike-marcacci]

I believe that in the following example, the variable a should be compatible with the parameter arg, seeing as it is a compatible subset of the union.

type FooUnion = string | { id: string };

function foo (arg: FooUnion[]) {
    console.log(arg);
}

const a: string[] = ['a'];

foo(a);

FooUnion (object type) This type is incompatible with const a: string[] = ['a'];

[nmn]

Problem due to variance. Use this instead:

type FooUnion = string | { id: string };

function foo (arg: $ReadOnlyArray<FooUnion>) {
    console.log(arg);
}

const a: string[] = ['a'];

foo(a);