It was would be nice for this to be automatic but filter does work like this it you pass a type guard as the predicate:

``` arr.filter((a: A): a is A & { kind: 'bar'} => a.kind === 'bar') ```

If you define:

``` function isKind<K extends A['kind']>(kind: K): (a: A) => a is A & { kind: K } { return (a): a is A & { kind: K } => a.kind === kind; } ```

Then you can write:

``` arr.filter(isKind('bar')) ```

and get a properly narrowed return type array