withDefaults and unions

[OliverJAsh]

Re. https://github.com/gcanti/typelevel-ts/blob/e1d718db887476f7b6b6e2c88542cc6ce3bd8265/examples/withDefaults.tsx

I have a use case for withDefaults that doesn't appear to work, but I'm not entirely clear why.

type Props =
    | {
          tag: 'a';
          href: string;
      }
    | {
          tag: 'button';
      };

declare const props: Props;

type DefaultProps = {
    tag: 'a';
};

declare const defaultProps: DefaultProps;

declare function withDefaults<Props extends DefaultProps, DefaultProps extends object>(
    props: Props,
    defaultProps: DefaultProps,
): void;

withDefaults(
    // Type '"button"' is not assignable to type '"a"'.
    props,
    defaultProps,
);

My thinking:

• withDefaults requires DefaultProps (defaultProps) to be a "supertype" of Props (props) (Props extends DefaultProps)
• In this example, DefaultProps is not a supertype of Props because Props is a union

Any thoughts?