title: I cannot set defaultProps with PartialType in Typescript
body: <p>I would like to pass to a Provider an orientation value (horizontal or vertical), but I have it declared as optional in case the user does not want to pass it.</p>
  <p>Through defaultProps I am trying to do the same that I could do with {orientation = 'horizontal'} but it's not working correctly.</p>
  <p>This gives me an error where it says orientation can be undefined.</p>
  <pre><code>export interface TabsProps extends Props {
      active: string;
      color?: DanaColor;
      orientation?: 'horizontal' | 'vertical';
      onTabChange: (tabKey: string) =&gt; void;
      children: ReactElement | ReactElement[];
  }
  
  const defaultProps: Partial&lt;TabsProps&gt; = {
      orientation: 'horizontal',
  };
  
  export const Tabs = (props: TabsProps) =&gt; {
      const { children, ...context } = { ...defaultProps, ...props };
  
      return (
          &lt;TabsProvider value={context}&gt;
              &lt;div css={tabs(context.orientation)}&gt;
                  &lt;div css={tabList(context.orientation)} role=&quot;tablist&quot;&gt;
                      {children}
                  &lt;/div&gt;
              &lt;/div&gt;
          &lt;/TabsProvider&gt;
      );
  };
  </code></pre>