static Body: typeof ModalBody;

is undefined, so it cannot be used as <MyModal.Body>.

In case it's inherited from wrapped component, it should be:

static Body = ModalBody;

Where Body type is inferred.

This problem could be avoided by using TypeScript strict (strictNullChecks) compiler option.

Thanks to Brady from Reactiflux I was able to find the answer.

The issue was that I was using React.FC

More info about React.FC here: https://github.com/typescript-cheatsheets/react-typescript-cheatsheet#function-components

Here is the answer:

interface ContentProps {
  children: ReactNode;
}
const Content = (props: ContentProps) => <div>{props.children}</div>;

interface Props {
  children: ReactNode;
}
const Nav = (props: Props) => <div>{props.children}</div>;

Nav.Content = Content;

export default Nav;