El.js

// this is exported by "some-package"
export default function El({ as: Component = "div", ...props }) {
  return <Component {...props} />
}

example.ts

import El from "some-package"

function Foo(props: { foo: string }) {
  return <div>{props.foo}</div>
}

const test = () => [
  <El as="div" className="foo" bar="baz" />,
  <El as={Foo} foo="bar" bar="baz" />,
]

some-package.d.ts

export interface PolymorphicProps<
  T extends keyof JSX.IntrinsicElements | React.ComponentType<any>,
> {
  as: T
}

type ComponentProps<T> = T extends keyof JSX.IntrinsicElements
  ? JSX.IntrinsicElements[T]
  : T extends React.ComponentType<infer P>
    ? P
    : never

declare module 'some-package' {
  export function El<
    T extends keyof JSX.IntrinsicElements | React.ComponentType<any>,
  >(props: PolymorphicProps<T> & ComponentProps<T>): JSX.Element
}