Type 'false' is not assignable to type 'true' in getServerSideProps

[kevinnammour]

There is a weird behavior when it comes to returning different props in getServerSideProps. For simplifying the question, I will give a small example:

Suppose I have this code:

import Search from "@/components/Search";
import { GetServerSideProps } from "next";
import { AuthAction, withAuthUser, withAuthUserTokenSSR } from "next-firebase-auth";

interface Props {
  even: boolean;
}

const Listings = () => {
  return <Search />;
};

export const getServerSideProps: GetServerSideProps<Props> = withAuthUserTokenSSR({
  whenUnauthed: AuthAction.REDIRECT_TO_LOGIN,
})(async () => { // red underline in here
  const a = Math.floor(Math.random());
  if (a % 2 == 0) {
    return {
      props: {
        even: true,
      },
    };
  } else {
    return {
      props: {
        even: false,
      },
    };
  }
});

export default withAuthUser({
  whenUnauthedAfterInit: AuthAction.REDIRECT_TO_LOGIN,
})(Listings);

There is a red highlight on the line where I wrote the comment. It's saying this:

Argument of type '() => Promise<{ props: { even: true; }; } | { props: { even: false; }; }>' is not assignable to parameter of type 'SSRPropGetter<{ even: true; }, ParsedUrlQuery, PreviewData>'.
  Type 'Promise<{ props: { even: true; }; } | { props: { even: false; }; }>' is not assignable to type 'Promise<GetServerSidePropsResult<{ even: true; }>>'.
    Type '{ props: { even: true; }; } | { props: { even: false; }; }' is not assignable to type 'GetServerSidePropsResult<{ even: true; }>'.
      Type '{ props: { even: false; }; }' is not assignable to type 'GetServerSidePropsResult<{ even: true; }>'.
        Type '{ props: { even: false; }; }' is not assignable to type '{ props: { even: true; } | Promise<{ even: true; }>; }'.
          Types of property 'props' are incompatible.
            Type '{ even: false; }' is not assignable to type '{ even: true; } | Promise<{ even: true; }>'.ype '{ even: false; }' is not assignable to type '{ even: true; }'.
                Types of property 'even' are incompatible.
                  Type 'false' is not assignable to type 'true'

Why is that?