New types for SSG break the GetServerSidePropsResult type

[PepijnSenders]

Bug report

Describe the bug

In this PR the type of GetServerSidePropsResult was updated which breaks it because the | chain is a collection of incompatible types.

This is the TS error:

Property 'props' does not exist on type 'GetServerSidePropsResult<ServerSidePropsResult>'.
  Property 'props' does not exist on type '{ redirect: Redirect; }'.

When you do something like this:

const { ssrPayload } = (serverSidePropsResult).props;

Even typecasting doesn't work:

const { ssrPayload } = (serverSidePropsResult as GetServerSidePropsResult<
  ServerSidePropsResult
>).props;

I'm not sure how this should be fixed, maybe we should add this GetServerSidePropsResult as generic, like so:

export type GetServerSideProps<
  P extends { [key: string]: any } = { [key: string]: any },
  ResultType = GetServerSidePropsResult<P>,
  Q extends ParsedUrlQuery = ParsedUrlQuery
> = (
  context: GetServerSidePropsContext<Q>
) => Promise<ResultType>

... and then have different types for notFound and redirect.

Expected behavior

The types not to trigger an error.

[ljosberinn]

Those new types seem absolutely correct. You'd need to check for presence instead:

if(!('props' in serverSidePropsResult)) {
  throw new Error('invalid getSSP result')
}

const { ssrPayload } = serverSidePropsResult

[PepijnSenders]

@ljosberinn right, weird I thought that it would work with casting as well, but I probably would need to cast as { props: serverSidePropsResult }

[balazsorban44]

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