Generic form of GetStaticProps and GetServerSideProps (vercel#10856)

* Generic form of GetStaticProps and GetServerSideProps

* Oops, fix type!

* Add constraint for P

* Update packages/next/types/index.d.ts

Co-authored-by: Joe Haddad <timer150@gmail.com>
Co-authored-by: Joe Haddad <joe.haddad@zeit.co>

packages/next/types/index.d.ts

@@ -64,12 +64,14 @@ export {
   NextApiHandler,
 }
 
-export type GetStaticProps = (ctx: {
+export type GetStaticProps<
+  P extends { [key: string]: any } = { [key: string]: any }
+> = (ctx: {
   params?: ParsedUrlQuery
   preview?: boolean
   previewData?: any
 }) => Promise<{
-  props: { [key: string]: any }
+  props: P
   revalidate?: number | boolean
 }>
 
@@ -78,13 +80,15 @@ export type GetStaticPaths = () => Promise<{
   fallback: boolean
 }>
 
-export type GetServerSideProps = (context: {
+export type GetServerSideProps<
+  P extends { [key: string]: any } = { [key: string]: any }
+> = (context: {
   req: IncomingMessage
   res: ServerResponse
   params?: ParsedUrlQuery
   query: ParsedUrlQuery
   preview?: boolean
   previewData?: any
-}) => Promise<{ [key: string]: any }>
+}) => Promise<{ props: P }>
 
 export default next