warn about useless prefetch if action is a function

packages/next/src/client/form.tsx

@@ -58,12 +58,13 @@ export type FormProps<RouteInferType = any> = InternalFormProps
 export default function Form({
   replace,
   scroll,
-  prefetch = null,
+  prefetch: prefetchProp,
   ref: externalRef,
   ...props
 }: FormProps) {
   const actionProp = props.action
   const isNavigatingForm = typeof actionProp === 'string'
+  const prefetch = prefetchProp ?? null
 
   for (const key of DISALLOWED_FORM_PROPS) {
     if (key in props) {
@@ -125,6 +126,11 @@ export default function Form({
             '  `router.replace()` - https://nextjs.org/docs/app/api-reference/functions/use-router#userouter\n'
         )
       }
+      if (prefetchProp !== undefined) {
+        console.error(
+          'Passing `prefetch` to a <Form> whose `action` is a function has no effect.'
+        )
+      }
     }
     return <form {...props} ref={ownRef} />
   }