Add err.sh for invalid getStaticPaths return value (#10605)

* Add err.sh for invalid getStaticPaths return value

* Update error link

* Apply suggestions from code review

Co-authored-by: Tim Neutkens <tim@timneutkens.nl>

errors/invalid-getstaticpaths-value.md

@@ -0,0 +1,18 @@
+# Invalid unstable_getStaticPaths Return Value
+
+#### Why This Error Occurred
+
+In one of the page's `unstable_getStaticPaths` the return value had the incorrect shape.
+
+#### Possible Ways to Fix It
+
+Make sure to return the following shape from `unstable_getStaticPaths`:
+
+```js
+export async function unstable_getStaticProps() {
+  return {
+    paths: Array<string | { params: { [key: string]: string } }>
+  }
+}
+```
+

packages/next/build/utils.ts

@@ -562,7 +562,9 @@ export async function isPageStatic(
 
       const staticPathsResult = await (mod.unstable_getStaticPaths as Unstable_getStaticPaths)()
 
-      const expectedReturnVal = `Expected: { paths: [] }`
+      const expectedReturnVal =
+        `Expected: { paths: [] }\n` +
+        `See here for more info: https://err.sh/zeit/next.js/invalid-getstaticpaths-value`
 
       if (
         !staticPathsResult ||