fixup! Throw error if res is accessed after gSSP returns

errors/gssp-no-mutating-res

@@ -1,8 +1,8 @@
-# Must not access ServerResponse after getServerSideProps() returns
+# Must not access ServerResponse after getServerSideProps() resolves
 
 #### Why This Error Occurred
 
-`getServerSideProps()` surfaces a `ServerResponse` object through the `res` property of its `context` arg. This object is not intended to be accessed or changed after `getServerSideProps()` returns. 
+`getServerSideProps()` surfaces a `ServerResponse` object through the `res` property of its `context` arg. This object is not intended to be accessed or changed after `getServerSideProps()` resolves. 
 
 This is because the framework tries to optimize when items like headers or status codes are flushed to the browser. If they are changed after `getServerSideProps()` completes, we can't guarantee that the changes will work.
 

packages/next/server/render.tsx

@@ -745,11 +745,12 @@ export async function renderToHTML(
     let canAccessRes = true
     let resOrProxy = res
     if (process.env.NODE_ENV !== 'production') {
-      resOrProxy = new Proxy(res, {
-        get: function (obj: ServerResponse, prop: string, receiver: any) {
+      resOrProxy = new Proxy<ServerResponse>(res, {
+        get: function (obj, prop, receiver) {
           if (!canAccessRes) {
             throw new Error(
-              `Must not access ServerResponse after getServerSideProps() returns! https://nextjs.org/docs/messages/gssp-no-mutating-res`
+              `You should not access 'res' after getServerSideProps resolves.` +
+              `\nRead more: https://nextjs.org/docs/messages/gssp-no-mutating-res`
             )
           }
           return Reflect.get(obj, prop, receiver)

test/integration/getserversideprops/pages/promise/mutate-res-props.js

@@ -0,0 +1,21 @@
+
+export async function getServerSideProps(context) {
+  return {
+    props: (async function () {
+      // Mimic some async work, like getting data from an API
+      await new Promise((resolve) => setTimeout(resolve)) 
+      context.res.setHeader('test-header', 'this is a test header')
+      return {
+        text: 'res',
+      }
+    })(),
+  }
+}
+
+export default ({ text }) => {  
+  return (
+    <>
+      <div>hello {text}</div>
+    </>
+  )
+}

test/integration/getserversideprops/test/index.test.js

@@ -147,6 +147,12 @@ const expectedManifestRoutes = () => [
       `^\\/_next\\/data\\/${escapeRegex(buildId)}\\/promise\\/mutate-res.json$`
     ),
     page: '/promise/mutate-res',
+  },  
+  {
+    dataRouteRegex: normalizeRegEx(
+      `^\\/_next\\/data\\/${escapeRegex(buildId)}\\/promise\\/mutate-res-props.json$`
+    ),
+    page: '/promise/mutate-res-props',
   },
   {
     dataRouteRegex: normalizeRegEx(
@@ -721,9 +727,17 @@ const runTests = (dev = false) => {
     it('should show error for accessing res after gssp returns', async () => {
       const html = await renderViaHTTP(appPort, '/promise/mutate-res')
       expect(html).toContain(
-        'Must not access ServerResponse after getServerSideProps() returns'
+        `You should not access 'res' after getServerSideProps resolves`
+      )
+    })
+
+    it('should show error for accessing res through props promise after gssp returns', async () => {
+      const html = await renderViaHTTP(appPort, '/promise/mutate-res-props')
+      expect(html).toContain(
+        `You should not access 'res' after getServerSideProps resolves`
       )
     })
+
   } else {
     it('should not fetch data on mount', async () => {
       const browser = await webdriver(appPort, '/blog/post-100')