Update to merge props from GSSP methods with _app pageProps

errors/gssp-pageprops-conflict.md

@@ -0,0 +1,11 @@
+# getStaticProps/getServerSideProps `pageProps` Conflict
+
+#### Why This Error Occurred
+
+In your `_app` you returned `pageProps` for a page that uses `getStaticProps` or `getServerSideProps`.
+
+The `pageProps` value will be overridden with the result from `getStaticProps` or `getServerSideProps`
+
+#### Possible Ways to Fix It
+
+Only return `pageProps` from your custom `_app` when needed for a page with `getInitialProps`

packages/next/next-server/server/render.tsx

@@ -503,6 +503,18 @@ export async function renderToHTML(
       props[STATIC_PROPS_ID] = true
     }
 
+    if (
+      (isSSG || getServerSideProps) &&
+      props.pageProps &&
+      process.env.NODE_ENV !== 'production'
+    ) {
+      console.warn(
+        `"pageProps" was returned from "_app" for page "${pathname}" with ${
+          isSSG ? 'getStaticProps' : 'getServerSideProps'
+        } and will be overridden.\nSee here for more info: https://err.sh/next.js/gssp-pageprops-conflict`
+      )
+    }
+
     let previewData: string | false | object | undefined
 
     if ((isSSG || getServerSideProps) && !isFallback) {

test/integration/gssp-pageProps-conflict/pages/_app.js

@@ -0,0 +1,9 @@
+const App = ({ Component, pageProps }) => <Component {...pageProps} />
+
+App.getInitialProps = () => ({
+  pageProps: {
+    hello: 'world',
+  },
+})
+
+export default App

test/integration/gssp-pageProps-conflict/test/index.test.js

@@ -0,0 +1,63 @@
+/* eslint-env jest */
+/* global jasmine */
+import fs from 'fs-extra'
+import { join } from 'path'
+import { findPort, launchApp, killApp, renderViaHTTP } from 'next-test-utils'
+
+jasmine.DEFAULT_TIMEOUT_INTERVAL = 1000 * 60 * 2
+const appDir = join(__dirname, '..')
+const indexPage = join(appDir, 'pages/index.js')
+
+let stderr = ''
+let appPort
+let app
+
+describe('pageProps GSSP conflict', () => {
+  it('should show warning for pageProps and getServerSideProps conflict', async () => {
+    appPort = await findPort()
+    app = await launchApp(appDir, appPort, {
+      onStderr(msg) {
+        stderr += msg || ''
+      },
+    })
+    await fs.writeFile(
+      indexPage,
+      `
+      export const getServerSideProps = () => ({ props: { hi: 'hi' } })
+      export default () => 'hello from gssp'
+    `
+    )
+    const html = await renderViaHTTP(appPort, '/')
+    await fs.remove(indexPage)
+    await killApp(app)
+
+    expect(html).toContain('hello from gssp')
+    expect(stderr).toContain(
+      `"pageProps" was returned from "_app" for page "/" with getServerSideProps`
+    )
+  })
+
+  it('should show warning for pageProps and getStaticProps conflict', async () => {
+    appPort = await findPort()
+    app = await launchApp(appDir, appPort, {
+      onStderr(msg) {
+        stderr += msg || ''
+      },
+    })
+    await fs.writeFile(
+      indexPage,
+      `
+      export const getStaticProps = () => ({ props: { hi: 'hi' } })
+      export default () => 'hello from gsp'
+    `
+    )
+    const html = await renderViaHTTP(appPort, '/')
+    await fs.remove(indexPage)
+    await killApp(app)
+
+    expect(html).toContain('hello from gsp')
+    expect(stderr).toContain(
+      `"pageProps" was returned from "_app" for page "/" with getStaticProps`
+    )
+  })
+})