first commit

test/integration/config-devtool-dev/next.config.js

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+module.exports = {
+  webpack: (config) => {
+    config.devtool = 'cheap-module-source-map'
+    return config
+  },
+}

test/integration/config-devtool-dev/pages/index.js

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+import { useEffect } from 'react'
+
+export default function Index(props) {
+  useEffect(() => {
+    throw new Error('this should render')
+  }, [])
+  return <div>Index Page</div>
+}

test/integration/config-devtool-dev/test/index.test.ts

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+/* eslint-env jest */
+
+import {
+  waitForRedbox,
+  findPort,
+  getRedboxSource,
+  killApp,
+  launchApp,
+  retry,
+} from 'next-test-utils'
+import webdriver from 'next-webdriver'
+import { join } from 'path'
+
+const appDir = join(__dirname, '../')
+
+// Webpack specific config tests.
+;(process.env.IS_TURBOPACK_TEST ? describe.skip : describe)(
+  'devtool set in development mode in next config',
+  () => {
+    it('should warn and revert when a devtool is set in development mode', async () => {
+      let stderr = ''
+
+      const appPort = await findPort()
+      const app = await launchApp(appDir, appPort, {
+        onStderr(msg) {
+          stderr += msg || ''
+        },
+      })
+
+      await retry(async () => {
+        expect(stderr).toMatch(/Reverting webpack devtool to /)
+      })
+
+      const browser = await webdriver(appPort, '/')
+      await waitForRedbox(browser)
+      if (process.platform === 'win32') {
+        // TODO: add win32 snapshot
+      } else {
+        expect(await getRedboxSource(browser)).toMatchInlineSnapshot(`
+          "pages/index.js (5:11) @ Index.useEffect
+
+            3 | export default function Index(props) {
+            4 |   useEffect(() => {
+          > 5 |     throw new Error('this should render')
+              |           ^
+            6 |   }, [])
+            7 |   return <div>Index Page</div>
+            8 | }"
+        `)
+      }
+      await browser.close()
+
+      await killApp(app)
+    })
+  }
+)