Don't generate invalid CSS when migrating a complex screens config

CHANGELOG.md

@@ -10,6 +10,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 ### Fixed
 
 - _Upgrade (experimental)_: Speed up template migrations ([#14679](https://github.com/tailwindlabs/tailwindcss/pull/14679))
+- _Upgrade (experimental)_: Don't generate invalid CSS when migrating a complex `screens` config ([#14691](https://github.com/tailwindlabs/tailwindcss/pull/14691))
 
 ## [4.0.0-alpha.27] - 2024-10-15
 

integrations/upgrade/js-config.test.ts

@@ -369,3 +369,62 @@ test(
     `)
   },
 )
+
+test(
+  `does not upgrade JS config files with non-simple screens object`,
+  {
+    fs: {
+      'package.json': json`
+        {
+          "dependencies": {
+            "@tailwindcss/upgrade": "workspace:^"
+          }
+        }
+      `,
+      'tailwind.config.ts': ts`
+        import { type Config } from 'tailwindcss'
+
+        export default {
+          theme: {
+            screens: {
+              xl: { min: '1024px', max: '1279px' },
+              tall: { raw: '(min-height: 800px)' },
+            },
+          },
+        } satisfies Config
+      `,
+      'src/input.css': css`
+        @tailwind base;
+        @tailwind components;
+        @tailwind utilities;
+      `,
+    },
+  },
+  async ({ exec, fs }) => {
+    await exec('npx @tailwindcss/upgrade')
+
+    expect(await fs.dumpFiles('src/**/*.css')).toMatchInlineSnapshot(`
+      "
+      --- src/input.css ---
+      @import 'tailwindcss';
+      @config '../tailwind.config.ts';
+      "
+    `)
+
+    expect(await fs.dumpFiles('tailwind.config.ts')).toMatchInlineSnapshot(`
+      "
+      --- tailwind.config.ts ---
+      import { type Config } from 'tailwindcss'
+
+      export default {
+        theme: {
+          screens: {
+            xl: { min: '1024px', max: '1279px' },
+            tall: { raw: '(min-height: 800px)' },
+          },
+        },
+      } satisfies Config
+      "
+    `)
+  },
+)

packages/@tailwindcss-upgrade/src/migrate-js-config.ts

@@ -226,9 +226,9 @@ function canMigrateConfig(unresolvedConfig: Config, source: string): boolean {
   // migrated
   let theme = unresolvedConfig.theme
   if (theme && typeof theme === 'object') {
-    if (theme.extend && !onlyUsesAllowedTopLevelKeys(theme.extend)) return false
+    if (theme.extend && !onlyAllowedThemeValues(theme.extend)) return false
     let { extend: _extend, ...themeCopy } = theme
-    if (!onlyUsesAllowedTopLevelKeys(themeCopy)) return false
+    if (!onlyAllowedThemeValues(themeCopy)) return false
   }
 
   return true
@@ -239,12 +239,20 @@ const DEFAULT_THEME_KEYS = [
   // Used by @tailwindcss/container-queries
   'containers',
 ]
-function onlyUsesAllowedTopLevelKeys(theme: ThemeConfig): boolean {
+function onlyAllowedThemeValues(theme: ThemeConfig): boolean {
   for (let key of Object.keys(theme)) {
     if (!DEFAULT_THEME_KEYS.includes(key)) {
       return false
     }
   }
+
+  if ('screens' in theme && typeof theme.screens === 'object' && theme.screens !== null) {
+    for (let screen of Object.values(theme.screens)) {
+      if (typeof screen === 'object' && screen !== null && ('max' in screen || 'raw' in screen)) {
+        return false
+      }
+    }
+  }
   return true
 }