Build error upgrading from 5.0.0-alpha.20 to 5.0.0-alpha.21

After updating the following dependencies, the project fails at build time:

  "@emotion/cache": "11.1.3",
  "@emotion/react": "11.1.4",
  "@emotion/server": "11.0.0",
  "@emotion/styled": "11.0.0",
-  "@material-ui/core": "5.0.0-alpha.20",
-  "@material-ui/icons": "5.0.0-alpha.20",
-  "@material-ui/lab": "5.0.0-alpha.20",
+  "@material-ui/core": "5.0.0-alpha.21",
+  "@material-ui/icons": "5.0.0-alpha.21",
+  "@material-ui/lab": "5.0.0-alpha.21",

Building the app produces the following error. I went through the release changes, but I couldn’t find the reason.

$ next build
Loaded env from /Users/test/mui5-test/.env.production.local
info  - Creating an optimized production build  
info  - Compiled successfully

> Build error occurred
/Users/test/mui5-test/node_modules/@material-ui/core/styles/index.js:1
export { default as adaptV4Theme } from './adaptV4Theme';
^^^^^^

SyntaxError: Unexpected token 'export'
    at wrapSafe (internal/modules/cjs/loader.js:915:16)
    at Module._compile (internal/modules/cjs/loader.js:963:27)
    at Object.Module._extensions..js (internal/modules/cjs/loader.js:1027:10)
    at Module.load (internal/modules/cjs/loader.js:863:32)
    at Function.Module._load (internal/modules/cjs/loader.js:708:14)
    at Module.require (internal/modules/cjs/loader.js:887:19)
    at require (internal/modules/cjs/helpers.js:74:18)
    at Object.9Pu4 (/Users/test/mui5-test/.next/server/pages/_app.js:340:18)
    at __webpack_require__ (/Users/test/mui5-test/.next/server/pages/_app.js:23:31)
    at Module.hUgY (/Users/test/mui5-test/.next/server/pages/_app.js:1125:15)
    at __webpack_require__ (/Users/test/mui5-test/.next/server/pages/_app.js:23:31)
    at Object.0 (/Users/test/mui5-test/.next/server/pages/_app.js:106:18)
    at __webpack_require__ (/Users/test/mui5-test/.next/server/pages/_app.js:23:31)
    at /Users/test/mui5-test/.next/server/pages/_app.js:91:18
    at Object.<anonymous> (/Users/test/mui5-test/.next/server/pages/_app.js:94:10)
    at Module._compile (internal/modules/cjs/loader.js:999:30)
error Command failed with exit code 1.
info Visit https://yarnpkg.com/en/docs/cli/run for documentation about this command.

Process finished with exit code 1

• The issue is present in the latest release.
• I have searched the issues of this repository and believe that this is not a duplicate.

Current Behavior 😯

Expected Behavior 🤔

Steps to Reproduce 🕹

Steps:

Context 🔦

Your Environment 🌎

`npx @material-ui/envinfo`

  System:
    OS: macOS 11.1
  Binaries:
    Node: 12.20.0 - ~/.asdf/installs/nodejs/12.20.0/bin/node
    Yarn: 1.22.10 - /usr/local/bin/yarn
    npm: 6.14.8 - ~/.asdf/installs/nodejs/12.20.0/bin/npm
  Browsers:
    Chrome: 87.0.4280.88
    Edge: Not Found
    Firefox: 83.0
    Safari: 14.0.2
  npmPackages:
    @emotion/react: 11.1.4 => 11.1.4 
    @emotion/styled: 11.0.0 => 11.0.0 
    @material-ui/core: 5.0.0-alpha.21 => 5.0.0-alpha.21 
    @material-ui/icons: 5.0.0-alpha.21 => 5.0.0-alpha.21 
    @material-ui/lab: 5.0.0-alpha.21 => 5.0.0-alpha.21 
    @material-ui/styled-engine:  5.0.0-alpha.21 
    @material-ui/styles:  5.0.0-alpha.21 
    @material-ui/system:  5.0.0-alpha.21 
    @material-ui/types:  5.1.3 
    @material-ui/unstyled:  5.0.0-alpha.21 
    @material-ui/utils:  5.0.0-alpha.21 
    @types/react: 17.0.0 => 17.0.0 
    react: 17.0.1 => 17.0.1 
    react-dom: 17.0.1 => 17.0.1 
    typescript: 4.1.3 => 4.1.3 

[oliviertassinari]

When this going to solve?

@usama-asfar As soon as we make a new release. Maybe tomorrow so that developers coming back from holidays can have a working environment with the latest changes.

[oliviertassinari]

Would the following be enough for fixing it in the next time we release it from a Windows machine?

diff --git a/scripts/copy-files.js b/scripts/copy-files.js
index 146adead04..229b21d7da 100644
--- a/scripts/copy-files.js
+++ b/scripts/copy-files.js
@@ -31,7 +31,7 @@ async function createModulePackages({ from, to }) {
       const packageJson = {
         sideEffects: false,
         module: './index.js',
-        main: path.join('../node', directoryPackage, 'index.js'),
+        main: `../node/${directoryPackage}/index.js`,
         types: './index.d.ts',
       };

In anycase, we round-robin releases, the next release (v5.0.0-alpha.22) won’t happen from a Windows machine, but we still need to fix it for the next time.