HMR warnings after upgrading to Webpack 5.10.0

Bug report

After upgrading to Webpack 5.10.0, I see this warning a lot on the console —

warn  - HotModuleReplacementPlugin
The configured output.hotUpdateMainFilename doesn't lead to unique filenames per runtime and HMR update differs between runtimes.
This might lead to incorrect runtime behavior of the applied update.
To fix this, make sure to include [runtime] in the output.hotUpdateMainFilename option, or use the default config.

Everything seems to work fine, though.

To Reproduce

Steps to reproduce the behavior, please provide code snippets or a repository:

1. Upgrade Webpack to 5.10.0
2. Use next to run a Next.js app in dev mode.
3. Make changes to a SSR page that is currently open in a browser so that HMR can kick in and auto-reload the page.
4. See warning on console —

   warn  - HotModuleReplacementPlugin
   The configured output.hotUpdateMainFilename doesn't lead to unique filenames per runtime and HMR update differs between runtimes.
   This might lead to incorrect runtime behavior of the applied update.
   To fix this, make sure to include [runtime] in the output.hotUpdateMainFilename option, or use the default config.
   

Expected behavior

No warnings.

Screenshots

N/A

System information

• OS: macOS
• Browser (if applies): Microsoft Edge
• Version of Next.js: 10.0.3
• Version of Node.js: v15.3.0
• Deployment: next (Dev mode)

Additional context

N/A

[subwaymatch]

My workaround is to add .[runtime] in the config.output.hotUpdateMainFilename option.

Below is my next.config.js file:

module.exports = {
  future: {
    webpack5: true,
  },
  webpack: (config, { isServer, dev, webpack }) => {
    console.log(`Webpack version: ${webpack.version}`);

    config.output.hotUpdateMainFilename =
      "static/webpack/[fullhash].[runtime].hot-update.json";

    if (!isServer) {
      config.resolve.fallback.fs = false;
    }

    return config;
  },
};

This is based on Nextjs’ default config:

https://github.com/vercel/next.js/blob/cd9bb986eba25006d4872e30eedf24ef5680bc6b/packages/next/build/webpack-config.ts#L898-L900

[balazsorban44]

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