MDX `export const metadata` fails to build with webpack in 16.2.0 (regression from 16.1.6)

Link to the code that reproduces this issue

https://github.com/minamicode/my-mdx-repro

To Reproduce

1. Clone this repo: git clone https://github.com/minamicode/my-mdx-repro
2. Install dependencies: npm install
3. Run npm run build
4. Build fails with the error below

Current vs. Expected behavior

Current behavior Build fails with webpack in Next.js 16.2.0 with the following error: You are attempting to export "metadata" from a component marked with "use client", which is disallowed.

Expected behavior Build succeeds. This pattern is explicitly documented as supported in the Next.js MDX guide: https://nextjs.org/docs/app/guides/mdx The same code builds successfully in:

Next.js 16.1.6 with webpack Next.js 16.2.0 with Turbopack

Provide environment information

Operating System:
  Platform: darwin
  Arch: arm64
  Version: Darwin Kernel Version 25.3.0: Wed Jan 28 20:53:31 PST 2026; root:xnu-12377.91.3~2/RELEASE_ARM64_T8103
  Available memory (MB): 8192
  Available CPU cores: 8
Binaries:
  Node: 22.9.0
  npm: 11.10.1
  Yarn: N/A
  pnpm: N/A
Relevant Packages:
  next: 16.2.0 // Latest available version is detected (16.2.0).
  eslint-config-next: N/A
  react: 19.2.4
  react-dom: 19.2.4
  typescript: 5.9.3
Next.js Config:
  output: N/A

Which area(s) are affected? (Select all that apply)

Markdown (MDX), Webpack

Which stage(s) are affected? (Select all that apply)

next build (local), Other (Deployed)

Additional context

This pattern is explicitly documented as supported in the Next.js MDX guide: https://nextjs.org/docs/app/guides/mdx

The bug is webpack-specific — the same code builds successfully with Turbopack in 16.2.0. It also built successfully with webpack in 16.1.6, suggesting this is a regression introduced in 16.2.0.

Workaround: downgrade to [email protected]