Turbopack: Missing Optional PeerDependencies Fail Module Resolution (Webpack Ignores Them)

Turbopack's resolver does not check whether a failed resolution request corresponds to an optional peer dependency of the importing package. Webpack explicitly ignores such unresolvable optional peers, but Turbopack treats the missing module as a hard error. The underlying design flaw is the absence of a resolver hook or configuration in Turbopack to handle optional peer dependencies, which are a standard npm feature.

1. Clone https://github.com/scs0209/nextjs-optional-peer-turbopack-repro 2. Run `pnpm install` 3. Run `pnpm build` (uses Turbopack by default) 4. Observe build failure with `Module not found: Can't resolve 'foo'` 5. Optionally run `pnpm exec next build --webpack` to see the expected success

This fix may cause a silent failure if an optional peer is actually required at runtime but missing—the code path that uses it would receive an empty module and likely throw a runtime error, which is still better than a build failure but may be unexpected. Additionally, the resolution fallback adds a file I/O lookup on every failed resolve, potentially impacting performance in large projects. Rollback is straightforward: remove the added fallback code and revert to the original resolver. In concurrent or multi-threaded builds, ensure the package.json reads are cached or use a shared filesystem abstraction to avoid race conditions or excessive overhead.