A Minor Dependency Update Silently Broke TypeScript Types in a Monorepo

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A TypeScript monorepo running Next.js and npm workspaces experienced a subtle type system failure when a transitive dependency received a minor version update. The issue caused a function's parameter type to collapse to 'never', a TypeScript type indicating unreachable code, even though the function worked correctly at runtime. Because the 'never' type propagates silently through downstream types, the visible error appeared far from its actual source, making diagnosis difficult. Engineers traced the root cause by bisecting type aliases and reviewing the package lockfile rather than the source code, eventually isolating the specific dependency version responsible. The fix involved pinning the dependency with a documented explanation, highlighting that semantic versioning guarantees runtime compatibility but not type-inference stability for complex generic patterns.

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