[vzaidman]

Follow-up to #1984, which added flow-typed/sourcemap-codec.js with a declare module block.

declare module is only allowed in a library file. OSS Flow loads flow-typed/*.js as libdefs, so this passed OSS CI. Internal Flow only treats .flow files as library files, so the same file fails internally with unsupported-syntax, and the metro-transform-worker import of @jridgewell/sourcemap-codec falls back to untyped-import. Those two errors block landing the #1984 DiffTrain stack internally (flow-check-land on D129410423).

Renaming the file to flow-typed/sourcemap-codec.js.flow makes Flow treat it as a library file in both. No content change.

Validation on this branch:

• OSS flow focus-check on packages/metro-transform-worker/src/index.js: 0 errors, and an intentional type error in the renamed file is reported, proving OSS Flow still loads it as a libdef.
• prettier --check: pass.

Changelog: [Internal]

[github-actions]

Metro review: Looks good

Risk: low · Commit: bd4b5fa · 7 commands run

This renames the @jridgewell/sourcemap-codec libdef to flow-typed/sourcemap-codec.js.flow with no content change, matching the existing eventemitter3.js.flow. OSS Flow still loads it as a library file and the rename has no runtime effect, so it looks good; the internal Flow behavior it targets can't be checked here. Separately, once this lands internally, packages/metro-transform-worker/src/index.js:45 ($FlowFixMe[cannot-resolve-module] - No typings at Meta) looks stale: OSS Flow already reports it as an unused suppression, at base and head.

No findings.

Verified

• ✅ yarn typecheck — Head: No errors.
• ✅ flow check with a probe module assigning decode('AAAA') to a number (failed as expected) — Head: incompatible-type citing flow-typed/sourcemap-codec.js.flow:20, so OSS Flow loads the renamed file as a libdef. Probe removed afterwards.
• ✅ flow type-at-pos packages/metro-transform-worker/src/index.js 46 20 — Head: decodeMappings resolves to the typed signature, defined at (lib) sourcemap-codec.js.flow.
• ✅ flow check with the libdef's decode return type changed to string, at head and base — Both report No errors, so the worker call site doesn't catch this particular mutation. Behavior is the same before and after; cause not investigated. Libdef restored.
• ✅ flow check --include-warnings --show-all-errors, at head and base — Both warn at metro-transform-worker/src/index.js:45. Head shows it as an unused suppression; base was matched by location only.
• ✅ yarn prettier --check flow-typed/sourcemap-codec.js.flow — Clean on the restored file. An earlier run failed only because it overlapped my temporary libdef edit.
• ✅ yarn jest --no-watchman packages/metro-transform-worker — 2 suites, 35 tests.

Not run: Internal Flow: its library-file handling and the flow-check-land errors the PR targets can't be reproduced here, including why other flow-typed/*.js files with declare module don't hit the same error. Benchmarks: a type-only rename has no runtime cost. Windows: not relevant to this change. One Flow command using a custom temp dir was denied by the permission layer, so yarn typecheck and yarn flow check with default settings were used instead; coverage was not affected.

_{Automated review by Claude. Advisory only: it never approves or blocks, and a maintainer makes the call.}