Babel preset: Add Hermes V1 native runtime workarounds for hermes#1761

[ramonclaudio]

Summary:

Three Babel plugins for @react-native/babel-preset that work around runtime bugs in the bundled Hermes V1 (250829098.0.13, branch cut 2025-08-29). They rewrite the buggy code patterns at compile time so Hermes never sees them. Direct port of @kitten's plugins in babel-preset-expo (expo/expo#45601, merged 2026-05-11); same fix, applied at the @react-native/babel-preset layer so bare React Native consumers don't need babel-preset-expo to escape the bugs. All plugin design and implementation credit is his.

facebook/hermes#1761 was reported by trossimel-sc at Meta in August 2025, confirmed within the hour by tmikov, and fixed on Hermes static_h two weeks later in 68bfb3a48b31. The fix never got cherry-picked back to the 250829098.0.0-stable branch React Native pins. Two related codegen bugs sit in the same gap: class declarations inside finally blocks (1e94fbe0ebb4, 2026-02-12) and super.x lookups in object-literal getters and setters (18a963465944, 2025-11-04). The most recent V1 stable bump (0.13, #56728, 2026-05-08) carried only the Promise rejection-tracker backport, not these three.

• fix-hermes-v1-async-arrow-non-simple-params: rewrites async ({a}) => 42 into async _p => { var {a} = _p; return 42; }. Without it, await resolves with undefined while the body keeps running in the background. Gated on isHermesProfile && preserveAsync since @babel/plugin-transform-async-to-generator strips async otherwise. Without this plugin, unstable_preserveAsync (Babel preset: Add unstable_preserveAsync to experiment with disabling async transforms for SH (#56254) #56254) is silently broken on Hermes V1 for any user code with non-simple async-arrow params.
• fix-hermes-v1-class-in-finally: wraps class declarations inside finally blocks in an IIFE so the class lives in its own scope. The Hermes bug surfaces on call graphs that populate the IR variable cache between normal and exception code paths, which is recompilation-pattern dependent and hard to construct in a minimal repro. @kitten verified the workaround against Expo's internal test-suite.
• fix-hermes-v1-super-in-object-accessor: rewrites get name() into get ["name"]() so the accessor key is computed. Identifier-keyed accessors segfault Hermes V1 during IR generation.

All three gate on isHermesProfile and bail out fast when the trigger patterns aren't present. The workaround unblocks 0.84.x and 0.85.x consumers today and becomes a no-op once a future V1 build picks up the upstream fixes.

Empirical validation:

Built a minimal bare React Native 0.85.3 repro that ports the bug-trigger patterns from @kitten's apps/test-suite work, plus a race detector, engine-identity assertions, and feature coverage for harder class shapes (static blocks, async/generator methods, private fields, getters, super-in-extends). 55 tests at ramonclaudio/hermes-1761-repro.

Per-plugin A/B isolation on iPhone 17 Pro / iOS 26.5, Hermes V1 250829098.0.13:

Config	Result
All three plugins active (this PR)	55/55 PASS
Vanilla @react-native/babel-preset@0.85.3	App crashes (SIGSEGV in hermes::irgen::ESTreeIRGen::genMemberExpression)
Only super-in-object-accessor disabled	App crashes in same path
Only async-arrow-non-simple-params disabled	14 async tests fail with race-detector output showing awaiter-resumed:undefined while the body returned 40
Only class-in-finally disabled	55/55 still PASS (bug surface is recompilation-pattern dependent)
iOS Release build (precompiled bytecode, no Metro)	55/55 PASS

Downstream confirmation: babel-preset-expo@56.0.7 (which carries the same plugins) applied to vanilla @convex-dev/better-auth@0.12.2 fixes the useConvexAuth.isAuthenticated race on Hermes V1. Original user-facing repro at ramonclaudio/convex-better-auth-368-repro.

Caveats:

fix-hermes-v1-class-in-finally rewrites block-scoped class declarations inside finally blocks to function-scoped var initializers. The change is observable only when code references the class identifier outside the finally, e.g., a direct reference after the finally exits or a same-named outer var the original class would have shadowed. Both are uncommon, and the four scope-delta cases I could construct are pinned as passing tests in the linked repro. The same plugin shape has been live in babel-preset-expo@56.0.6+ since 2026-05-11. Flagging in case Meta's internal test fleet surfaces an edge case the OSS test suite does not.

Changelog:

[GENERAL] [FIXED] - Work around three Hermes V1 source-level codegen bugs in @react-native/babel-preset (async-arrow non-simple params, class-in-finally, super-in-object-accessor)

Test Plan:

Test repro at ramonclaudio/hermes-1761-repro. 55/55 PASS on iPhone 17 Pro / iOS 26.5 in both Debug and Release builds. Per-plugin A/B matrix above shows each plugin's role.

Locally on packages/react-native-babel-preset:

• 18 new inline-snapshot unit tests cover the transformed output and the skip-when-irrelevant path.
• yarn jest packages/react-native-babel-preset clean (65 tests, 24 snapshots).
• Full repo yarn test clean (279 suites, 5656 tests, 1816 snapshots).
• yarn lint, yarn format-check, yarn flow-check, yarn test-typescript, yarn featureflags --verify-unchanged, yarn build-types --validate all clean.
• Existing transform-snapshot-test is unchanged: the kitchen-sink fixture has none of the bug patterns so the new plugins don't alter its output.
• Happy to add the three trigger patterns to kitchen-sink-input.js in a follow-up if reviewers want snapshot coverage.

References:

• Await returns undefined in async arrow functions with parameter destructuring hermes#1761 (root-cause issue, fixed in static_h)
• fix(babel-preset-expo): Fix three Hermes v1 (250829098) IR/compiler syntax bugs with fix-up transforms expo/expo#45601 (source of the ported plugins, by @kitten)
• 🐛 [SDK 56 preview.7] await of an async function's Promise resolves to undefined while the underlying function runs to completion later — breaks RTK Query queryFn dispatch on iOS expo/expo#45592 (user-facing bug report on SDK 56 preview)

[ramonclaudio]

Thanks to @kitten for the original plugin work in expo/expo#45601. The original design and implementation credit on these three transforms belongs to him. This PR just ports his work over to @react-native/babel-preset so bare React Native consumers benefit from the same fix without depending on babel-preset-expo.