Transform fails on async arrow with rest param inside a conditional expression (VariableDeclarator id got SpreadElement)

[itaihanski]

Description

Bundling a module that contains an async arrow function with a rest parameter and a block body, placed as a branch of a conditional (ternary) expression, fails during Metro's transform step with:

SyntaxError: <file>: Property id of VariableDeclarator expected node to be of
a type ["LVal","VoidPattern"] but instead got "SpreadElement"

This is not a parse error. hermesParser.parse(code, { babel: true }) succeeds on the input; the failure happens later, while Metro's Babel transform processes the hermes-produced AST. It is a @babel/types builder validation, so it carries no source location and Metro prints no code frame.

Minimal reproduction

A module whose entire contents are:

const g = e => (e ? async (...r) => { return r } : 0);
export default { g };

Import it from the entry and bundle for any platform (e.g. npx expo export --platform ios, or metro build). It fails with the error above.

Every one of the following variants bundles fine — all four conditions are required to trigger it:

Variant	Result
e ? async (...r) => { return r } : 0	❌ fails
async (...r) => { return r } (no ternary)	✅ ok
e ? (...r) => { return r } : 0 (not async)	✅ ok
e ? async (...r) => r : 0 (concise body, no block)	✅ ok

So the trigger is precisely: async + rest parameter + block body + conditional-expression position.

Regression

The byte-identical input bundles successfully on Metro 0.82.x (Expo SDK 53 / RN 0.79) and fails on Metro 0.84.4 (Expo SDK 56 / RN 0.85.3). Pure toolchain bisect — only the Metro/@react-native/babel-preset/hermes-parser versions changed.

Analysis

• hermes-parser parse stage succeeds ({ babel: true }).
• The error is thrown from @babel/types while a transform builds a VariableDeclarator whose id is a SpreadElement — i.e. a node is synthesized incorrectly during the async-to-generator / parameter handling of the hermes AST.
• Likely an interaction between hermes-parser's Babel AST output and @react-native/babel-preset's async transform, surfacing in metro-transform-worker.

Environment

• metro 0.84.4
• hermes-parser 0.33.3
• react-native 0.85.3 (Expo SDK 56.0.9)
• node v24.1.0, macOS
• Default Metro config (Hermes parser enabled by default, inlineRequires on)

Expected

The module bundles successfully — the syntax is valid and runs natively on Hermes.