Guard private syntax preservation by profile

Author: OskarEichler

packages/react-native-babel-preset/src/__tests__/transform-snapshot-test.js

@@ -444,6 +444,25 @@ describe('react-native-babel-preset transform snapshots', () => {
       expect(result).toContain('_classPrivateFieldLooseKey');
     });
 
+    it('transforms private class fields when the profile lowers classes', () => {
+      const code = `
+        class Counter {
+          #count = 0;
+          #privateMethod() { return this.#count; }
+        }
+      `;
+      const result = transformCode(code, {
+        dev: false,
+        unstable_transformProfile: 'hermes-legacy',
+        customTransformOptions: {
+          unstable_preserveClassPrivate: true,
+        },
+      });
+      expect(result).not.toContain('#count');
+      expect(result).not.toContain('#privateMethod');
+      expect(result).toContain('_classPrivateFieldLooseKey');
+    });
+
     it('preserves async/await with unstable_preserveAsync', () => {
       const code = `
         async function fetchData() {

packages/react-native-babel-preset/src/configs/main.js

@@ -94,10 +94,13 @@ const getPreset = (src, options, babel) => {
   // Preserve class syntax and related features for Hermes V1 profiles.
   const preserveClasses = isHermesProfile;
 
-  // Preserve private class fields and methods if the experiment is enabled.
-  const preserveClassPrivate = TRUE_VALS.has(
-    options?.customTransformOptions?.unstable_preserveClassPrivate,
-  );
+  // Private fields can only be preserved when the surrounding class syntax is
+  // also preserved. Babel's class transform requires the private transforms.
+  const preserveClassPrivate =
+    preserveClasses &&
+    TRUE_VALS.has(
+      options?.customTransformOptions?.unstable_preserveClassPrivate,
+    );
 
   // Preserve async/await syntax if the experiment is enabled.
   const preserveAsync = TRUE_VALS.has(