Bug Report: Duplicate RCTSwiftUI Symbol Leakage Causes lowercaseString Crash in RN 0.84 Prebuilt Frameworks

Description

Description
In React Native 0.84 (New Architecture), apps that include custom Swift modules or import React in AppDelegate.swift crash deterministically on iOS launch before the JS bundle evaluates. The crash is -[NSMallocBlock lowercaseString]: unrecognized selector sent to instance.

Root Cause Analysis:
This is a framework-level linker bug caused by the new prebuilt React.framework and ENABLE_DEBUG_DYLIB features.

React.framework (and the .debug.dylib) comes prebuilt with exposed Swift symbols, specifically RCTSwiftUI and RCTEventEmitter.

When the main app target compiles AppDelegate.swift (which imports React), the Swift compiler statically links those exposed RCTSwiftUI symbols directly into the main app executable.

At runtime, dyld loads the main executable AND the React.framework (or .debug.dylib). Both binaries now contain the exact same Swift classes.

The Objective-C runtime duplicate class registration corrupts the dispatch table. When the TurboModule system initializes, it sends a string method (lowercaseString) to a corrupted pointer (an NSMallocBlock), causing a fatal crash.

Steps to reproduce

Steps to Reproduce:

Initialize a React Native 0.84 project with New Architecture enabled.

Add a basic Swift file to the main iOS target that imports React (or just rely on the default AppDelegate.swift).

Build for an iOS physical device (xcodebuild -workspace ...).

Crash occurs immediately upon boot.
(Note: Setting ENABLE_DEBUG_DYLIB=NO strips the dylib, but the crash persists because the duplicate symbols still exist between the main executable and the prebuilt React.framework).

Expected Behavior
Prebuilt React frameworks should have their internal Swift symbols (like RCTSwiftUI) stripped or marked with hidden visibility so they do not leak into the consumer's main app binary during compilation.

React Native Version

0.84.x

Affected Platforms

Runtime - iOS

Output of npx @react-native-community/cli info

System:
  OS: macOS 15.7.4
  CPU: (10) arm64 Apple M4
  Memory: 2.81 GB / 24.00 GB
  Shell:
    version: "5.9"
    path: /bin/zsh
Binaries:
  Node:
    version: 24.12.0
    path: /Users/avacado/.nvm/versions/node/v24.12.0/bin/node
  Yarn: Not Found
  npm:
    version: 11.6.2
    path: /Users/avacado/.nvm/versions/node/v24.12.0/bin/npm
  Watchman:
    version: 2026.01.12.00
    path: /opt/homebrew/bin/watchman
Managers:
  CocoaPods:
    version: 1.16.2
    path: /opt/homebrew/bin/pod
SDKs:
  iOS SDK:
    Platforms:
      - DriverKit 25.2
      - iOS 26.2
      - macOS 26.2
      - tvOS 26.2
      - visionOS 26.2
      - watchOS 26.2
  Android SDK: Not Found
IDEs:
  Android Studio: 2025.3 AI-253.29346.138.2531.14876573
  Xcode:
    version: 26.3/17C529
    path: /usr/bin/xcodebuild
Languages:
  Java:
    version: 17.0.18
    path: /opt/homebrew/opt/openjdk@17/bin/javac
  Ruby:
    version: 2.6.10
    path: /usr/bin/ruby
npmPackages:
  "@react-native-community/cli":
    installed: 20.1.0
    wanted: 20.1.0
  react:
    installed: 19.2.3
    wanted: 19.2.3
  react-native:
    installed: 0.84.1
    wanted: ^0.84.1
  react-native-macos: Not Found
npmGlobalPackages:
  "*react-native*": Not Found
Android:
  hermesEnabled: true
  newArchEnabled: true
iOS:
  hermesEnabled: true
  newArchEnabled: true

Stacktrace or Logs

Fatal Exception: NSInvalidArgumentException
-[__NSMallocBlock__ lowercaseString]: unrecognized selector sent to instance

Crash Timeline Context:
1. iOS launches the app binary successfully.
2. React Native's prebuilt React.framework loads (registers RCTSwiftUI / RCTEventEmitter classes).
3. LocalMind.debug.dylib loads (registers duplicate RCTSwiftUI / RCTEventEmitter classes).
4. Duplicate class registration corrupts the Objective-C runtime dispatch table.
5. React Native attempts to evaluate the JavaScript bundle.
6. A TurboModule or Event Emitter attempts string manipulation, routing the method to a corrupted pointer (a memory block instead of an NSString).
7. Hard crash before any application-level JS or native code can execute.

Note: Setting ENABLE_DEBUG_DYLIB=NO in the xcodebuild command successfully strips the .debug.dylib, but the crash persists because the main application executable still statically links the duplicate RCTSwiftUI symbols from AppDelegate.swift importing the prebuilt React.framework.

MANDATORY Reproducer

https://github.com/marinoonthelake-blip/test

Screenshots and Videos

N/A - This is a deterministic compiler/linker crash that occurs at launch before the React Native view hierarchy is mounted.

Comienza con el reproducer obligatorio y la ruta predeterminada de AppDelegate.swift que importa React; después, inspecciona cómo se producen y enlazan mediante xcodebuild el React.framework precompilado y .debug.dylib. Verifica los símbolos duplicados RCTSwiftUI y RCTEventEmitter en las salidas de la app y del framework; se considera terminado cuando el reproducer se inicia sin el fallo de lowercaseString y los binarios consumidores ya no contienen símbolos duplicados.