[0.59rc2 blocking] Can't compile React Native on iOS using `-force_load` instead of `-ObjC` any more

🐛 Bug Report

Our project can’t use the -ObjC linker flag to discover/load all symbols as it causes “duplicate symbol” errors for some of the static libraries we use (ObjC is all-or-nothing flag, so we can’t opt these out of it, and the “duplicate symbols” are valid as we depend on the same shared library from multiple other libraries). As RN depends on this behaviour, the alternative is to use -force_load for each library that needs to be loaded in this way and specify the path to the compiled (.a) library.

With 0.58 this compiles fine, but with 0.59rc2 it results in “duplicate symbol” errors for some Yoga references in libReact.a:

Please let me know if you’d like more help looking into this issue or reproducing it - this would be a blocking issue for our project as we can’t compile with -ObjC.

To Reproduce

1. Remove -ObjC from and add -force_load libReact.a to the “Linker Flags” section of Xcode, with the correct path to libReact.a (see below)

2. Clean and build, error will be displayed

I’ve created a repro case: https://github.com/tomduncalf/rn0.59-force-load-bug - in the interest of keeping the project simple, the path to libReact.a needs to be manually changed in “Linker Flags”, you can discover it by going to ~/Library/Developer/Xcode/DerivedData and looking for the directory starting forceLoad59rc2-.... then replacing the existing one with this path (feels like there should be a $(..) variable pointing to this location but I’ve never found it!).

There is also a rn@0.58 branch in that repo to show it compiles OK in the current release version - you may need to “Clean Build Folder” and/or clear the DerivedData folder to see the change when switching branches.

Expected Behavior

Project compiles with no errors as in 0.58

Code Example

https://github.com/tomduncalf/rn0.59-force-load-bug

Environment

    System:
      OS: macOS 10.14.2
      CPU: (8) x64 Intel(R) Core(TM) i7-7820HQ CPU @ 2.90GHz
      Memory: 126.19 MB / 16.00 GB
      Shell: 5.3 - /bin/zsh
    Binaries:
      Node: 10.7.0 - ~/.nvm/versions/node/v10.7.0/bin/node
      Yarn: 1.10.1 - ~/.yarn/bin/yarn
      npm: 6.1.0 - ~/.nvm/versions/node/v10.7.0/bin/npm
      Watchman: 4.7.0 - /usr/local/bin/watchman
    SDKs:
      iOS SDK:
        Platforms: iOS 12.1, macOS 10.14, tvOS 12.1, watchOS 5.1
      Android SDK:
        API Levels: 10, 16, 23, 24, 26, 27, 28
        Build Tools: 28.0.0, 28.0.3
        System Images: android-23 | Intel x86 Atom_64, android-23 | Google APIs Intel x86 Atom_64, android-28 | Google APIs Intel x86 Atom
    IDEs:
      Android Studio: 3.3 AI-182.5107.16.33.5264788
      Xcode: 10.1/10B61 - /usr/bin/xcodebuild
    npmPackages:
      react: 16.8.1 => 16.8.1
      react-native: 0.59.0-rc.2 => 0.58.5
    npmGlobalPackages:
      react-native-cli: 2.0.1```

[hramos]

Thank you!

[tomduncalf]

@kelset It still happens but I think I might know what is wrong… testing something out and I’ll submit a PR if I can fix