Xcode Build fails upon adding new target

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Description

Upon adding a new target to a fresh react-native app (in my case an iOS 14 Widget). Xcode fails to build the app. With two errors:

1. Undefined symbol: _swift_getOpaqueTypeConformance
2. Undefined symbol: _swift_getTypeByMangledNameInContextInMetadataState

The latter error points to #29246. However the suggested fix did not help.

LIBRARY_SEARCH_PATHS = (
   "\"$(TOOLCHAIN_DIR)/usr/lib/swift/$(PLATFORM_NAME)\"",
-  "\"$(TOOLCHAIN_DIR)/usr/lib/swift-5.0/$(PLATFORM_NAME)\"",
+  "\"$(TOOLCHAIN_DIR)/usr/lib/swift-5.2/$(PLATFORM_NAME)\"",
   "\"$(inherited)\"",
 );

_Originally posted by @hengkx in https://github.com/facebook/react-native/issues/29246#issuecomment-663986438_

Bumping the Swift version to 5.2 led to 23 further undefined symbol errors mostly of type FORCE_LOAD… eg: Undefined symbol: __swift_FORCE_LOAD_$_swiftObjectiveC

React Native version:

    OS: macOS 11.0
    CPU: (8) x64 Intel(R) Core(TM) i5-8257U CPU @ 1.40GHz
    Memory: 36.74 MB / 8.00 GB
    Shell: 5.8 - /bin/zsh
  Binaries:
    Node: 14.4.0 - ~/.nvm/versions/node/v14.4.0/bin/node
    Yarn: 1.22.10 - /usr/local/bin/yarn
    npm: 6.14.5 - ~/.nvm/versions/node/v14.4.0/bin/npm
    Watchman: 4.9.0 - /usr/local/bin/watchman
  Managers:
    CocoaPods: 1.9.1 - /usr/local/bin/pod
  SDKs:
    iOS SDK:
      Platforms: iOS 14.1, DriverKit 19.0, macOS 10.15, tvOS 14.0, watchOS 7.0
    Android SDK: Not Found
  IDEs:
    Android Studio: 4.0 AI-193.6911.18.40.6821437
    Xcode: 12.1/12A7403 - /usr/bin/xcodebuild
  Languages:
    Java: Not Found
    Python: 2.7.16 - /usr/bin/python
  npmPackages:
    @react-native-community/cli: Not Found
    react: 16.13.1 => 16.13.1 
    react-native: ~0.63.3 => 0.63.3 
  npmGlobalPackages:
    *react-native*: Not Found

Steps To Reproduce

Provide a detailed list of steps that reproduce the issue.

1. Instantiate a new react-native app: react-native init myProj
2. In Xcode, file > new > target > Widget Extension
3. Try to build app an observe described behaviour

Expected Results

The app should continue to build normally after adding a new target

Snack, code example, screenshot, or link to a repository:

n/a

[doylemark]

Resolved, solution is to make the following changes to the WidgetExtension target not project.

LIBRARY_SEARCH_PATHS = (
   "\"$(TOOLCHAIN_DIR)/usr/lib/swift/$(PLATFORM_NAME)\"",
-  "\"$(TOOLCHAIN_DIR)/usr/lib/swift-5.0/$(PLATFORM_NAME)\"",
+  "\"$(TOOLCHAIN_DIR)/usr/lib/swift-5.2/$(PLATFORM_NAME)\"",
   "\"$(inherited)\"",
 );

[timothyac]

I might be missing it, but it doesn’t look my widget target has the LIBRARY_SEARCH_PATHS. I do see two in the settings, but one is for the debug version of the app target, and one is for the release version of the app target. Not sure if I’m doing something wrong when adding the widget to the project.