To this end, I have created an XCFramework version of Google Sign. See https://github.com/SyncServerII/GSignIn.git. So far, that is enabling testing and development with Xcode iOS Simulator and on actual iOS Devices.

Once built, this is used in a Package.swift manifest like:

        .binaryTarget(
            // This was generated using https://github.com/SyncServerII/GSignIn.git
            name: "GSignIn",
            path: "Frameworks/GSignIn.xcframework"
        ),

(See also https://github.com/SyncServerII/iOSGoogle/blob/main/Package.swift for a complete example).

Issues so far:

1. I am apparently unable to access the bundle assets. I have had to create a new Google Sign In button. Not a big deal, but for completeness I'd like to have access to the Google button the Google Sign In framework provides.

2. This package depends on a specific version of Google Sign In. Updating it will require going into https://github.com/SyncServerII/GSignIn.git and manually updating it based on updates to Google's Sign In framework.

I have yet to deploy an app using this package to the app store so that's an unknown at this point.

-- Updates --

12/28/20 (Resolved-- see 1/1/21 update below): I should also note that I'm getting a warning for this library in my sharing extension:

ld: warning: linking against a dylib which is not safe for use in application extensions: /Users/chris/Library/Developer/Xcode/DerivedData/Neebla-aybzuiaoyytaypbvqtqeummvcssh/Build/Products/Debug-iphoneos/GSignIn.framework/GSignIn

when I build with this framework in my target app, which has a sharing extension. I have not tried to upload this app to the app store yet, so don't know what will happen then.

12/29/20 (Resolved-- see 1/1/21 update below): I was building previously with my Google Sign In package dragged into my app from a local directory. I have just tried moving from not dragged in, but to just relying on it as a swift package in my app, referenced using its Github repo. And that breaks the build with various obscure linker errors like the following:

Showing All Messages
Ld /Users/chris/Library/Developer/Xcode/DerivedData/Neebla-aybzuiaoyytaypbvqtqeummvcssh/Build/Products/Debug-iphoneos/Share\ Extension.appex/Share\ Extension normal (in target 'Share Extension' from project 'Neebla')
    cd /Users/chris/Developer/Neebla
    /Applications/Xcode-12.3.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/clang -target arm64-apple-ios14.0 -isysroot /Applications/Xcode-12.3.app/Contents/Developer/Platforms/iPhoneOS.platform/Developer/SDKs/iPhoneOS14.3.sdk -L/Users/chris/Library/Developer/Xcode/DerivedData/Neebla-aybzuiaoyytaypbvqtqeummvcssh/Build/Products/Debug-iphoneos -F/Users/chris/Library/Developer/Xcode/DerivedData/Neebla-aybzuiaoyytaypbvqtqeummvcssh/Build/Products/Debug-iphoneos -filelist /Users/chris/Library/Developer/Xcode/DerivedData/Neebla-aybzuiaoyytaypbvqtqeummvcssh/Build/Intermediates.noindex/Neebla.build/Debug-iphoneos/Share\ Extension.build/Objects-normal/arm64/Share\ Extension.LinkFileList -Xlinker -rpath -Xlinker @executable_path/Frameworks -Xlinker -rpath -Xlinker @executable_path/../../Frameworks -dead_strip -Xlinker -object_path_lto -Xlinker /Users/chris/Library/Developer/Xcode/DerivedData/Neebla-aybzuiaoyytaypbvqtqeummvcssh/Build/Intermediates.noindex/Neebla.build/Debug-iphoneos/Share\ Extension.build/Objects-normal/arm64/Share\ Extension_lto.o -Xlinker -export_dynamic -Xlinker -no_deduplicate -fembed-bitcode-marker -fapplication-extension -fobjc-link-runtime -L/Applications/Xcode-12.3.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/lib/swift/iphoneos -L/usr/lib/swift -Xlinker -add_ast_path -Xlinker /Users/chris/Library/Developer/Xcode/DerivedData/Neebla-aybzuiaoyytaypbvqtqeummvcssh/Build/Intermediates.noindex/Neebla.build/Debug-iphoneos/Share\ Extension.build/Objects-normal/arm64/Share_Extension.swiftmodule -lc++ -framework Accelerate -e _NSExtensionMain -Xlinker -no_adhoc_codesign -Xlinker -dependency_info -Xlinker /Users/chris/Library/Developer/Xcode/DerivedData/Neebla-aybzuiaoyytaypbvqtqeummvcssh/Build/Intermediates.noindex/Neebla.build/Debug-iphoneos/Share\ Extension.build/Objects-normal/arm64/Share\ Extension_dependency_info.dat -o /Users/chris/Library/Developer/Xcode/DerivedData/Neebla-aybzuiaoyytaypbvqtqeummvcssh/Build/Products/Debug-iphoneos/Share\ Extension.appex/Share\ Extension -Xlinker -add_ast_path -Xlinker /Users/chris/Library/Developer/Xcode/DerivedData/Neebla-aybzuiaoyytaypbvqtqeummvcssh/Build/Intermediates.noindex/iOSFacebook.build/Debug-iphoneos/iOSFacebook.build/Objects-normal/arm64/iOSFacebook.swiftmodule -Xlinker -add_ast_path -Xlinker /Users/chris/Library/Developer/Xcode/DerivedData/Neebla-aybzuiaoyytaypbvqtqeummvcssh/Build/Intermediates.noindex/iOSSignIn.build/Debug-iphoneos/iOSSignIn.build/Objects-normal/arm64/iOSSignIn.swiftmodule -Xlinker -add_ast_path -Xlinker /Users/chris/Library/Developer/Xcode/DerivedData/Neebla-aybzuiaoyytaypbvqtqeummvcssh/Build/Intermediates.noindex/ServerShared.build/Debug-iphoneos/ServerShared.build/Objects-normal/arm64/ServerShared.swiftmodule -Xlinker -add_ast_path -Xlinker /Users/chris/Library/Developer/Xcode/DerivedData/Neebla-aybzuiaoyytaypbvqtqeummvcssh/Build/Intermediates.noindex/PersistentValue.build/Debug-iphoneos/PersistentValue.build/Objects-normal/arm64/PersistentValue.swiftmodule -Xlinker -add_ast_path -Xlinker /Users/chris/Library/Developer/Xcode/DerivedData/Neebla-aybzuiaoyytaypbvqtqeummvcssh/Build/Intermediates.noindex/SwiftyUserDefaults.build/Debug-iphoneos/SwiftyUserDefaults.build/Objects-normal/arm64/SwiftyUserDefaults.swiftmodule -Xlinker -add_ast_path -Xlinker /Users/chris/Library/Developer/Xcode/DerivedData/Neebla-aybzuiaoyytaypbvqtqeummvcssh/Build/Intermediates.noindex/KeychainAccess.build/Debug-iphoneos/KeychainAccess.build/Objects-normal/arm64/KeychainAccess.swiftmodule -Xlinker -add_ast_path -Xlinker /Users/chris/Library/Developer/Xcode/DerivedData/Neebla-aybzuiaoyytaypbvqtqeummvcssh/Build/Intermediates.noindex/iOSShared.build/Debug-iphoneos/iOSShared.build/Objects-normal/arm64/iOSShared.swiftmodule -Xlinker -add_ast_path -Xlinker /Users/chris/Library/Developer/Xcode/DerivedData/Neebla-aybzuiaoyytaypbvqtqeummvcssh/Build/Intermediates.noindex/swift-log.build/Debug-iphoneos/Logging.build/Objects-normal/arm64/Logging.swiftmodule -Xlinker -add_ast_path -Xlinker /Users/chris/Library/Developer/Xcode/DerivedData/Neebla-aybzuiaoyytaypbvqtqeummvcssh/Build/Intermediates.noindex/SQLite.swift.build/Debug-iphoneos/SQLite.build/Objects-normal/arm64/SQLite.swiftmodule -Xlinker -add_ast_path -Xlinker /Users/chris/Library/Developer/Xcode/DerivedData/Neebla-aybzuiaoyytaypbvqtqeummvcssh/Build/Intermediates.noindex/Facebook.build/Debug-iphoneos/FacebookLogin.build/Objects-normal/arm64/FacebookLogin.swiftmodule -Xlinker -add_ast_path -Xlinker /Users/chris/Library/Developer/Xcode/DerivedData/Neebla-aybzuiaoyytaypbvqtqeummvcssh/Build/Intermediates.noindex/Facebook.build/Debug-iphoneos/FacebookCore.build/Objects-normal/arm64/FacebookCore.swiftmodule -Xlinker -add_ast_path -Xlinker /Users/chris/Library/Developer/Xcode/DerivedData/Neebla-aybzuiaoyytaypbvqtqeummvcssh/Build/Intermediates.noindex/Facebook.build/Debug-iphoneos/FacebookShare.build/Objects-normal/arm64/FacebookShare.swiftmodule -Xlinker -add_ast_path -Xlinker /Users/chris/Library/Developer/Xcode/DerivedData/Neebla-aybzuiaoyytaypbvqtqeummvcssh/Build/Intermediates.noindex/iOSDropbox.build/Debug-iphoneos/iOSDropbox.build/Objects-normal/arm64/iOSDropbox.swiftmodule -Xlinker -add_ast_path -Xlinker /Users/chris/Library/Developer/Xcode/DerivedData/Neebla-aybzuiaoyytaypbvqtqeummvcssh/Build/Intermediates.noindex/SwiftyDropbox.build/Debug-iphoneos/SwiftyDropbox.build/Objects-normal/arm64/SwiftyDropbox.swiftmodule -Xlinker -add_ast_path -Xlinker /Users/chris/Library/Developer/Xcode/DerivedData/Neebla-aybzuiaoyytaypbvqtqeummvcssh/Build/Intermediates.noindex/Alamofire.build/Debug-iphoneos/Alamofire.build/Objects-normal/arm64/Alamofire.swiftmodule -Xlinker -add_ast_path -Xlinker /Users/chris/Library/Developer/Xcode/DerivedData/Neebla-aybzuiaoyytaypbvqtqeummvcssh/Build/Intermediates.noindex/ImageIOSwift.build/Debug-iphoneos/ImageIOUIKit.build/Objects-normal/arm64/ImageIOUIKit.swiftmodule -Xlinker -add_ast_path -Xlinker /Users/chris/Library/Developer/Xcode/DerivedData/Neebla-aybzuiaoyytaypbvqtqeummvcssh/Build/Intermediates.noindex/ImageIOSwift.build/Debug-iphoneos/ImageIOSwift.build/Objects-normal/arm64/ImageIOSwift.swiftmodule -Xlinker -add_ast_path -Xlinker /Users/chris/Library/Developer/Xcode/DerivedData/Neebla-aybzuiaoyytaypbvqtqeummvcssh/Build/Intermediates.noindex/SMLinkPreview.build/Debug-iphoneos/SMLinkPreview.build/Objects-normal/arm64/SMLinkPreview.swiftmodule -Xlinker -add_ast_path -Xlinker /Users/chris/Library/Developer/Xcode/DerivedData/Neebla-aybzuiaoyytaypbvqtqeummvcssh/Build/Intermediates.noindex/swift-log-file.build/Debug-iphoneos/FileLogging.build/Objects-normal/arm64/FileLogging.swiftmodule -Xlinker -add_ast_path -Xlinker /Users/chris/Library/Developer/Xcode/DerivedData/Neebla-aybzuiaoyytaypbvqtqeummvcssh/Build/Intermediates.noindex/iOSBasics.build/Debug-iphoneos/iOSBasics.build/Objects-normal/arm64/iOSBasics.swiftmodule -Xlinker -add_ast_path -Xlinker /Users/chris/Library/Developer/Xcode/DerivedData/Neebla-aybzuiaoyytaypbvqtqeummvcssh/Build/Intermediates.noindex/Version.build/Debug-iphoneos/Version.build/Objects-normal/arm64/Version.swiftmodule -Xlinker -add_ast_path -Xlinker /Users/chris/Library/Developer/Xcode/DerivedData/Neebla-aybzuiaoyytaypbvqtqeummvcssh/Build/Intermediates.noindex/ChangeResolvers.build/Debug-iphoneos/ChangeResolvers.build/Objects-normal/arm64/ChangeResolvers.swiftmodule -Xlinker -add_ast_path -Xlinker /Users/chris/Library/Developer/Xcode/DerivedData/Neebla-aybzuiaoyytaypbvqtqeummvcssh/Build/Intermediates.noindex/ServerAccount.build/Debug-iphoneos/ServerAccount.build/Objects-normal/arm64/ServerAccount.swiftmodule

ld: warning: linking against a dylib which is not safe for use in application extensions: /Users/chris/Library/Developer/Xcode/DerivedData/Neebla-aybzuiaoyytaypbvqtqeummvcssh/Build/Products/Debug-iphoneos/GSignIn.framework/GSignIn
Undefined symbols for architecture arm64:
  "iOSGoogle.GoogleHashing.init() -> iOSGoogle.GoogleHashing", referenced from:
      Share_Extension.ServerInterface.addHashingForCloudStorageSignIns(hashingManager: iOSShared.HashingManager) throws -> () in ServerInterface+CloudHashing.o
  "type metadata for iOSGoogle.GoogleHashing", referenced from:
      Share_Extension.ServerInterface.addHashingForCloudStorageSignIns(hashingManager: iOSShared.HashingManager) throws -> () in ServerInterface+CloudHashing.o
  "protocol witness table for iOSGoogle.GoogleHashing : iOSShared.CloudStorageHashing in iOSGoogle", referenced from:
      Share_Extension.ServerInterface.addHashingForCloudStorageSignIns(hashingManager: iOSShared.HashingManager) throws -> () in ServerInterface+CloudHashing.o
  "type metadata accessor for iOSGoogle.GoogleSyncServerSignIn", referenced from:
      Share_Extension.Services.getSignIns(configPlist: Share_Extension.ConfigPlist) -> [(iOSSignIn.GenericSignIn, iOSSignIn.SignInDescription)] in Services+SignIns.o
  "iOSGoogle.GoogleSyncServerSignIn.__allocating_init(serverClientId: Swift.String, appClientId: Swift.String, signInDelegate: iOSGoogle.GoogleSignInDelegate) -> iOSGoogle.GoogleSyncServerSignIn", referenced from:
      Share_Extension.Services.getSignIns(configPlist: Share_Extension.ConfigPlist) -> [(iOSSignIn.GenericSignIn, iOSSignIn.SignInDescription)] in Services+SignIns.o
  "direct field offset for iOSGoogle.GoogleSyncServerSignIn.userType : ServerShared.UserType", referenced from:
      Share_Extension.Services.getSignIns(configPlist: Share_Extension.ConfigPlist) -> [(iOSSignIn.GenericSignIn, iOSSignIn.SignInDescription)] in Services+SignIns.o
  "protocol witness table for iOSGoogle.GoogleSyncServerSignIn : iOSSignIn.GenericSignIn in iOSGoogle", referenced from:
      Share_Extension.Services.getSignIns(configPlist: Share_Extension.ConfigPlist) -> [(iOSSignIn.GenericSignIn, iOSSignIn.SignInDescription)] in Services+SignIns.o
  "type metadata accessor for iOSApple.AppleSignIn", referenced from:
      Share_Extension.Services.getSignIns(configPlist: Share_Extension.ConfigPlist) -> [(iOSSignIn.GenericSignIn, iOSSignIn.SignInDescription)] in Services+SignIns.o
  "iOSApple.AppleSignIn.__allocating_init() -> iOSApple.AppleSignIn", referenced from:
      Share_Extension.Services.getSignIns(configPlist: Share_Extension.ConfigPlist) -> [(iOSSignIn.GenericSignIn, iOSSignIn.SignInDescription)] in Services+SignIns.o
  "protocol descriptor for iOSGoogle.GoogleSignInDelegate", referenced from:
      l_got.$s9iOSGoogle20GoogleSignInDelegateMp in Services+SignIns.o
  "protocol witness table for iOSApple.AppleSignIn : iOSSignIn.GenericSignIn in iOSApple", referenced from:
      Share_Extension.Services.getSignIns(configPlist: Share_Extension.ConfigPlist) -> [(iOSSignIn.GenericSignIn, iOSSignIn.SignInDescription)] in Services+SignIns.o
ld: symbol(s) not found for architecture arm64
clang: error: linker command failed with exit code 1 (use -v to see invocation)

Note that this is not specific to building for a real device (e.g., arm64). If I build for the simulator, I get the same kind of messages, but the heading is Undefined symbols for architecture x86_64:.

When I build and still include my Google Sign In package (with references in my main app), but have no references to this package in my sharing extension, it builds cleanly. I'm taking this as more evidence that this package cannot be used in a sharing extension.

1/1/21: I now have the both above linker issues resolved. The first was just a switch I needed to change in my https://github.com/SyncServerII/GSignIn.git Xcode project (see warning message linking against a dylib which is not safe for use in application extensions). The second was just that I hadn't added my framework reference (for iOSGoogle in my case) into my sharing extension. See, for example, https://stackoverflow.com/a/61848359/1675875

2/3/21: I ran into some other issues, with using XCFrameworks in a sharing extension when I uploaded my app to iTunes Connect:

1. https://developer.apple.com/forums/thread/662247

2. App Store Connect Operation Error ERROR ITMS-90680: "Invalid directory. The bundle Payload/Neebla.app/PlugIns/GSignIn.framework is not contained in a correctly named directory. It should be under "Frameworks"." See https://forums.swift.org/t/swift-package-binary-framework-issue/41922/2 and https://bugs.swift.org/browse/SR-13840