I experienced the same. The check in this case is too strict since optional is an header only library so it should be available indenpendently of the avaiable runtime, and compilation would fail anyway. Just looking at where the macro is defined[1] I discovered you can just define:_LIBCPP_DISABLE_AVAILABILITYto disable the availability mechanism as a whole, effectively working around the problem.By the way, I recommend you to post your development questions also to StackOverflow to have more chances to find answers quickly. It's so unconfortable to find posts on forum with exactly the same problem and zero answers.[1] https://github.com/llvm-mirror/libcxx/blob/master/include/__config

Since some people asked for exact steps on how to create the fat framework with lipo, here is how I did it: # Create a template framework starting from the arm64 ios simulator one mkdir -p build-iossim cp -r build-iossim-arm64-release/target/MyFramework.framework build-iossim # Merge the arm64 and x86_64 binaries with lipo xcrun lipo -create -output build-iossim/MyFramework.framework/MyFramework build-iossim-x86_64-release/target/MyFramework.framework/MyFramework build-iossim-arm64-release/target/MyFramework.framework/MyFramework # Codesign the fat binary xcrun codesign --sign - build-iossim/MyFramework.framework/MyFramework # Create the xcframework merging the ios_arm64 framework and the fat simulator framework mkdir -p build-xcframework xcodebuild -create-xcframework \ -framework build-ios-arm64-release/target/MyFramework.framework \ -framework build-iossim/MyFramework.framework \ -output "build-xcframework/MyFramework.xcframework"