I can't understand what you are trying. Compile framework originally written in Swift 5 with Swift 4.2. What version of Xcode are you using? Xcode Version 10.1 (10B61)

I tested in 12.4 (Swift 5.2), and got error in The situation can be reproduced in Xcode 10.1 (Swift 4.2.1) Also, I would like to attach the screenshot of the Xcode with error, but the forum does not have such functionality.

That means you cannot build a binary for App Store, are you OK with it? I did not know that. Could you please provide an official document confirm that ?

Thank you very much!

Thank you!

So, what's your choice? Continue developing with Xcode 10.1? Or migrate your framework to Xcode 12? For now I have no reason to continue develop in deprecated version and I am going to rollback all of the changes and continue develop on Swift 5.

Why can't you use Xcode 12 ? That would be the simplest and safest way. Because binaries compiled in swift 4.2 are not compatible with swift 5. I have a binary framework dependency compiled with swift 4.2.1. Code in attach. AES.swift - https://developer.apple.com/forums/content/attachment/7b4cc77a-0224-4819-b666-a74b2f5765c6