Speaking as a PMC of Apache Flex, but not as a lawyer...

> If someone makes a from-scratch implementation of Swift, or any other language with similar features, can Apple then sue them over these patents?

Yes, Apple can sue you for making use of their patents w/o a formal license in place!

> Or does AL prevent this?

No! The Apache License protects users who use code from the project in question, but does not allow you to use those patents for other purposes.

To give a parallel example:

I license a photograph of two kids in overalls to put it on a poster and sell it.

That does not give me permission to use that image on playing cards, porcelain dolls, or other avenues.

https://www.farmanddairy.com/columns/the-story-behind-the-fa...

Apple could try, sure, and you wouldn't have something like the Apache 2 license to point to in order to attempt to resolve things quicker.

Any number of people could try to sue you for any number of things. Thats unfortunately how lawsuits work.

You could also attempt to get another license from Apple for your work. They might not be willing to do this unless you somehow could guarantee all patent licensees for your version of Swift would be unmotivated (by some legal clause or agreement) to sue them as well.

I doubt Apple would go through the work of getting a patent for something that they already openly give out licenses for people to use (via Apache 2) unless it was meant purely for defensive purposes. So I'd be shocked if they went after a from-scratch Swift implementation except for trademark purposes (aka, you can't call name it Swift if it is a partial implementation)