I think the issue is not that it’s trained on open source code but that it’s trained on code whose licenses may not permit it. If you license your project in a permissive way then I don’t see a problem.
I think there might be some in this thread who don't consider these derivatives, for whatever reason, but it seems to be that if rangeCheck() passes de minimis, then the output from Copilot almost certainly does, too. That a tool is doing the copying and mutating, as opposed to a human, seems immaterial to it all. (Now, I don't know that I agree with rangeCheck() not being de minimis … and yet.) Or they think that Copilot is "thinking", which, ha, no.