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.
(IANAL) It's a tool, transforming source code. The result thus seems like a derivative work; whether you are or are not allowed to use that in your work depends on the originating license. (And perhaps, your license. E.g., you can't derive from a GPL project and license it as MIT, as the GPL doesn't permit that. But to license as GPL would be fine. But this minimal example assumes all the input to Copilot was GPL, which I rather doubt is true, and I don't think we even know what the input was.)
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.
