[CamTin]

Perl almost does this. Various CPAN packages have different "use" lines where they can declare themselves to be written in a particular version. These can all live together on an interpreter recent enough to know all the dialects. This is at the module level, rather than the function level though.

[nixpulvis]

I'm imagining something like `foo@v2.3.1(x)`. Of course, `foo(x)` would still work, but it would probably just choose the top version, like how things generally work today. Integrate with signed VCS and call it a day.