Even back in March, you could tell it "give me a function that adds two numbers" and you could be 100% confident that it would write the correct function.

There's a lot of languages where that's nowhere near as simple as you make it sound. Floating point, decimals, etc.

Then there's other little quirks like rounding rules: https://en.wikipedia.org/wiki/Rounding

Basically, adding numbers together is exactly the sort of thing AIs still muck up spectacularly, precisely because they either fail to understand the context of the problem, or fail to ask about an assumption they make.

That you've had so many replies and no-one else has even mentioned this is in itself worrying.

Your own example proves your point is wrong.