The best I have ever seen were obscure languages with very strong type safety. Some researcher at a sibling org to my own told me to try it with the Lean language, and it basically gave flawless suggestions.

I'm guessing this is because the only training material was blogs from uber-nerdy CS researchers on a language where "mistakes" are basically impossible to write, and not a bunch of people flailing on forums asking about hello world-ish stuff and segfaulting examples.