Tuple constructor takes Iterable[T] where T is a type variable. Mypy cannot infer any value of T such that int is a subtype of Iterable[T] and so has to say Never - that's what we do with all unsolvable type variables. I agree this is questionable from usability standpoint.

Ah, yes, it's the same for any user-level function as well...

from typing import Iterable, Never

def f[T](a: Iterable[T]) -> None:
    pass

f(1) # main.py:6: error: Argument 1 to "f" has incompatible type "int"; expected "Iterable[Never]"  [arg-type]

It's hard to say what the type checker could usefully and accurately say here. Never is too narrow, Any is too broad, int (the actual value of T in this case) is also unhelpful). I guess the best option might be just printing the name of the typevar instead.

But typevar name is also weird, some people use long-ish typevar names, and that can look like some "good" type one has to look up. Even short names like T won't really friendly for beginners who aren't familiar with typevars (or at least not proficient with typing to expect them in such contexts).

We used to use Unknown <nothing> in the error message IIRC and maybe that's cleaner? Alternatively maybe we can also emit a "unable to infer type vars" just like with variable assignment if a function call fails?

(I think any sort of proper solution for this is really hard, I would love an interface to explore a program and find what constraints are used for inference etc, but that's a massive undertaking...)

Maybe instead of <nothing>, it should print <something>. Probably kind of hard to get it to know the difference between Nevers that are something not given and Nevers that are real Nevers, though...

It's very easy, the Never is explicitly assigned upon not finding anything to infer. We would just need to add some sort of "kind" parameter like Any I think.