error: Value of type variable "_AnyPath" of "copyfile" cannot be "Union[str, Path]"

[petsuter]

I have this code which was accepted in a older version (0.6xx?) of mypy:

import pathlib
import shutil
from typing import Union

def f(x: Union[str, pathlib.Path]):
    shutil.copyfile("bla", x)

But is not accepted by mypy 0.710:

error: Value of type variable "_AnyPath" of "copyfile" cannot be "Union[str, Path]"

How should I fix my type annotations?
I tried this:

import os
import shutil
from typing import TypeVar

_AnyPath = TypeVar("_AnyPath", str, os.PathLike)

def f(x: _AnyPath):
    shutil.copyfile("bla", x)

And that seems to work.

---

I asked this on StackOverflow but it was suggested to open a "bug/missing feature issue" here.

[Michael0x2a]

More specifically, I think the root cause is that we don't properly perform union math on functions containing TypeVars with value restrictions:

from typing import TypeVar, Union

T = TypeVar('T', int, str)

def test(x: T) -> T:
    return x

y: Union[int, str]
test(y)

We get a Value of type variable "T" of "test" cannot be "Union[int, str]" error on the final line, which I think is suboptimal -- test is morally equivalent to an overload, and we'd accept a call of test(Union[int, str]) if the signature was written as such.

(Note that shutil.copyfile(...) accepts a TypeVar with value restriction for the second arg, and fails when passed a union.)

...it's a little surprising to me that we don't have an issue for this already, actually. I tried searching for one before suggesting OP file an issue here, but no dice.

[ilevkivskyi]

Duplicate of #1533