isinstance with TypeVar in Tuple doesn't accurately determine TypeVar

[ikelos]

Mypy version: 0.590
Python version: 3.6.5

I don't know if I've described this accurately, but it's probably best described by some code. I've reduced this to the simple scenario I can find:

T = TypeVar('T', bound = A)

def check_subtype(instance: A, instance_type: Tuple[Type[T], ...]) -> Optional[T]:
    if isinstance(instance, instance_type):
      return instance
    return None

returns

test.py:15: error: Incompatible return value type (got "A", expected "Optional[T]")

Removing the Tuple and only allowing one Type to be passed to isinstance gives no errors:

T = TypeVar('T', bound = A)

def check_subtype(instance: A, instance_type: Type[T]) -> Optional[T]:
    if isinstance(instance, instance_type):
      return instance
    return None

My understanding was that the signature for isinstance was Union[Type[T], Tuple[Type[T], ...]], so I'd have expected similar outputs when providing it a Type[T] versus a Tuple[Type[T], ...]?

[gvanrossum]

The code that understands isinstance is separate, I think it missed this possibility. So it's a bug.

[roosemberth]

I believe to have stumbled with another instance of this problem:

from typing import TypeVar, Dict, Tuple

TT = TypeVar('TT', bound=Tuple)
KT = TypeVar('KT', bound=Dict)

def k1() -> Tuple[Tuple[int, int], Dict[str, int]]:
    return (23, 42), dict(ds=42)  # No problem


def k2() -> Tuple[TT, KT]:
    # vv Incompatible return value type (got "Tuple[Tuple[int, int], Dict[str, int]]", expected "Tuple[TT, KT]")
    return (23, 231231), dict(ds=42)

[JelleZijlstra]

@roosemberth I don't think that code is correct, because you use TypeVars without anything to constrain them. Usually you only want to use TypeVars in the signature of a function if you're using them in multiple places, because the point of a TypeVar is to constrain multiple types to be the same.

Maybe there's still a bug here (mypy's error message could be better), but in that case it's unrelated to the original bug in this issue, which is about isinstance.