TypeVar resolved to a concrete type using isinstance causes mypy to become confused at the equivalence

[alexkoay]

Bug Report

To Reproduce

from typing import Any, Generic, TypeVar
T = TypeVar("T")

class Foo(Generic[T]):
 def __init__(self, value: T):
 self.value = value

def identity(value: T) -> Foo[T]:
 if isinstance(value, str):
 return Foo(value)
 return Foo(value)

Expected Behavior

mypy should be able to tell that in the type-guarded block, T == str and use that to qualify the types.

Actual Behavior

mypy reports an error Argument 1 to "Foo" has incompatible type "str"; expected "T" in the type-guarded block in identity().

Your Environment

• Mypy version used: 0.910
• Mypy command-line flags: N/A
• Mypy configuration options from mypy.ini (and other config files): N/A
• Python version used: 3.9
• Operating system and version: Windows 10