Type narrowing of 'TypeVar' causes '[return-value]' error

[nikitagashkov]

Bug Report

mypy reports a type incompatibility error when the TypeVars are narrowed within a generic function but the generic arg is not changed in any way.

To Reproduce

from typing import TypeVar

T = TypeVar("T")


def foo(v: T) -> T:
    if isinstance(v, str):
        return v  # error: Incompatible return value type (got "str", expected "T")  [return-value]
    return v

…and a mypy-play link.

Expected Behavior

No errors are thrown since the variable wasn't patched in any way (e.g., it was not returned as str(v)).

Actual Behavior

main.py:8: error: Incompatible return value type (got "str", expected "T")  [return-value]
Found 1 error in 1 file (checked 1 source file)

Your Environment

• Mypy version used: 1.2.0
• Mypy command-line flags: None
• Mypy configuration options from mypy.ini (and other config files): None
• Python version used: 3.10

[ikonst]

One way to think about it is that "v didn't change" suggesting that v should narrow not to an Instance but to a TypeVarType bound to str. Or maybe it should "narrow" not v itself but T so that the function itself, within that branch, would now be happy with return "foo".

[JelleZijlstra]

Or maybe it should "narrow" not v itself but T so that the function itself, within that branch, would now be happy with return "foo".

That would be incorrect, since foo can be a subtype of str.

[ikonst]

That would be incorrect, since foo can be a subtype of str.

What am I missing?

from typing import TypeVar

T = TypeVar('T')

class mystr(str):
  pass

def f(v: T) -> T:
    if isinstance(v, str):
        return mystr("bar")  # at this point, T is narrowed to TypeVar('T', bound=str)
    return v

value = f("foo")
reveal_type(value)  # N: Revealed type is "builtins.str"

IIUC, value being deemed a str here is fine, even though it might end up a mystr.

[hauntsaninja]

from typing import TypeVar

T = TypeVar('T')

class mystr(str): ...
  
class otherstr(str): ...

def f(v: T) -> T:
    if isinstance(v, str):
        return mystr("bar")
    return v

value = f(otherstr("foo"))
reveal_type(value)  # oops, revealed type is "__main__.otherstr"