Narrowing doesn't work on a typevar with a union bound

[eltoder]

Bug Report

not isinstance check fails to narrow type for a TypeVar with a union bound.

To Reproduce

from typing import TypeVar

T = TypeVar("T", bound=int | str)

def foo(x: T) -> None:
    if not isinstance(x, int):
        reveal_type(x)

https://mypy-play.net/?mypy=latest&python=3.11&gist=2be94d37f0b12222d62a7f40e59fb152

Expected Behavior

Revealed type is "builtins.str".

Actual Behavior

Revealed type is "T`-1".

Your Environment

• Mypy version used: 1.4.1
• Python version used: 3.10.6

[A5rocks]

By the way, the expected behavior is not correct:

Imagine T is bound as some string subclass! If T were accepted as str then the following should probably be valid, correct?:

from typing import TypeVar

T = TypeVar("T", bound=int | str)

def foo(x: T) -> T:
    if not isinstance(x, int):
        return "h"
    else:
        # whatever
        return x

But that's not the case! The main problem here is typevar reveal_types cannot really express the narrowing as shown here. While mypy can do it under the hood (and you presumably have a case that shows this isn't the case?) that's not exactly what you're asking here!

Mind clarifying a bit?

[eltoder]

Good point, my example is not 100% correct. Let me clarify and also add a second problem case.

1. A problem from your example that already happens with a positive isinstance check:

from typing import TypeVar

T = TypeVar("T", bound=int | str)

def foo(x: T) -> T:
    if isinstance(x, int):
        reveal_type(x)
        return x
    else:
        return x

This produces:

typevar_narrow.py:7: note: Revealed type is "builtins.int"
typevar_narrow.py:8: error: Incompatible return value type (got "int", expected "T")  [return-value]

The type of x was narrowed to int and became incompatible with T even though the code is obviously correct. Here we did too much narrowing.

2. A problem with not enough narrowing:

from typing import TypeVar

T = TypeVar("T", bound=int | str)

def foo(x: T) -> None:
    if not isinstance(x, int):
        bar(x)

def bar(x: str) -> None:
    pass

This produces

typevar_narrow.py:7: error: Argument 1 to "bar" has incompatible type "T"; expected "str"  [arg-type]

but we know the type should be (a subclass of) str, because of the bound on T.

To make it work in both cases, I think, rather than narrowing the type of x we should narrow the bound on T.

[brianschubert]

Duplicate of #15235

[brianschubert]

Actually, this issue is different from #15235. #15235 is only about the negative-narrowing behavior, whereas this issue is also about the positive-narrowing behavior. My bad, sorry for the noise!