Wrong 'incompatible return value type' when using TypeVar with constraints

Bug Report

Mypy throws an error where everything should be alright.

To Reproduce

from typing import TypeVar

class Hello:
    pass

class World:
    pass

HelloWorld = TypeVar("HelloWorld", Hello, World)

def fn(x: HelloWorld) -> HelloWorld:
    if isinstance(x, Hello):
        return Hello()
    if isinstance(x, World):
        return World()

Expected Behavior

No issues found.

Actual Behavior

Got error in line "return Hello()":

error: Incompatible return value type (got "Hello", expected "World")

Your Environment

• Mypy version used: 0.961
• Mypy command-line flags: none
• Mypy configuration options from mypy.ini (and other config files):

[mypy]
plugins = numpy.typing.mypy_plugin
show_absolute_path = True

[mypy-numpy]
ignore_errors = True

[mypy-numpy.*]
ignore_errors = True

• Python version used: 3.9.12
• Operating system and version: Ubuntu 22.04

Inizia eseguendo mypy sulla riproduzione di TypeVar fornita e conferma l’errore segnalato sul valore restituito da Hello(). Segui il percorso del controllo dei tipi per i TypeVars vincolati e il narrowing con isinstance; il lavoro è completato quando l’esempio non produce problemi come descritto in Expected Behavior.