mypy incorrectly flags a type-var error when a descriptor overrides a protocol property in a different file.

Bug Report

To Reproduce

• Create an app.py file with this inside:

from __future__ import annotations

from typing import Protocol

# PROTOCOL
class ServiceProto(Protocol):
    @property
    def value(self) -> int: ...

# DESCRIPTOR
class Descriptor:
    def __get__(self, instance: ServiceImpl, owner: type[ServiceImpl]) -> int:
        return 5

# IMPLEMENTATION
class ServiceImpl:
    value = Descriptor()

class App[ServiceT: ServiceProto]:
    def __init__(self, service: ServiceT) -> None:
        self.service = service

• create a second file (important) with this inside (mine is named run.py):

from __future__ import annotations

from .app import App, ServiceImpl

def bootstrap() -> App[ServiceImpl]:
    return App(ServiceImpl())

Expected Behavior
No errors

Actual Behavior

mypy src/
run.py:5: error: Type argument "ServiceImpl" of "App" must be a subtype of "ServiceProto"  [type-var]
run.py:6: error: Value of type variable "ServiceT" of "App" cannot be "ServiceImpl"  [type-var]
Found 2 errors in 1 file (checked 3 source files)

Your Environment

• Mypy version used: mypy 1.18.2 (compiled: yes)
• Mypy command-line flags: none
• Mypy configuration options from mypy.ini (and other config files): none
• Python version used: Python 3.13.7 (via uv)

Weird Behavior

Merge both files, remove redundant imports. No more errors.

PS: Feel free to edit the title. This was a hard one to describe in one sentence without knowing the root cause.
PPS: This is a toy example with integers to illustrate the bug. My real use case is more complex: the descriptor is a lazy instantiator that works like cached_property (the descriptor is given a class object on instantiation, it instantiates and caches an instance of that class on the owning service instance, then serves the cached instance on further accesses).

Reproduziere den Bericht mit den getrennten Dateien app.py und run.py und führe anschließend mypy src/ mit Python 3.13.7 und mypy 1.18.2 aus. Verfolge, wie der descriptor-gestützte ServiceImpl dateiübergreifend gegen ServiceProto geprüft wird und wie App[ServiceImpl] die type-var errors erzeugt. Als erledigt gilt die Aufgabe, wenn das Zwei-Dateien-Beispiel keine Fehler meldet und das Verhalten weiterhin durch einen geeigneten Regressionstest abgedeckt ist.