MyPy infers descriptors are always ClassVars when testing assignability to protocols

Beginne mit dem eigenständigen Descriptor und dem Protocol-Beispiel in err_report.py und reproduziere die Ergebnisse mit mypy 1.17.1 ohne Flags. Verfolge anschließend das Verhalten der Protocol-Zuweisbarkeit für Descriptor-Attribute und überprüfe dann, dass der erwartete Fall mit einer Instanzvariablen erfolgreich ist, während der ClassVar-Fall wie beschrieben abgelehnt wird.

Bug Report

When defining a Protocol for objects containing descriptors, mypy appears to assume descriptors are always ClassVars. Annotating the protocol's descriptor attributes with ClassVar silences mypy, but raises errors in other type checkers (e.g. pyright). Given descriptors are allowed to be used as both instance and class variables (and __get__ / __set__ provide obj: object | None semantics to support this), I think this is a mypy bug.

A real-world example of this is defining a protocol for an SQLAlchemy model, e.g. a protocol might define id: orm.Mapped[int] to match models with an id integer attribute.

To Reproduce
Given the complexity of SQLAlchemy, here's an entirely self-contained example:

from typing import ClassVar, Protocol


class MyDescriptor:
    def __init__(self):
        self.value = "test"

    def __get__(self, obj: object | None, objtype: type[object]) -> str:
        return self.value

    def __set__(self, obj: object | None, value: str) -> None:
        self.value = value


class Foo:
    bar = MyDescriptor()


class HasBar(Protocol):
    bar: MyDescriptor


class HasBarWithClassVar(Protocol):
    bar: ClassVar[MyDescriptor]


instance1: HasBar = Foo()  # fails in mypy, passes in pyright
model1: type[HasBar] = Foo  # fails in mypy, passes in pyright

instance2: HasBarWithClassVar = Foo()  # fails in pyright, passes in mypy
model2: type[HasBarWithClassVar] = Foo  # fails in pyright, passes in mypy

Expected Behavior

Assigning Foo to HasBar should type check correctly, and assigning to HasBarWithClassVar should fail.

Actual Behavior

Assigning Foo to HasBar errors in mypy, but checks in pyright. Conversely, assigning to HasBarWithClassVar checks in mypy, but fails in pyright. They seem to fundamentally disagree.

$ mypy err_report.py
err_report.py:27: error: Incompatible types in assignment (expression has type "Foo", variable has type "HasBar")  [assignment]
err_report.py:27: note: Following member(s) of "Foo" have conflicts:
err_report.py:27: note:     bar: expected "MyDescriptor", got "str"
err_report.py:28: error: Incompatible types in assignment (expression has type "type[Foo]", variable has type "type[HasBar]")  [assignment]
Found 2 errors in 1 file (checked 1 source file)

$ pyright err_report.py
/path/to/my/err_report.py
  /path/to/my/err_report.py:30:33 - error: Type "Foo" is not assignable to declared type "HasBarWithClassVar"
    "Foo" is incompatible with protocol "HasBarWithClassVar"
      "bar" is defined as a ClassVar in protocol (reportAssignmentType)
  /path/to/my/err_report.py:31:36 - error: Type "type[Foo]" is not assignable to declared type "type[HasBarWithClassVar]"
    "Foo" is incompatible with protocol "HasBarWithClassVar"
    Type "type[Foo]" is not assignable to type "type[HasBarWithClassVar]"
      "bar" is defined as a ClassVar in protocol (reportAssignmentType)
2 errors, 0 warnings, 0 informations

If I were to annotate Foo.bar with ClassVar, then pyright accepts assigning Foo to HasBarWithClassVar (which is what I would expect), but now mypy won't:

class Foo:
    bar: ClassVar = MyDescriptor()

$ mypy err_report.py
err_report.py:27: error: Incompatible types in assignment (expression has type "Foo", variable has type "HasBar")  [assignment]
err_report.py:27: note: Protocol member HasBar.bar expected instance variable, got class variable
err_report.py:28: error: Incompatible types in assignment (expression has type "type[Foo]", variable has type "type[HasBar]")  [assignment]
err_report.py:30: error: Incompatible types in assignment (expression has type "Foo", variable has type "HasBarWithClassVar")  [assignment]
err_report.py:30: note: Protocol member HasBarWithClassVar.bar expected instance variable, got class variable
err_report.py:31: error: Incompatible types in assignment (expression has type "type[Foo]", variable has type "type[HasBarWithClassVar]")  [assignment]
Found 4 errors in 1 file (checked 1 source file)

Your Environment

• Mypy version used: 1.17.1
• Mypy command-line flags: none
• Mypy configuration options from mypy.ini (and other config files): n/a
• Python version used: 3.13.5