mypy thinks that instance variables that implement the descriptor protocol are invoked when accessing them

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Bug Report

__get__ should only be understood to be invoked on descriptors stored in a type's dictionary. But mypy type-checks as if there is a default present. This makes any infrastructure that wraps a decorator impossible to type correctly.

To Reproduce

https://mypy-play.net/?mypy=latest&python=3.11&flags=strict&gist=9483880764b49d2ce31d4156c9f4190a

from dataclasses import dataclass

class A:
    pass

class B:
    def __get__(self, instance: A, owner: None=None) -> int:
        return 3

    def __set__(self, instance: A, value: int) -> None:
        print("set:", value)


@dataclass
class W:
    b: B

    def set_b(self, b: B) -> None:
        self.b = b

w = W(B())
w.set_b(B())
print(w.b)

Expected Behavior

It should type-check without errors, as Python runs it fine.

Actual Behavior

main.py:19: error: Argument 1 to "__get__" of "B" has incompatible type "W"; expected "A"  [arg-type]
main.py:19: error: Argument 2 to "__get__" of "B" has incompatible type "type[W]"; expected "None"  [arg-type]
main.py:19: error: Argument 1 to "__set__" of "B" has incompatible type "W"; expected "A"  [arg-type]
main.py:19: error: Incompatible types in assignment (expression has type "B", variable has type "int")  [assignment]
Found 4 errors in 1 file (checked 1 source file)

Your Environment

• Mypy version used: 1.4.1
• Mypy command-line flags: --strict
• Python version used: 3.11

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Apologies if this is a dup; I did look through the descriptors topic and there's lots of stuff adjacent to this, but I didn't see it reported as such.

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(Also worth noting that the @dataclass seems to be irrelevant; this happens with regular type annotations too with a manual __init__.)

[jace]

I've run into the same issue trying to use a Protocol to declare a function's namespace (as is usually set by decorators):

https://mypy-play.net/?mypy=latest&python=3.11&gist=3dc9a75bf220a34c461fe2ed2523f61b

IMO, Mypy should invoke the descriptor protocol only when the variable is a ClassVar (implicit or explicit):

class Foo:
    a: ClassVar[Descriptor]  # Variable type is the return type of Descriptor.__get__
    b = Descriptor()  # Implicit ClassVar descriptor
    c: Descriptor  # The type is just `Descriptor`