Handle corner case: protocol vs classvar vs descriptor (#19277)

Ref #19274

This is a bit ugly. But I propose to have this "hot-fix" until we have a
proper overhaul of instance vs class variables. To be clear: attribute
access already works correctly (on both `P` and `Type[P]`), but
subtyping returns false because of
```python
                elif (IS_CLASSVAR in subflags) != (IS_CLASSVAR in superflags):
                    return False
```

Author: ilevkivskyi

docs/source/protocols.rst

@@ -352,6 +352,53 @@ the parameters are positional-only. Example (using the legacy syntax for generic
    copy_a = copy_b  # OK
    copy_b = copy_a  # Also OK
 
+Binding of types in protocol attributes
+***************************************
+
+All protocol attributes annotations are treated as externally visible types
+of those attributes. This means that for example callables are not bound,
+and descriptors are not invoked:
+
+.. code-block:: python
+
+   from typing import Callable, Protocol, overload
+
+   class Integer:
+       @overload
+       def __get__(self, instance: None, owner: object) -> Integer: ...
+       @overload
+       def __get__(self, instance: object, owner: object) -> int: ...
+       # <some implementation>
+
+   class Example(Protocol):
+       foo: Callable[[object], int]
+       bar: Integer
+
+   ex: Example
+   reveal_type(ex.foo)  # Revealed type is Callable[[object], int]
+   reveal_type(ex.bar)  # Revealed type is Integer
+
+In other words, protocol attribute types are handled as they would appear in a
+``self`` attribute annotation in a regular class. If you want some protocol
+attributes to be handled as though they were defined at class level, you should
+declare them explicitly using ``ClassVar[...]``. Continuing previous example:
+
+.. code-block:: python
+
+   from typing import ClassVar
+
+   class OtherExample(Protocol):
+       # This style is *not recommended*, but may be needed to reuse
+       # some complex callable types. Otherwise use regular methods.
+       foo: ClassVar[Callable[[object], int]]
+       # This may be needed to mimic descriptor access on Type[...] types,
+       # otherwise use a plain "bar: int" style.
+       bar: ClassVar[Integer]
+
+   ex2: OtherExample
+   reveal_type(ex2.foo)  # Revealed type is Callable[[], int]
+   reveal_type(ex2.bar)  # Revealed type is int
+
 .. _predefined_protocols_reference:
 
 Predefined protocol reference

mypy/subtypes.py

@@ -1457,14 +1457,24 @@ def get_member_flags(name: str, itype: Instance, class_obj: bool = False) -> set
         flags = {IS_VAR}
         if not v.is_final:
             flags.add(IS_SETTABLE)
-        if v.is_classvar:
+        # TODO: define cleaner rules for class vs instance variables.
+        if v.is_classvar and not is_descriptor(v.type):
             flags.add(IS_CLASSVAR)
         if class_obj and v.is_inferred:
             flags.add(IS_CLASSVAR)
         return flags
     return set()
 
 
+def is_descriptor(typ: Type | None) -> bool:
+    typ = get_proper_type(typ)
+    if isinstance(typ, Instance):
+        return typ.type.get("__get__") is not None
+    if isinstance(typ, UnionType):
+        return all(is_descriptor(item) for item in typ.relevant_items())
+    return False
+
+
 def find_node_type(
     node: Var | FuncBase,
     itype: Instance,

test-data/unit/check-protocols.test

@@ -4602,3 +4602,47 @@ def deco(fn: Callable[[], T]) -> Callable[[], list[T]]: ...
 @deco
 def defer() -> int: ...
 [builtins fixtures/list.pyi]
+
+[case testProtocolClassValDescriptor]
+from typing import Any, Protocol, overload, ClassVar, Type
+
+class Desc:
+    @overload
+    def __get__(self, instance: None, owner: object) -> Desc: ...
+    @overload
+    def __get__(self, instance: object, owner: object) -> int: ...
+    def __get__(self, instance, owner):
+        pass
+
+class P(Protocol):
+    x: ClassVar[Desc]
+
+class C:
+    x = Desc()
+
+t: P = C()
+reveal_type(t.x)  # N: Revealed type is "builtins.int"
+tt: Type[P] = C
+reveal_type(tt.x)  # N: Revealed type is "__main__.Desc"
+
+bad: P = C  # E: Incompatible types in assignment (expression has type "type[C]", variable has type "P") \
+            # N: Following member(s) of "C" have conflicts: \
+            # N:     x: expected "int", got "Desc"
+
+[case testProtocolClassValCallable]
+from typing import Any, Protocol, overload, ClassVar, Type, Callable
+
+class P(Protocol):
+    foo: Callable[[object], int]
+    bar: ClassVar[Callable[[object], int]]
+
+class C:
+    foo: Callable[[object], int]
+    bar: ClassVar[Callable[[object], int]]
+
+t: P = C()
+reveal_type(t.foo)  # N: Revealed type is "def (builtins.object) -> builtins.int"
+reveal_type(t.bar)  # N: Revealed type is "def () -> builtins.int"
+tt: Type[P] = C
+reveal_type(tt.foo)  # N: Revealed type is "def (builtins.object) -> builtins.int"
+reveal_type(tt.bar)  # N: Revealed type is "def (builtins.object) -> builtins.int"