Add docs (also one test just in case)

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 re-use
+       # 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

test-data/unit/check-protocols.test

@@ -4608,9 +4608,9 @@ from typing import Any, Protocol, overload, ClassVar, Type
 
 class Desc:
     @overload
-    def __get__(self, instance: None, owner: Any) -> Desc: ...
+    def __get__(self, instance: None, owner: object) -> Desc: ...
     @overload
-    def __get__(self, instance: object, owner: Any) -> int: ...
+    def __get__(self, instance: object, owner: object) -> int: ...
     def __get__(self, instance, owner):
         pass
 
@@ -4628,3 +4628,21 @@ 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"