Fix class objects falling back to metaclass for callback protocol (#14121)

Fixes #10482 

This is not very principled, but should work except people will want to
explicitly check some metaclass `__call__`.

Author: ilevkivskyi

mypy/messages.py

@@ -1899,7 +1899,7 @@ def report_protocol_problems(
         missing = get_missing_protocol_members(subtype, supertype, skip=skip)
         if (
             missing
-            and len(missing) < len(supertype.type.protocol_members)
+            and (len(missing) < len(supertype.type.protocol_members) or missing == ["__call__"])
             and len(missing) <= MAX_ITEMS
         ):
             if missing == ["__call__"] and class_obj:

mypy/subtypes.py

@@ -1005,6 +1005,10 @@ def named_type(fullname: str) -> Instance:
                 subtype: ProperType | None = mypy.checkmember.type_object_type(
                     left.type, named_type
                 )
+            elif member == "__call__" and left.type.is_metaclass():
+                # Special case: we want to avoid falling back to metaclass __call__
+                # if constructor signature didn't match, this can cause many false negatives.
+                subtype = None
             else:
                 subtype = get_proper_type(find_member(member, left, left, class_obj=class_obj))
             # Useful for debugging:

test-data/unit/check-protocols.test

@@ -3526,6 +3526,23 @@ test(B)  # OK
 test(C)  # E: Argument 1 to "test" has incompatible type "Type[C]"; expected "P" \
          # N: "C" has constructor incompatible with "__call__" of "P"
 
+[case testProtocolClassObjectPureCallback]
+from typing import Any, ClassVar, Protocol
+
+class P(Protocol):
+    def __call__(self, x: int, y: int) -> Any: ...
+
+class B:
+    def __init__(self, x: int, y: int) -> None: ...
+class C:
+    def __init__(self, x: int, y: str) -> None: ...
+
+def test(arg: P) -> None: ...
+test(B)  # OK
+test(C)  # E: Argument 1 to "test" has incompatible type "Type[C]"; expected "P" \
+         # N: "C" has constructor incompatible with "__call__" of "P"
+[builtins fixtures/type.pyi]
+
 [case testProtocolTypeTypeAttribute]
 from typing import ClassVar, Protocol, Type
 

test-data/unit/fixtures/type.pyi

@@ -1,8 +1,9 @@
 # builtins stub used in type-related test cases.
 
-from typing import Generic, TypeVar, List, Union
+from typing import Any, Generic, TypeVar, List, Union
 
-T = TypeVar('T')
+T = TypeVar("T")
+S = TypeVar("S")
 
 class object:
     def __init__(self) -> None: pass
@@ -12,13 +13,16 @@ class list(Generic[T]): pass
 
 class type(Generic[T]):
     __name__: str
+    def __call__(self, *args: Any, **kwargs: Any) -> Any: pass
     def __or__(self, other: Union[type, None]) -> type: pass
     def __ror__(self, other: Union[type, None]) -> type: pass
     def mro(self) -> List['type']: pass
 
 class tuple(Generic[T]): pass
+class dict(Generic[T, S]): pass
 class function: pass
 class bool: pass
 class int: pass
 class str: pass
 class unicode: pass
+class ellipsis: pass