Allow generic decorators on abstarct classes (#13398)

Fixes #5374

As discussed in the issue, we should allow this common use case, although it may be technically unsafe (e.g. if one would instantiate a class in a class decorator body).

mypy/checker.py

@@ -409,6 +409,11 @@ def __init__(
         # of giving a note on possibly missing "await". It is used to avoid infinite recursion.
         self.checking_missing_await = False
 
+        # While this is True, allow passing an abstract class where Type[T] is expected.
+        # although this is technically unsafe, this is desirable in some context, for
+        # example when type-checking class decorators.
+        self.allow_abstract_call = False
+
     @property
     def type_context(self) -> List[Optional[Type]]:
         return self.expr_checker.type_context
@@ -2027,9 +2032,12 @@ def visit_class_def(self, defn: ClassDef) -> None:
 
                     # TODO: Figure out how to have clearer error messages.
                     # (e.g. "class decorator must be a function that accepts a type."
+                    old_allow_abstract_call = self.allow_abstract_call
+                    self.allow_abstract_call = True
                     sig, _ = self.expr_checker.check_call(
                         dec, [temp], [nodes.ARG_POS], defn, callable_name=fullname
                     )
+                    self.allow_abstract_call = old_allow_abstract_call
                 # TODO: Apply the sig to the actual TypeInfo so we can handle decorators
                 # that completely swap out the type.  (e.g. Callable[[Type[A]], Type[B]])
         if typ.is_protocol and typ.defn.type_vars:

mypy/checkexpr.py

@@ -1914,6 +1914,7 @@ def check_arg(
             and (caller_type.type_object().is_abstract or caller_type.type_object().is_protocol)
             and isinstance(callee_type.item, Instance)
             and (callee_type.item.type.is_abstract or callee_type.item.type.is_protocol)
+            and not self.chk.allow_abstract_call
         ):
             self.msg.concrete_only_call(callee_type, context)
         elif not is_subtype(caller_type, callee_type, options=self.chk.options):

test-data/unit/check-abstract.test

@@ -1018,3 +1018,15 @@ d = my_abstract_types['B']()  # E: Cannot instantiate abstract class "MyAbstract
 d.do()
 
 [builtins fixtures/dict.pyi]
+
+[case testAbstractClassesWorkWithGenericDecorators]
+from abc import abstractmethod, ABCMeta
+from typing import Type, TypeVar
+
+T = TypeVar("T")
+def deco(cls: Type[T]) -> Type[T]: ...
+
+@deco
+class A(metaclass=ABCMeta):
+    @abstractmethod
+    def foo(self, x: int) -> None: ...