2 files changed, 15 insertions, 5 deletions
diff --git a/synapse/handlers/auth.py b/synapse/handlers/auth.py
index 57a6854b1e..cf12b55d21 100644
--- a/synapse/handlers/auth.py
+++ b/synapse/handlers/auth.py
@@ -201,7 +201,7 @@ class AuthHandler:
         for auth_checker_class in INTERACTIVE_AUTH_CHECKERS:
             inst = auth_checker_class(hs)
             if inst.is_enabled():
-                self.checkers[inst.AUTH_TYPE] = inst  # type: ignore
+                self.checkers[inst.AUTH_TYPE] = inst
 
         self.bcrypt_rounds = hs.config.registration.bcrypt_rounds
 
diff --git a/synapse/handlers/ui_auth/checkers.py b/synapse/handlers/ui_auth/checkers.py
index 332edcca24..78a75bfed6 100644
--- a/synapse/handlers/ui_auth/checkers.py
+++ b/synapse/handlers/ui_auth/checkers.py
@@ -13,7 +13,8 @@
 # limitations under the License.
 
 import logging
-from typing import TYPE_CHECKING, Any
+from abc import ABC, abstractmethod
+from typing import TYPE_CHECKING, Any, ClassVar, Sequence, Type
 
 from twisted.web.client import PartialDownloadError
 
@@ -27,19 +28,28 @@ if TYPE_CHECKING:
 logger = logging.getLogger(__name__)
 
 
-class UserInteractiveAuthChecker:
+class UserInteractiveAuthChecker(ABC):
     """Abstract base class for an interactive auth checker"""
 
-    def __init__(self, hs: "HomeServer"):
+    # This should really be an "abstract class property", i.e. it should
+    # be an error to instantiate a subclass that doesn't specify an AUTH_TYPE.
+    # But calling this a `ClassVar` is simpler than a decorator stack of
+    # @property @abstractmethod and @classmethod (if that's even the right order).
+    AUTH_TYPE: ClassVar[str]
+
+    def __init__(self, hs: "HomeServer"):  # noqa: B027
         pass
 
+    @abstractmethod
     def is_enabled(self) -> bool:
         """Check if the configuration of the homeserver allows this checker to work
 
         Returns:
             True if this login type is enabled.
         """
+        raise NotImplementedError()
 
+    @abstractmethod
     async def check_auth(self, authdict: dict, clientip: str) -> Any:
         """Given the authentication dict from the client, attempt to check this step
 
@@ -304,7 +314,7 @@ class RegistrationTokenAuthChecker(UserInteractiveAuthChecker):
             )
 
 
-INTERACTIVE_AUTH_CHECKERS = [
+INTERACTIVE_AUTH_CHECKERS: Sequence[Type[UserInteractiveAuthChecker]] = [
     DummyAuthChecker,
     TermsAuthChecker,
     RecaptchaAuthChecker,