diff --git a/hgext/sqlitestore.py b/hgext/sqlitestore.py
--- a/hgext/sqlitestore.py
+++ b/hgext/sqlitestore.py
@@ -86,7 +86,6 @@ from mercurial import (
 )
 from mercurial.interfaces import (
     repository,
-    util as interfaceutil,
 )
 from mercurial.utils import (
     hashutil,
@@ -1282,8 +1281,7 @@ def newreporequirements(orig, ui, create
     return requirements
 
 
-@interfaceutil.implementer(repository.ilocalrepositoryfilestorage)
-class sqlitefilestorage:
+class sqlitefilestorage(repository.ilocalrepositoryfilestorage):
     """Repository file storage backed by SQLite."""
 
     def file(self, path):
diff --git a/mercurial/interfaces/repository.py b/mercurial/interfaces/repository.py
--- a/mercurial/interfaces/repository.py
+++ b/mercurial/interfaces/repository.py
@@ -1658,6 +1658,7 @@ class ilocalrepositoryfilestorage(Protoc
     tracked file path.
     """
 
+    @abc.abstractmethod
     def file(self, f):
         """Obtain a filelog for a tracked path.
 
diff --git a/mercurial/localrepo.py b/mercurial/localrepo.py
--- a/mercurial/localrepo.py
+++ b/mercurial/localrepo.py
@@ -1241,7 +1241,7 @@ def makemain(**kwargs):
     return localrepository
 
 
-class revlogfilestorage:  # (repository.ilocalrepositoryfilestorage)
+class revlogfilestorage(repository.ilocalrepositoryfilestorage):
     """File storage when using revlogs."""
 
     def file(self, path):
@@ -1256,7 +1256,7 @@ class revlogfilestorage:  # (repository.
         return filelog.filelog(self.svfs, path, try_split=try_split)
 
 
-class revlognarrowfilestorage:  # (repository.ilocalrepositoryfilestorage)
+class revlognarrowfilestorage(repository.ilocalrepositoryfilestorage):
     """File storage when using revlogs and narrow files."""
 
     def file(self, path):