diff --git a/mercurial/cext/bdiff.pyi b/mercurial/cext/bdiff.pyi
--- a/mercurial/cext/bdiff.pyi
+++ b/mercurial/cext/bdiff.pyi
@@ -5,7 +5,7 @@ from typing import (
 
 version: int
 
-def bdiff(a: bytes, b: bytes): bytes
+def bdiff(a: bytes, b: bytes) -> bytes: ...
 def blocks(a: bytes, b: bytes) -> List[Tuple[int, int, int, int]]: ...
 def fixws(s: bytes, allws: bool) -> bytes: ...
 def splitnewlines(text: bytes) -> List[bytes]: ...