diff --git a/mercurial/pycompat.py b/mercurial/pycompat.py
--- a/mercurial/pycompat.py
+++ b/mercurial/pycompat.py
@@ -154,7 +154,7 @@
     bytechr = struct.Struct('>B').pack
     byterepr = b'%r'.__mod__
 
-    class bytestr(bytes):
+    class bytestr(bytes): # type: Callable[[Union[bytes, str], bytestr]
         """A bytes which mostly acts as a Python 2 str
 
         >>> bytestr(), bytestr(bytearray(b'foo')), bytestr(u'ascii'), bytestr(1)
@@ -208,7 +208,7 @@
         """
 
         def __new__(cls, s=b''):
-            if isinstance(s, bytestr):
+            if isinstance(s, _bytestr):
                 return s
             if not isinstance(
                 s, (bytes, bytearray)
@@ -398,7 +398,7 @@
     unicode = unicode
     bytechr = chr
     byterepr = repr
-    bytestr = str
+    bytestr = str # type: Callable[[Union[bytes, str], bytestr]
     iterbytestr = iter
     maybebytestr = identity
     sysbytes = identity
@@ -504,3 +504,12 @@
     return tempfile.NamedTemporaryFile(
         mode, bufsize, suffix=suffix, prefix=prefix, dir=dir, delete=delete
     )
+
+
+try:
+    if ispy3:
+        import typing
+
+        typing.Union
+except ImportError:
+    pass