diff --git a/mercurial/__init__.py b/mercurial/__init__.py
--- a/mercurial/__init__.py
+++ b/mercurial/__init__.py
@@ -308,12 +308,23 @@
     import os
     import struct
     from hgdemandimport import py3tokenize as tokenize
+    from hgdemandimport import py3token as token
 
     rootpath = os.path.normpath(os.path.join(os.path.dirname(__file__), '..'))
 
     def replacetokens(tokens, fullname):
-        for i, t in enumerate(tokens):
+        i = 0
+        while i < len(tokens):
+            t = tokens[i]
+
+            # Function return type annotation.
+            if t.type == token.OP and t.string == '->':
+                t2 = tokens[i + 1]
+                i += 2
+                continue
+
             yield t
+            i += 1
 
     # Header to add to bytecode files. This MUST be changed when
     # ``replacetokens`` or any mechanism that changes semantics of module
diff --git a/mercurial/dispatch.py b/mercurial/dispatch.py
--- a/mercurial/dispatch.py
+++ b/mercurial/dispatch.py
@@ -177,7 +177,7 @@
 def _formatargs(args):
     return ' '.join(procutil.shellquote(a) for a in args)
 
-def dispatch(req):
+def dispatch(req) -> int:
     """run the command specified in req.args; returns an integer status code"""
     with tracing.log('dispatch.dispatch'):
         if req.ferr: