gh-140911: Ensure that UserString.rindex(), UserString.index() accept UserString as sub argument.

Lib/collections/__init__.py

@@ -1542,6 +1542,8 @@ def format_map(self, mapping):
         return self.data.format_map(mapping)
 
     def index(self, sub, start=0, end=_sys.maxsize):
+        if isinstance(sub, UserString):
+            sub = sub.data
         return self.data.index(sub, start, end)
 
     def isalpha(self):
@@ -1610,6 +1612,8 @@ def rfind(self, sub, start=0, end=_sys.maxsize):
         return self.data.rfind(sub, start, end)
 
     def rindex(self, sub, start=0, end=_sys.maxsize):
+        if isinstance(sub, UserString):
+            sub = sub.data
         return self.data.rindex(sub, start, end)
 
     def rjust(self, width, *args):

Misc/NEWS.d/next/Library/2025-11-03-17-13-00.gh-issue-140911.7KFvSQ.rst

@@ -0,0 +1,3 @@
+:mod:`collections`: Ensure that the methods ``UserString.rindex()`` and
+``UserString.index()`` accept :class:`collections.UserString` instances as the
+sub argument.