diff --git a/mercurial/context.py b/mercurial/context.py
--- a/mercurial/context.py
+++ b/mercurial/context.py
@@ -2526,6 +2526,7 @@
         return len(self._cache) == 0
 
     def clean(self):
+        self._mergestate = None
         self._cache = {}
 
     def _compact(self):
@@ -2586,6 +2587,11 @@
             self._repo, path, parent=self, filelog=filelog
         )
 
+    def mergestate(self, clean=False):
+        if clean or self._mergestate is None:
+            self._mergestate = mergestatemod.memmergestate(self._repo, self)
+        return self._mergestate
+
 
 class overlayworkingfilectx(committablefilectx):
     """Wrap a ``workingfilectx`` but intercepts all writes into an in-memory