diff -r ff72bd52d56a -r 9d2b2df2c2ba mercurial/match.py
--- a/mercurial/match.py	Fri Mar 06 10:52:44 2020 +0100
+++ b/mercurial/match.py	Fri Mar 06 13:27:41 2020 -0500
@@ -772,7 +772,7 @@
         candidates = self._fileset | self._dirs - {b''}
         if dir != b'':
             d = dir + b'/'
-            candidates = set(c[len(d) :] for c in candidates if c.startswith(d))
+            candidates = {c[len(d) :] for c in candidates if c.startswith(d)}
         # self._dirs includes all of the directories, recursively, so if
         # we're attempting to match foo/bar/baz.txt, it'll have '', 'foo',
         # 'foo/bar' in it. Thus we can safely ignore a candidate that has a