diff -r 96fcc37a9c80 -r f90a5c211251 mercurial/cmdutil.py
--- a/mercurial/cmdutil.py	Thu Oct 22 23:10:06 2020 -0700
+++ b/mercurial/cmdutil.py	Tue Oct 27 15:33:15 2020 -0700
@@ -1222,8 +1222,16 @@
     if not spec:
         spec = default_spec
     if not spec:
-        # TODO: Pick a default we can agree on. This isn't used yet.
-        raise error.ProgrammingError(b"no default one-line summary defined yet")
+        spec = (
+            b'{separate(" ", '
+            b'label("log.changeset", "{rev}:{node|short}")'
+            b', '
+            b'label("log.tag", tags)'
+            b', '
+            b'label("log.bookmark", bookmarks)'
+            b')} '
+            b'"{label("log.desc", desc|firstline)}"'
+        )
     text = rendertemplate(ctx, spec)
     return text.split(b'\n')[0]