--- a/mercurial/templateutil.py	Sat Apr 21 17:00:21 2018 +0900
+++ b/mercurial/templateutil.py	Sat Apr 21 17:15:11 2018 +0900
@@ -189,12 +189,8 @@
         yield self.tomap()
 
     def join(self, context, mapping, sep):
-        # TODO: just copies the old behavior where a value was a generator
-        # yielding one item, but reconsider about it. join() over a string
-        # has no consistent result because a string may be a bytes, or a
-        # generator yielding an item, or a generator yielding multiple items.
-        # Preserving all of the current behaviors wouldn't make any sense.
-        return self.show(context, mapping)
+        w = makewrapped(context, mapping, self._value)
+        return w.join(context, mapping, sep)
 
     def show(self, context, mapping):
         # TODO: switch gen to (context, mapping) API?

--- a/tests/test-command-template.t	Sat Apr 21 17:00:21 2018 +0900
+++ b/tests/test-command-template.t	Sat Apr 21 17:15:11 2018 +0900
@@ -3242,12 +3242,12 @@
   $ hg log -R latesttag -l1 -T '{max(revset("0:9")) % "{rev}:{node|short}\n"}'
   9:fbc7cd862e9c
 
-Test manifest/get() can be join()-ed as before, though it's silly:
-
-  $ hg log -R latesttag -r tip -T '{join(manifest, "")}\n'
-  11:2bc6e9006ce2
-  $ hg log -R latesttag -r tip -T '{join(get(extras, "branch"), "")}\n'
-  default
+Test manifest/get() can be join()-ed as string, though it's silly:
+
+  $ hg log -R latesttag -r tip -T '{join(manifest, ".")}\n'
+  1.1.:.2.b.c.6.e.9.0.0.6.c.e.2
+  $ hg log -R latesttag -r tip -T '{join(get(extras, "branch"), ".")}\n'
+  d.e.f.a.u.l.t
 
 Test join() over string