diff --git a/mercurial/templater.py b/mercurial/templater.py
--- a/mercurial/templater.py
+++ b/mercurial/templater.py
@@ -162,8 +162,13 @@ def buildfilter(exp, context):
 
 def runfilter(context, mapping, data):
     func, data, filt = data
+    # func() may return string, generator of strings or arbitrary object such
+    # as date tuple, but filter does not want generator.
+    thing = func(context, mapping, data)
+    if isinstance(thing, types.GeneratorType):
+        thing = stringify(thing)
     try:
-        return filt(func(context, mapping, data))
+        return filt(thing)
     except (ValueError, AttributeError, TypeError):
         if isinstance(data, tuple):
             dt = data[1]
diff --git a/tests/test-command-template.t b/tests/test-command-template.t
--- a/tests/test-command-template.t
+++ b/tests/test-command-template.t
@@ -1901,6 +1901,11 @@ Thrown an error if a template function d
   hg: parse error: unknown function 'foo'
   [255]
 
+Pass generator object created by template function to filter
+
+  $ hg log -l 1 --template '{if(author, author)|user}\n'
+  test
+
 Test diff function:
 
   $ hg diff -c 8