1 files changed, 12 insertions, 0 deletions
diff --git a/tests/test_runtime.py b/tests/test_runtime.py
index 1b24b40..1afcb3f 100644
--- a/tests/test_runtime.py
+++ b/tests/test_runtime.py
@@ -1,3 +1,5 @@
+import itertools
+
 from jinja2 import Template
 from jinja2.runtime import LoopContext
 
@@ -46,3 +48,13 @@ def test_loopcontext2():
     in_lst = [10, 11]
     l = LoopContext(reversed(in_lst), None)
     assert l.length == len(in_lst)
+
+
+def test_iterator_not_advanced_early():
+    t = Template("{% for _, g in gs %}{{ loop.index }} {{ g|list }}\n{% endfor %}")
+    out = t.render(gs=itertools.groupby(
+        [(1, "a"), (1, "b"), (2, "c"), (3, "d")], lambda x: x[0]
+    ))
+    # groupby groups depend on the current position of the iterator. If
+    # it was advanced early, the lists would appear empty.
+    assert out == "1 [(1, 'a'), (1, 'b')]\n2 [(2, 'c')]\n3 [(3, 'd')]\n"