Fix stream cycle over empty enumerable (#6253)

2 files changed, 14 insertions, 0 deletions

diff --git a/lib/elixir/lib/stream.ex b/lib/elixir/lib/stream.ex
index ae7f22cc2..e797c0239 100644
--- a/lib/elixir/lib/stream.ex
+++ b/lib/elixir/lib/stream.ex
@@ -1155,6 +1155,10 @@ defmodule Stream do
   @spec cycle(Enumerable.t) :: Enumerable.t
   def cycle(enumerable)
 
+  def cycle([]) do
+    raise ArgumentError, "cannot cycle over empty enumerable"
+  end
+
   def cycle(enumerable) when is_list(enumerable) do
     unfold {enumerable, enumerable}, fn
       {source, [h | t]}      -> {h, {source, t}}
@@ -1185,6 +1189,8 @@ defmodule Stream do
       {:stream_cycle, acc} ->
         {:halted, acc}
     else
+      {state, []} when state in [:done, :halted] ->
+        raise ArgumentError, "cannot cycle over empty enumerable"
       {state, acc} when state in [:done, :halted] ->
         do_cycle(cycle, cycle, {:cont, acc})
       {:suspended, acc, continuation} ->
diff --git a/lib/elixir/test/elixir/stream_test.exs b/lib/elixir/test/elixir/stream_test.exs
index 69d7f9efc..4dd905e30 100644
--- a/lib/elixir/test/elixir/stream_test.exs
+++ b/lib/elixir/test/elixir/stream_test.exs
@@ -168,6 +168,14 @@ defmodule StreamTest do
     stream = Stream.cycle([1, 2, 3])
     assert is_function(stream)
 
+    assert_raise ArgumentError, "cannot cycle over empty enumerable", fn ->
+      Stream.cycle([])
+    end
+
+    assert_raise ArgumentError, "cannot cycle over empty enumerable", fn ->
+      Stream.cycle(%{}) |> Enum.to_list()
+    end
+
     assert Stream.cycle([1, 2, 3]) |> Stream.take(5) |> Enum.to_list == [1, 2, 3, 1, 2]
     assert Enum.take(stream, 5) == [1, 2, 3, 1, 2]
   end