Make sure to return nil if enumerable halts

Fix issue #5004

Signed-off-by: José Valim <[email protected]>

Author: Troush

lib/elixir/lib/enum.ex

@@ -861,13 +861,13 @@ defmodule Enum do
 
   def find_index(enumerable, fun) do
     res =
-      Enumerable.reduce(enumerable, {:cont, 0}, fn(entry, acc) ->
-        if fun.(entry), do: {:halt, acc}, else: {:cont, acc + 1}
+      Enumerable.reduce(enumerable, {:cont, {:not_found, 0}}, fn(entry, {status, index}) ->
+        if fun.(entry), do: {:halt, {:found, index}}, else: {:cont, {:not_found, index + 1}}
       end)
 
     case res do
-      {:halted, entry} -> entry
-      {:done, _} -> nil
+      {_, {:found, index}} -> index
+      {_, {:not_found, _}} -> nil
     end
   end
 

lib/elixir/test/elixir/enum_test.exs

@@ -195,6 +195,8 @@ defmodule EnumTest do
   test "find_index/2" do
     assert Enum.find_index([2, 4, 6], fn(x) -> rem(x, 2) == 1 end) == nil
     assert Enum.find_index([2, 3, 4], fn(x) -> rem(x, 2) == 1 end) == 1
+    assert Stream.take(1..3, 3) |> Enum.find_index(fn _ -> false end) == nil
+    assert Stream.take(1..6, 6) |> Enum.find_index(fn x -> x == 5 end) == 4
   end
 
   test "find_value/2" do