From ae48ec59c5f15e3f439fe4d351f140786f96479c Mon Sep 17 00:00:00 2001
From: Eric Entin <eric@ericent.in>
Date: Wed, 3 Aug 2016 11:59:04 -0400
Subject: [PATCH] Do not wait for self in parallel compiler

---
 lib/elixir/lib/kernel/parallel_compiler.ex               | 4 +++-
 lib/elixir/test/elixir/fixtures/parallel_struct/undef.ex | 5 +++++
 lib/elixir/test/elixir/kernel/parallel_compiler_test.exs | 7 +++++++
 3 files changed, 15 insertions(+), 1 deletion(-)
 create mode 100644 lib/elixir/test/elixir/fixtures/parallel_struct/undef.ex

diff --git a/lib/elixir/lib/kernel/parallel_compiler.ex b/lib/elixir/lib/kernel/parallel_compiler.ex
index 34c9bb8fe9e..e6baf5b9f33 100644
--- a/lib/elixir/lib/kernel/parallel_compiler.ex
+++ b/lib/elixir/lib/kernel/parallel_compiler.ex
@@ -206,8 +206,10 @@ defmodule Kernel.ParallelCompiler do
 
       {:waiting, kind, child, ref, on, defining} ->
         # Oops, we already got it, do not put it on waiting.
+        # OR
+        # We're waiting on ourselves, send :found so that we can crash with a better error
         waiting =
-          if :lists.any(&match?({^kind, ^on}, &1), result) do
+          if :lists.any(&match?({^kind, ^on}, &1), result) or on in defining do
             send child, {ref, :found}
             waiting
           else
diff --git a/lib/elixir/test/elixir/fixtures/parallel_struct/undef.ex b/lib/elixir/test/elixir/fixtures/parallel_struct/undef.ex
new file mode 100644
index 00000000000..8d35959a82b
--- /dev/null
+++ b/lib/elixir/test/elixir/fixtures/parallel_struct/undef.ex
@@ -0,0 +1,5 @@
+defmodule Undef do
+  def undef do
+    %__MODULE__{}
+  end
+end
diff --git a/lib/elixir/test/elixir/kernel/parallel_compiler_test.exs b/lib/elixir/test/elixir/kernel/parallel_compiler_test.exs
index 7c4a009d529..4b475d10448 100644
--- a/lib/elixir/test/elixir/kernel/parallel_compiler_test.exs
+++ b/lib/elixir/test/elixir/kernel/parallel_compiler_test.exs
@@ -28,6 +28,13 @@ defmodule Kernel.ParallelCompilerTest do
     end
   end
 
+  test "emits struct undefined error when local struct is undefined" do
+    fixtures = [fixture_path("parallel_struct/undef.ex")]
+    assert capture_io(fn ->
+      assert catch_exit(Kernel.ParallelCompiler.files(fixtures)) == {:shutdown, 1}
+    end) =~ "Undef.__struct__/1 is undefined, cannot expand struct Undef"
+  end
+
   test "does not hang on missing dependencies" do
     fixtures = [fixture_path("parallel_compiler/bat.ex")]
     assert capture_io(fn ->