Add test on protocols

lib/elixir/test/elixir/protocol_test.exs

@@ -87,6 +87,29 @@ defmodule ProtocolTest do
     end
   end
 
+  test "built-in protocols" do
+    builtin_protocols =
+      for app <- ~w[eex elixir ex_unit iex logger mix]a,
+          Application.ensure_loaded(app),
+          module <- Application.spec(app, :modules),
+          Code.ensure_loaded(module),
+          function_exported?(module, :__protocol__, 1),
+          do: module
+
+    # If this test fails, update:
+    # * lib/elixir/lib/module/types/apply.ex
+    # * lib/elixir/scripts/elixir_docs.ex
+    assert Enum.sort(builtin_protocols) == [
+             Collectable,
+             Enumerable,
+             IEx.Info,
+             Inspect,
+             JSON.Encoder,
+             List.Chars,
+             String.Chars
+           ]
+  end
+
   test "protocol implementations without any" do
     assert is_nil(Sample.impl_for(:foo))
     assert is_nil(Sample.impl_for(fn x -> x end))