Improve error message when passing a non-list

Author: sabiwara

lib/elixir/lib/inspect.ex

@@ -235,6 +235,13 @@ defprotocol Inspect do
   end
 
   defp validate_option(option, option_list, fields, module) do
+    if not is_list(option_list) do
+      raise ArgumentError,
+            "invalid value #{Kernel.inspect(option_list)} in #{Kernel.inspect(option)} " <>
+              "when deriving the Inspect protocol for #{Kernel.inspect(module)} " <>
+              "(expected a list)"
+    end
+
     case option_list -- fields do
       [] ->
         :ok

lib/elixir/test/elixir/inspect_test.exs

@@ -715,6 +715,17 @@ defmodule Inspect.MapTest do
                  end
   end
 
+  test "passing a non-list to the :only option" do
+    assert_raise ArgumentError,
+                 "invalid value :not_a_list in :only when deriving the Inspect protocol for Inspect.MapTest.StructInvalidListInOnlyOption (expected a list)",
+                 fn ->
+                   defmodule StructInvalidListInOnlyOption do
+                     @derive {Inspect, only: :not_a_list}
+                     defstruct [:a, :b]
+                   end
+                 end
+  end
+
   defmodule StructWithExceptOption do
     @derive {Inspect, except: [:b, :c]}
     defstruct [:a, :b, :c, :d]