Restrict support for binary message in the default impl of exception

lib/elixir/lib/kernel.ex

@@ -3162,26 +3162,26 @@ defmodule Kernel do
       @behaviour Exception
       fields = defstruct unquote(fields)
 
-      @spec exception(String.t) :: Exception.t
-      def exception(msg) when is_binary(msg) do
-        exception(message: msg)
-      end
-
-      @spec exception(Keyword.t) :: Exception.t
-      def exception(args) when is_list(args) do
-        Kernel.struct(__struct__, args)
-      end
-
-      defoverridable exception: 1
-
       if Map.has_key?(fields, :message) do
         @spec message(Exception.t) :: String.t
         def message(exception) do
           exception.message
         end
 
         defoverridable message: 1
+
+        @spec exception(String.t) :: Exception.t
+        def exception(msg) when is_binary(msg) do
+          exception(message: msg)
+        end
       end
+
+      @spec exception(Keyword.t) :: Exception.t
+      def exception(args) when is_list(args) do
+        Kernel.struct(__struct__, args)
+      end
+
+      defoverridable exception: 1
     end
   end