Add a small note to UndefinedFunctionError when the module is not available

Author: José Valim

lib/elixir/lib/exception.ex

@@ -615,10 +615,15 @@ end
 defmodule UndefinedFunctionError do
   defexception [module: nil, function: nil, arity: nil]
 
-  def message(exception) do
-    if exception.function do
-      formatted = Exception.format_mfa exception.module, exception.function, exception.arity
-      "undefined function: #{formatted}"
+  def message(%{function: function, module: module, arity: arity}) do
+    if function do
+      formatted = Exception.format_mfa module, function, arity
+      suffix = if nil?(module) || :code.is_loaded(module) do
+        ""
+      else
+        " (module #{inspect module} is not available)"
+      end
+      "undefined function: #{formatted}" <> suffix
     else
       "undefined function"
     end

lib/elixir/test/elixir/exception_test.exs

@@ -308,8 +308,10 @@ defmodule Kernel.ExceptionTest do
 
   test "undefined function message" do
     assert %UndefinedFunctionError{} |> message == "undefined function"
+    assert %UndefinedFunctionError{module: Kernel, function: :bar, arity: 1} |> message ==
+           "undefined function: Kernel.bar/1"
     assert %UndefinedFunctionError{module: Foo, function: :bar, arity: 1} |> message ==
-           "undefined function: Foo.bar/1"
+           "undefined function: Foo.bar/1 (module Foo is not available)"
     assert %UndefinedFunctionError{module: nil, function: :bar, arity: 0} |> message ==
            "undefined function: nil.bar/0"
   end