Ensure we don't look up inexisting variables for default args

Author: José Valim

lib/elixir/src/elixir_def.erl

@@ -176,7 +176,7 @@ translate_definition(Kind, Line, Module, Name, Args, Guards, Body, E) when is_in
   Arity = length(Args),
 
   {EArgs, EGuards, EBody, _} = elixir_exp_clauses:def(fun elixir_def_defaults:expand/2,
-                                   Args, Guards, expr_from_body(Line, Body), E),
+                                Args, Guards, expr_from_body(Line, Body), E),
 
   Body == nil andalso check_args_for_bodyless_clause(Line, EArgs, E),
 

lib/elixir/src/elixir_def_defaults.erl

@@ -4,10 +4,11 @@
 -include("elixir.hrl").
 
 expand(Args, E) ->
+  NoContext = E#{context := nil},
   lists:mapfoldl(fun
     ({'\\\\', Meta, [Left, Right]}, Acc) ->
       {ELeft, EL} = elixir_exp:expand(Left, Acc),
-      {ERight, _} = elixir_exp:expand(Right, Acc#{context := nil}),
+      {ERight, _} = elixir_exp:expand(Right, NoContext),
       {{'\\\\', Meta, [ELeft, ERight]}, EL};
     (Left, Acc) ->
       elixir_exp:expand(Left, Acc)

lib/elixir/test/elixir/kernel/errors_test.exs

@@ -150,6 +150,15 @@ defmodule Kernel.ErrorsTest do
         def hello(arg), do: arg
       end
       '''
+
+    assert_compile_fail CompileError,
+      "nofile:2: function foo/0 undefined",
+      ~C'''
+      defmodule ErrorsTest do
+        def hello(foo, bar \\ foo)
+        def hello(foo, bar), do: foo + bar
+      end
+      '''
   end
 
   test :invalid_match_pattern do