Avoid conflicts between internal vars and empty vars, closes #15246

Author: josevalim

lib/elixir/src/elixir_erl_var.erl

@@ -45,7 +45,7 @@ build(Key, #elixir_erl{counter=Counter} = S) ->
   {build_name(Key, Count),
    S#elixir_erl{counter=Counter#{Key => Count}}}.
 
-build_name('_', Count) -> list_to_atom("_@" ++ integer_to_list(Count));
+build_name('_', Count) -> list_to_atom("_" ++ integer_to_list(Count));
 build_name(Name, Count) -> list_to_atom("_" ++ atom_to_list(Name) ++ "@" ++ integer_to_list(Count)).
 
 %% BINDINGS

lib/elixir/test/elixir/kernel/macros_test.exs

@@ -16,6 +16,17 @@ defmodule Kernel.MacrosTest.Nested do
       require Integer
     end
   end
+
+  defmacro wrap_empty_var(expr) do
+    empty = Macro.var(:"", nil)
+
+    quote do
+      (fn x ->
+         unquote(empty) = List.wrap(x)
+         unquote(empty)
+       end).(unquote(expr))
+    end
+  end
 end
 
 defmodule Kernel.MacrosTest do
@@ -91,4 +102,9 @@ defmodule Kernel.MacrosTest do
               1
             end) == 1
   end
+
+  test "do not collide on empty variable name", config do
+    import Kernel.MacrosTest.Nested
+    assert wrap_empty_var(config) == [config]
+  end
 end