Do not leak vars between default args

Closes #15463.

Author: josevalim

lib/elixir/src/elixir_def.erl

@@ -310,7 +310,8 @@ unpack_expanded(_Kind, _Meta, _Name, [], _Counter, Acc, Clauses) ->
 
 expand_defaults([{'\\\\', Meta, [Expr, Default]} | Args], S, E, Acc) ->
   {ExpandedDefault, SE, _} = elixir_expand:expand(Default, S, E),
-  expand_defaults(Args, SE, E, [{'\\\\', Meta, [Expr, ExpandedDefault]} | Acc]);
+  expand_defaults(Args, elixir_env:reset_read(SE, S), E,
+                  [{'\\\\', Meta, [Expr, ExpandedDefault]} | Acc]);
 expand_defaults([Arg | Args], S, E, Acc) ->
    expand_defaults(Args, S, E, [Arg | Acc]);
 expand_defaults([], S, _E, Acc) ->

lib/elixir/test/elixir/kernel/defaults_test.exs

@@ -52,6 +52,14 @@ defmodule Kernel.DefaultsTest do
     end)
   end
 
+  test "errors on accessing variable from another default" do
+    assert_compile_error(~r/undefined variable \"x\"/, fn ->
+      defmodule VarDefaultScopeBetweenDefaults do
+        def test(a \\ x = 1, b \\ x), do: {a, b}
+      end
+    end)
+  end
+
   test "errors on multiple defaults" do
     message = ~r"def hello/1 defines defaults multiple times"