Correct Macro.to_string/1 formatting for capture operator

Signed-off-by: José Valim <jose.valim@plataformatec.com.br>

Author: lexmag

lib/elixir/lib/macro.ex

@@ -571,6 +571,10 @@ defmodule Macro do
     fun.(ast, "&" <> to_string(mod, fun) <> "." <> Atom.to_string(name) <> "/" <> to_string(arity, fun))
   end
 
+  def to_string({:&, _, [arg]} = ast, fun) when not is_integer(arg) do
+    fun.(ast, "&(" <> to_string(arg, fun) <> ")")
+  end
+
   # Unary ops
   def to_string({unary, _, [{binary, _, [_, _]} = arg]} = ast, fun)
       when unary in unquote(@unary_ops) and binary in unquote(@binary_ops) do

lib/elixir/lib/string.ex

@@ -8,10 +8,10 @@ defmodule String do
 
   The functions in this module act according to the Unicode
   Standard, version 6.3.0.
-  
+
   As per the standard, a codepoint is a single Unicode Character,
   which may be represented by one or more bytes.
-  
+
   For example, the codepoint "é" is two bytes:
 
       iex> byte_size("é")

lib/elixir/test/elixir/macro_test.exs

@@ -387,6 +387,7 @@ defmodule MacroTest do
     assert Macro.to_string(quote do: &foo/0) == "&foo/0"
     assert Macro.to_string(quote do: &Foo.foo/0) == "&Foo.foo/0"
     assert Macro.to_string(quote do: & &1 + &2) == "&(&1 + &2)"
+    assert Macro.to_string(quote do: & &1) == "&(&1)"
   end
 
   test "containers to string" do
@@ -422,7 +423,6 @@ defmodule MacroTest do
     assert Macro.to_string(quote do: !(foo > bar)) == "!(foo > bar)"
     assert Macro.to_string(quote do: @foo(bar)) == "@foo(bar)"
     assert Macro.to_string(quote do: identity(&1)) == "identity(&1)"
-    assert Macro.to_string(quote do: identity(&foo)) == "identity(&foo)"
   end
 
   test "access to string" do