Resolve variable references in mixin defaults to other args first, then globals.

lib/sass/engine.rb

@@ -396,7 +396,7 @@ END
         default_arg_found ||= default
         raise SyntaxError.new("Invalid variable \"#{arg}\".", @line) unless arg =~ Script::VALIDATE
         raise SyntaxError.new("Required arguments must not follow optional arguments \"#{arg}\".", @line) if default_arg_found && !default
-        default = Script.parse(default, @line).perform(@environment) if default
+        default = Script.parse(default, @line) if default
         { :name => arg[1..-1], :default_value => default }
       end
       mixin = @mixins[name] = Mixin.new(name, args, [])

lib/sass/tree/mixin_node.rb

@@ -15,8 +15,8 @@ module Sass::Tree
       perform_children(@mixin.args.zip(@args).inject(environment.dup) do |env, (arg, value)|
           env[arg[:name]] = if value
                               value.perform(environment)
-                            else
-                              arg[:default_value]
+                            elsif arg[:default_value]
+                              arg[:default_value].perform(env)
                             end
           raise Sass::SyntaxError.new("Mixin #{@mixin.name} is missing parameter !#{arg[:name]}.") unless env[arg[:name]]
           env

test/sass/engine_test.rb

@@ -387,12 +387,12 @@ SASS
 one {
   color: #ffffff;
   padding: 1px;
-  margin: 8px; }
+  margin: 4px; }
 
 two {
   color: #ffffff;
   padding: 2px;
-  margin: 8px; }
+  margin: 5px; }
 
 three {
   color: #ffffff;
@@ -400,7 +400,7 @@ three {
   margin: 3px; }
 CSS
 !a = 5px
-=foo(!a, !b = 1px, !c = 3px + !a)
+=foo(!a, !b = 1px, !c = 3px + !b)
   :color= !a
   :padding= !b
   :margin= !c