class Sass::Script::Parser

def defn_arglist!(must_have_default)
  return [] unless try_tok(:lparen)
  return [] if try_tok(:rparen)
  res = []
  loop do
    line = @lexer.line
    offset = @lexer.offset + 1
    c = assert_tok(:const)
    var = Script::Variable.new(c.value)
    if tok = (try_tok(:colon) || try_tok(:single_eq))
      val = assert_expr(:concat)
      if tok.type == :single_eq
        val.context = :equals
        val.options = @options
        Script.equals_warning("mixin argument defaults", "$#{c.value}",
          val.to_sass, false, line, offset, @options[:filename])
      end
      must_have_default = true
    elsif must_have_default
      raise SyntaxError.new("Required argument #{var.inspect} must come before any optional arguments.")
    end
    res << [var, val]
    break unless try_tok(:comma)
  end
  assert_tok(:rparen)
  res
end