class RuboCop::Cop::Layout::SpaceAfterMethodName

def on_def(node)
  args = node.arguments
  return unless args.loc.begin && args.loc.begin.is?('(')
  expr = args.source_range
  pos_before_left_paren = range_between(expr.begin_pos - 1,
                                        expr.begin_pos)
  return unless pos_before_left_paren.source =~ /\s/
  add_offense(pos_before_left_paren, pos_before_left_paren)
end