class RuboCop::Cop::Style::InPatternThen

def alternative_pattern_source(pattern)
  return pattern.children.map(&:source) unless pattern.children.first.match_alt_type?
  pattern_sources = alternative_pattern_source(pattern.children.first)
  (pattern_sources << pattern.children[1].source).join(' | ')
end

def on_in_pattern(node)
  return if node.multiline? || node.then? || !node.body
  pattern = node.pattern
  pattern_source = if pattern.match_alt_type?
                     alternative_pattern_source(pattern)
                   else
                     pattern.source
                   end
  add_offense(node.loc.begin, message: format(MSG, pattern: pattern_source)) do |corrector|
    corrector.replace(node.loc.begin, ' then')
  end
end