class RuboCop::Cop::Style::MultilineWhenThen


end
arg2)
when bar then do_something(arg1,
case foo
# good
end
when bar then do_something
case foo
# good
end
when bar
case foo
# good
end
when bar then
case foo
# bad
@example
in multi-line when statements.
Checks uses of the ‘then` keyword

def on_when(node) 

def on_when(node)
  return if !node.then? || require_then?(node)
  range = node.loc.begin
  add_offense(range) do |corrector|
    corrector.remove(range_with_surrounding_space(range, side: :left, newlines: false))
  end
end

def require_then?(when_node)

Requires `then` for write `when` and its body on the same line. 
def require_then?(when_node)
  unless when_node.conditions.first.first_line == when_node.conditions.last.last_line
    return true
  end
  return false unless when_node.body
  same_line?(when_node, when_node.body)
end