Style/MultilineWhenThen autocorrect generates invalid Ruby for certain case statements

[leklund]

The autocorrect for Style/MultilineWhenThen generates invalid ruby when correcting a case statement with the following format (then on the line after the when):

case foo
when :bar
  then :value

Definitely not a standard way to write a case statement but it is valid Ruby.

---

Full Example code:

def flavor(flave)
  case flave
  when 'chocolate'
    then %w[dark white milk].sample
  when 'fruit'
    then %w[apple banana orange].sample
  when 'null'
    then nil
  end
end

Expected behavior

I would expect autocorrect to just remove the unnecessary then like so:

def flavor(flave)
  case flave
  when 'chocolate'
   %w[dark white milk].sample
  when 'fruit'
   %w[apple banana orange].sample
  when 'null'
   nil
  end
end

Actual behavior

Instead, what we get is invalid ruby:

def flavor(flave)
  case flave
  when 'chocolate' %w[dark white milk].sample
  when 'fruit' %w[apple banana orange].sample
  when 'null' nil
  end
end

Steps to reproduce the problem

The sample code provided above can reproduce the problem.

RuboCop version

0.75.1 (using Parser 2.6.5.0, running on ruby 2.4.2 x86_64-darwin17)