[Fix #7434] Fix an incorrect autocorrect for Style/MultilineWhenThen

[koic]

Fixes #7434.

This PR fixes an incorrect autocorrect for Style/MultilineWhenThen when the body of when branch starts with then.

The following is a reproduction procedure.

% cat example.rb
# frozen_string_literal: true

case foo
when bar
  then do_something
end

% ruby -c example.rb
Syntax OK

Before

% rubocop example.rb
Inspecting 1 file
C

Offenses:

example.rb:5:3: C: Style/MultilineWhenThen: Do not use then for
multiline when statement.
  then do_something
  ^^^^

It is changed to the code with syntax error as follows.

# frozen_string_literal: true

case foo
when bar do_something
end

% ruby -c example.rb
example.rb:4: syntax error, unexpected tIDENTIFIER, expecting do or '{'
or '('
when bar do_something

After

% rubocop example.rb -a
Inspecting 1 file
C

Offenses:

example.rb:5:1: C: [Corrected] Layout/IndentationWidth: Use 2 (not 1)
spaces for indentation.
 do_something
^
example.rb:5:3: C: [Corrected] Style/MultilineWhenThen: Do not use then
for multiline when statement.
  then do_something
  ^^^^

1 file inspected, 2 offenses detected, 2 offenses corrected

It will be changed to valid Ruby code as follows.

# frozen_string_literal: true

case foo
when bar
  do_something
end

There is a slight difference in indentation using only this cop. That indentation is corrected using Layout/IndentationWidth cop.

 case foo
 when bar
- do_something
+  do_something
 end

---

Before submitting the PR make sure the following are checked:

• Wrote good commit messages.
• Commit message starts with [Fix #issue-number] (if the related issue exists).
• Feature branch is up-to-date with master (if not - rebase it).
• Squashed related commits together.
• Added tests.
• Added an entry to the Changelog if the new code introduces user-observable changes. See changelog entry format.
• The PR relates to only one subject with a clear title and description in grammatically correct, complete sentences.
• Run bundle exec rake default. It executes all tests and RuboCop for itself, and generates the documentation.

[koic]

📝 There is a slight difference in indentation using only this cop. That indentation is corrected using Layout/IndentationWidth cop.

[bbatsov]

🚀