Add new Style/IfWithBooleanLiteralBranches cop

[koic]

This PR adds new Style/IfWithBooleanLiteralBranches cop that checks for redundant if with boolean literal branches.

It checks only conditions to return boolean value (true or false) for safe detection.
The conditions to be checked are comparison methods, predicate methods, and double negative.

# bad
if foo == bar
  true
else
  false
end

# bad
foo == bar ? true : false

# good
foo == bar

And this PR auto-corrected the following offense by the new cop.

% bundle exec rake
(snip)

Offenses:

lib/rubocop/cop/style/ternary_parentheses.rb:123:47: C: [Correctable]
Style/IfWithBooleanLiteralBranches: Remove redundant ternary operator
with boolean literal branches.
            non_complex_expression?(condition) ? false : true
                                              ^^^^^^^^^^^^^^^

1246 files inspected, 1 offense detected, 1 offense auto-correctable
RuboCop failed!

---

Before submitting the PR make sure the following are checked:

• The PR relates to only one subject with a clear title and description in grammatically correct, complete sentences.
• 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.
• Ran bundle exec rake default. It executes all tests and runs RuboCop on its own code.
• Added an entry (file) to the changelog folder named {change_type}_{change_description}.md if the new code introduces user-observable changes. See changelog entry format for details.

[marcandre]

That's a great cop imo

[marcandre]

Why do you prefer not simplifying this one?

[koic]

This is because the return value changes from true to "hi" if bar is not a boolean value. Example:

def foo?
  false
end

def bar
  "hi"
end

def baz?
  false
end

foo? || bar || baz? ? true : false #=> true
foo? || bar || baz? #=> "hi"

So this aims for safe detection, which keeps the return value as a boolean value.

[marcandre]

Ah, got it. Thanks @koic

[bbatsov]

Great addition indeed! Technically speaking we can extend this to cover case as well, but I guess it's quite uncommon to write such code.