Add AllowIfModifier option to Style/IfInsideElse cop

This PR adds `AllowIfModifier` option to `Style/IfInsideElse` cop.

This new option is based on rails/rails repo use case shown at the following URL.
rails/rails#36495.

This option works as follows.

### AllowIfModifier: false (default)

`AllowIfModifier` is false by default. So breaking change will not happen.

```ruby
# bad
if condition_a
  action_a
else
  action_b if condition_b
end
```

### AllowIfModifier: true

It allows the following case when `AllowIfModifier` is true.

```ruby
# good
if condition_a
  action_a
else
  action_b if condition_b
end
```

CHANGELOG.md

@@ -2,19 +2,23 @@
 
 ## master (unreleased)
 
+### New features
+
+* [#7150](https://github.com/rubocop-hq/rubocop/pull/7150): Add `AllowIfModifier` option to `Style/IfInsideElse` cop. ([@koic][])
+
 ### Bug fixes
 
 * [#7063](https://github.com/rubocop-hq/rubocop/issues/7063): Fix autocorrect in `Style/TernaryParentheses` cop. ([@parkerfinch][])
 * [#7106](https://github.com/rubocop-hq/rubocop/issues/7106): Fix an error for `Lint/NumberConversion` when `#to_i` called on a variable on a hash. ([@koic][])
 * [#7107](https://github.com/rubocop-hq/rubocop/issues/7107): Fix parentheses offence for numeric arguments with an operator in `Style/MethodCallWithArgsParentheses`. ([@gsamokovarov][])
 * [#7119](https://github.com/rubocop-hq/rubocop/pull/7119): Fix cache with non UTF-8 offense message. ([@pocke][])
 * [#7118](https://github.com/rubocop-hq/rubocop/pull/7118): Fix `Style/WordArray` with `encoding: binary` magic comment and non-ASCII string. ([@pocke][])
-* [#7113](https://github.com/rubocop-hq/rubocop/pull/7113): This PR renames `EnforcedStyle: rails` to `EnabledStyle: outdented_access_modifiers` for `Layout/IndentationConsistency`. ([@koic][])
 
 ### Changes
 
 * [#5976](https://github.com/rubocop-hq/rubocop/issues/5976): Remove Rails cops. ([@koic][])
 * [#5976](https://github.com/rubocop-hq/rubocop/issues/5976): Remove `rubocop -R/--rails` option. ([@koic][])
+* [#7113](https://github.com/rubocop-hq/rubocop/pull/7113): Rename `EnforcedStyle: rails` to `EnabledStyle: outdented_access_modifiers` for `Layout/IndentationConsistency`. ([@koic][])
 
 ## 0.71.0 (2019-05-30)
 

config/default.yml

@@ -2751,6 +2751,7 @@ Style/IdenticalConditionalBranches:
 Style/IfInsideElse:
   Description: 'Finds if nodes inside else, which can be converted to elsif.'
   Enabled: true
+  AllowIfModifier: false
   VersionAdded: '0.36'
 
 Style/IfUnlessModifier:

lib/rubocop/cop/style/if_inside_else.rb

@@ -27,6 +27,37 @@ module Style
       #   else
       #     action_c
       #   end
+      #
+      # @example AllowIfModifier: false (default)
+      #   # bad
+      #   if condition_a
+      #     action_a
+      #   else
+      #     action_b if condition_b
+      #   end
+      #
+      #   # good
+      #   if condition_a
+      #     action_a
+      #   elsif condition_b
+      #     action_b
+      #   end
+      #
+      # @example AllowIfModifier: true
+      #   # good
+      #   if condition_a
+      #     action_a
+      #   else
+      #     action_b if condition_b
+      #   end
+      #
+      #   # good
+      #   if condition_a
+      #     action_a
+      #   elsif condition_b
+      #     action_b
+      #   end
+      #
       class IfInsideElse < Cop
         MSG = 'Convert `if` nested inside `else` to `elsif`.'
 
@@ -36,9 +67,20 @@ def on_if(node)
           else_branch = node.else_branch
 
           return unless else_branch&.if_type? && else_branch&.if?
+          return if allow_if_modifier_in_else_branch?(else_branch)
 
           add_offense(else_branch, location: :keyword)
         end
+
+        private
+
+        def allow_if_modifier_in_else_branch?(else_branch)
+          allow_if_modifier? && else_branch&.modifier_form?
+        end
+
+        def allow_if_modifier?
+          cop_config['AllowIfModifier']
+        end
       end
     end
   end

manual/cops_style.md

@@ -2455,6 +2455,46 @@ else
   action_c
 end
 ```
+#### AllowIfModifier: false (default)
+
+```ruby
+# bad
+if condition_a
+  action_a
+else
+  action_b if condition_b
+end
+
+# good
+if condition_a
+  action_a
+elsif condition_b
+  action_b
+end
+```
+#### AllowIfModifier: true
+
+```ruby
+# good
+if condition_a
+  action_a
+else
+  action_b if condition_b
+end
+
+# good
+if condition_a
+  action_a
+elsif condition_b
+  action_b
+end
+```
+
+### Configurable attributes
+
+Name | Default value | Configurable values
+--- | --- | ---
+AllowIfModifier | `false` | Boolean
 
 ## Style/IfUnlessModifier
 

spec/rubocop/cop/style/if_inside_else_spec.rb

@@ -1,7 +1,11 @@
 # frozen_string_literal: true
 
-RSpec.describe RuboCop::Cop::Style::IfInsideElse do
-  subject(:cop) { described_class.new }
+RSpec.describe RuboCop::Cop::Style::IfInsideElse, :config do
+  subject(:cop) { described_class.new(config) }
+
+  let(:cop_config) do
+    { 'AllowIfModifier' => false }
+  end
 
   it 'catches an if node nested inside an else' do
     expect_offense(<<~RUBY)
@@ -31,15 +35,33 @@
     RUBY
   end
 
-  it 'catches a modifier if nested inside an else' do
-    expect_offense(<<~RUBY)
-      if a
-        blah
-      else
-        foo if b
-            ^^ Convert `if` nested inside `else` to `elsif`.
-      end
-    RUBY
+  context 'when AllowIfModifier is false' do
+    it 'catches a modifier if nested inside an else' do
+      expect_offense(<<~RUBY)
+        if a
+          blah
+        else
+          foo if b
+              ^^ Convert `if` nested inside `else` to `elsif`.
+        end
+      RUBY
+    end
+  end
+
+  context 'when AllowIfModifier is true' do
+    let(:cop_config) do
+      { 'AllowIfModifier' => true }
+    end
+
+    it 'accepts a modifier if nested inside an else' do
+      expect_no_offenses(<<~RUBY)
+        if a
+          blah
+        else
+          foo if b
+        end
+      RUBY
+    end
   end
 
   it "isn't offended if there is a statement following the if node" do