[Fix rubocop#4886] Fix false offense for Style/CommentedKeyword

Commit also contains cop improvements:
Instead of going through each line of the processed_source we can easily take comments only and cop only those lines, where comments are located.
Instead of splitting line with `#` symbol we take column values from `comment.location`.

CHANGELOG.md

@@ -8,6 +8,7 @@
 * [#4883](https://github.com/bbatsov/rubocop/pull/4883): Fix auto-correction for `Performance/HashEachMethods`. ([@pocke][])
 * [#4888](https://github.com/bbatsov/rubocop/pull/4888): Improve offense message of `Style/StderPuts`. ([@jaredbeck][])
 * [#4896](https://github.com/bbatsov/rubocop/pull/4896): Fix Style/DateTime wrongly triggered on classes `...::DateTime`. ([@dpostorivo][])
+* [#4886](https://github.com/bbatsov/rubocop/issues/4886): Fix false offense for Style/CommentedKeyword. ([@michniewicz][])
 
 ## 0.51.0 (2017-10-18)
 
@@ -2983,3 +2984,4 @@
 [@nelsonjr]: https://github.com/nelsonjr
 [@jonatas]: https://github.com/jonatas
 [@jaredbeck]: https://www.jaredbeck.com
+[@michniewicz]: https://github.com/michniewicz

lib/rubocop/cop/style/commented_keyword.rb

@@ -39,13 +39,15 @@ class CommentedKeyword < Cop
         def investigate(processed_source)
           heredoc_lines = extract_heredoc_lines(processed_source.ast)
 
-          processed_source.lines.each_with_index do |line, index|
-            next if heredoc_lines.any? { |r| r.include?(index + 1) }
+          processed_source.comments.each do |comment|
+            location = comment.location
+            line_position = location.line
+            line = processed_source.lines[line_position - 1]
+            next if heredoc_lines.any? { |r| r.include?(line_position) }
             next unless offensive?(line)
-
             range = source_range(processed_source.buffer,
-                                 index + 1,
-                                 (line.index('#'))...(line.length))
+                                 line_position,
+                                 (location.column)...(location.last_column))
 
             add_offense(range, location: range)
           end
@@ -57,7 +59,8 @@ def investigate(processed_source)
         ALLOWED_COMMENTS = %w[:nodoc: rubocop:disable].freeze
 
         def offensive?(line)
-          KEYWORDS.any? { |k| line =~ /^\s*#{k}\s+.*#/ } &&
+          line = line.lstrip
+          line.start_with?(*KEYWORDS) &&
             ALLOWED_COMMENTS.none? { |c| line =~ /#\s*#{c}/ }
         end
 

spec/rubocop/cop/style/commented_keyword_spec.rb

@@ -141,4 +141,17 @@ def x # rubocop:disable Metrics/MethodLength
       end
     RUBY
   end
+
+  it 'does not register an offense if AST contains # symbol' do
+    expect_no_offenses(<<-RUBY.strip_indent)
+      def x(y = "#value")
+        y
+      end
+    RUBY
+    expect_no_offenses(<<-RUBY.strip_indent)
+      def x(y: "#value")
+        y
+      end
+    RUBY
+  end
 end