[Fix rubocop#5648] Suggest valid memoized instance variable for predi…

…cate method

CHANGELOG.md

@@ -2,6 +2,10 @@
 
 ## master (unreleased)
 
+### Bug fixes
+
+* [#5648](https://github.com/bbatsov/rubocop/issues/5648): Suggest valid memoized instance variable for predicate method. ([@satyap][])
+
 ## 0.53.0 (2018-03-05)
 
 ### New features

lib/rubocop/cop/naming/memoized_instance_variable_name.rb

@@ -34,7 +34,7 @@ module Naming
       #
       class MemoizedInstanceVariableName < Cop
         MSG = 'Memoized variable `%<var>s` does not match ' \
-          'method name `%<method>s`. Use `@%<method>s` instead.'.freeze
+          'method name `%<method>s`. Use `@%<suggested_var>s` instead.'.freeze
 
         def self.node_pattern
           memo_assign = '(or_asgn $(ivasgn _) _)'
@@ -55,6 +55,7 @@ def on_def(node)
           msg = format(
             MSG,
             var: ivar_assign.children.first.to_s,
+            suggested_var: method_name.to_s.chomp('?'),
             method: method_name
           )
           add_offense(node, location: ivar_assign.source_range, message: msg)

spec/rubocop/cop/naming/memoized_instance_variable_name_spec.rb

@@ -61,6 +61,18 @@ def foo
     end
   end
 
+  context 'memoized variable after other code does not match method name' do
+    it 'registers an offense' do
+      expect_offense(<<-RUBY.strip_indent)
+        def foo?
+          helper_variable = something_we_need_to_calculate_foo
+          @bar ||= calculate_expensive_thing(helper_variable)
+          ^^^^ Memoized variable `@bar` does not match method name `foo?`. Use `@foo` instead.
+        end
+      RUBY
+    end
+  end
+
   context 'memoized variable matches method name' do
     it 'does not register an offense' do
       expect_no_offenses(<<-RUBY.strip_indent)