[Fix #10581] Fix a false positive for Style/FetchEnvVar

Fixes #10581.

This PR fixes a false positive for `Style/FetchEnvVar` when comparing with `ENV['TERM']`.
`ENV['DISABLE_DOTENV'] != '1'` could be treated the same as the already allowed in `used_as_flag?`
(that is, used as a flag).

changelog/fix_a_false_positive_for_style_fetch_env_var_cop.md

@@ -0,0 +1 @@
+* [#10581](https://github.com/rubocop/rubocop/issues/10581): Fix a false positive for `Style/FetchEnvVar` when comparing with `ENV['TERM']`. ([@koic][])

lib/rubocop/cop/style/fetch_env_var.rb

@@ -53,8 +53,9 @@ def allowed_var?(expression)
 
         def used_as_flag?(node)
           return false if node.root?
+          return true if node.parent.if_type?
 
-          node.parent.if_type? || (node.parent.send_type? && node.parent.prefix_bang?)
+          node.parent.send_type? && (node.parent.prefix_bang? || node.parent.comparison_method?)
         end
 
         # Check if the node is a receiver and receives a message with dot syntax.

spec/rubocop/cop/style/fetch_env_var_spec.rb

@@ -49,15 +49,18 @@
     end
   end
 
-  context 'when it is compared with other object' do
-    it 'registers an offense' do
-      expect_offense(<<~RUBY)
+  context 'when it is compared `==` with other object' do
+    it 'does not register an offense' do
+      expect_no_offenses(<<~RUBY)
         ENV['X'] == 1
-        ^^^^^^^^ Use `ENV.fetch('X')` or `ENV.fetch('X', nil)` instead of `ENV['X']`.
       RUBY
+    end
+  end
 
-      expect_correction(<<~RUBY)
-        ENV.fetch('X', nil) == 1
+  context 'when it is compared `!=` with other object' do
+    it 'does not register an offense' do
+      expect_no_offenses(<<~RUBY)
+        ENV['X'] != 1
       RUBY
     end
   end