[Fix #12236] Fix an error for Lint/ShadowedArgument

Fixes #12236.

This PR fixes an error for `Lint/ShadowedArgument`
when self assigning to a block argument in `for`.

changelog/fix_an_error_for_lint_shadowed_argument.md

@@ -0,0 +1 @@
+* [#12236](https://github.com/rubocop/rubocop/issues/12236): Fix an error for `Lint/ShadowedArgument` when self assigning to a block argument in `for`. ([@koic][])

lib/rubocop/cop/lint/shadowed_argument.rb

@@ -123,6 +123,7 @@ def assignment_without_argument_usage(argument)
 
             # Shorthand assignments always use their arguments
             next false if assignment_node.shorthand_asgn?
+            next false unless assignment_node.parent
 
             node_within_block_or_conditional =
               node_within_block_or_conditional?(assignment_node.parent, argument.scope.node)

spec/rubocop/cop/lint/shadowed_argument_spec.rb

@@ -97,6 +97,16 @@ def do_something(*items)
         end
       end
 
+      context 'when self assigning to a block argument in `for`' do
+        it 'does not register an offense' do
+          expect_no_offenses(<<~RUBY)
+            for item in items
+              do_something { |arg| arg = arg }
+            end
+          RUBY
+        end
+      end
+
       context 'when binding is used' do
         it 'registers an offense' do
           expect_offense(<<~RUBY)