[Fix #12832] Fix an error for Style/ArgumentsForwarding

Fixes #12832.

This PR fixes an error for `Style/ArgumentsForwarding`
when using block arg in nested method definitions.

changelog/fix_an_error_for_style_arguments_forwarding.md

@@ -0,0 +1 @@
+* [#12832](https://github.com/rubocop/rubocop/issues/12832): Fix an error for `Style/ArgumentsForwarding` when using block arg in nested method definitions. ([@koic][])

lib/rubocop/cop/style/arguments_forwarding.rb

@@ -312,7 +312,8 @@ def register_forward_kwargs_offense(add_parens, def_arguments_or_send, kwrest_ar
         end
 
         def register_forward_block_arg_offense(add_parens, def_arguments_or_send, block_arg)
-          return if target_ruby_version <= 3.0 || block_arg.source == '&' || explicit_block_name?
+          return if target_ruby_version <= 3.0 ||
+                    block_arg.nil? || block_arg.source == '&' || explicit_block_name?
 
           add_offense(block_arg, message: BLOCK_MSG) do |corrector|
             add_parens_if_missing(def_arguments_or_send, corrector) if add_parens

spec/rubocop/cop/style/arguments_forwarding_spec.rb

@@ -205,6 +205,18 @@ def foo(&)
       RUBY
     end
 
+    it 'does not register an offense when using block arg in nested method definitions', :ruby32 do
+      expect_no_offenses(<<~RUBY)
+        def foo(x)
+          class << x
+            def bar(y, &)
+              baz.qux(&)
+            end
+          end
+        end
+      RUBY
+    end
+
     context 'when `RedundantBlockArgumentNames: [meaningless_block_name]`' do
       let(:redundant_block_argument_names) { ['meaningless_block_name'] }