[Fix rubocop#12693] Fix an incorrect autocorrect for Style/ObjectThen

Fixes rubocop#12693.

This PR fixes an incorrect autocorrect for `Style/ObjectThen`
when using `yield_self` without receiver.

changelog/fix_an_incorrect_for_style_object_then.md

@@ -0,0 +1 @@
+* [#12693](https://github.com/rubocop/rubocop/issues/12693): Fix an incorrect autocorrect for `Style/ObjectThen` when using `yield_self` without receiver. ([@koic][])

lib/rubocop/cop/style/object_then.rb

@@ -46,15 +46,17 @@ def on_send(node)
         private
 
         def check_method_node(node)
-          return unless preferred_method(node)
+          return unless preferred_method?(node)
 
           message = message(node)
           add_offense(node.loc.selector, message: message) do |corrector|
-            corrector.replace(node.loc.selector, style.to_s)
+            prefer = style == :then && node.receiver.nil? ? 'self.then' : style
+
+            corrector.replace(node.loc.selector, prefer)
           end
         end
 
-        def preferred_method(node)
+        def preferred_method?(node)
           case style
           when :then
             node.method?(:yield_self)

spec/rubocop/cop/style/object_then_spec.rb

@@ -36,6 +36,17 @@
           obj.then { _1.test }
         RUBY
       end
+
+      it 'registers an offense for `yield_self` without receiver' do
+        expect_offense(<<~RUBY)
+          yield_self { |obj| obj.test }
+          ^^^^^^^^^^ Prefer `then` over `yield_self`.
+        RUBY
+
+        expect_correction(<<~RUBY)
+          self.then { |obj| obj.test }
+        RUBY
+      end
     end
 
     it 'registers an offense for yield_self with proc param' do