[Fix #9128] Fix an incorrect auto-correct for `Style/ClassAndModuleCh…

…ildren`

Fixes #9128.

This PR fixes an incorrect auto-correct for `Style/ClassAndModuleChildren`
when namespace is defined as a class in the same file.

It is still undetectable when class definition exists in the different file,
but it can be detected in the same file. This PR solves the latter.

changelog/fix_incorrect_autocorrect_for_class_and_module_children.md

@@ -0,0 +1 @@
+* [#9128](https://github.com/rubocop-hq/rubocop/issues/9128): Fix an incorrect auto-correct for `Style/ClassAndModuleChildren` when namespace is defined as a class in the same file. ([@koic][])

lib/rubocop/cop/style/class_and_module_children.rb

@@ -55,13 +55,18 @@ def nest_definition(corrector, node)
           padding = ((' ' * indent_width) + leading_spaces(node)).to_s
           padding_for_trailing_end = padding.sub(' ' * node.loc.end.column, '')
 
-          replace_keyword_with_module(corrector, node)
+          replace_namespace_keyword(corrector, node)
           split_on_double_colon(corrector, node, padding)
           add_trailing_end(corrector, node, padding_for_trailing_end)
         end
 
-        def replace_keyword_with_module(corrector, node)
-          corrector.replace(node.loc.keyword, 'module')
+        def replace_namespace_keyword(corrector, node)
+          class_definition = node.left_sibling&.each_node(:class)&.find do |class_node|
+            class_node.identifier == node.identifier.namespace
+          end
+          namespace_keyword = class_definition ? 'class' : 'module'
+
+          corrector.replace(node.loc.keyword, namespace_keyword)
         end
 
         def split_on_double_colon(corrector, node, padding)

spec/rubocop/cop/style/class_and_module_children_spec.rb

@@ -18,6 +18,48 @@ class BarClass
       RUBY
     end
 
+    it 'registers an offense for not nested classes when namespace is defined as a class' do
+      expect_offense(<<~RUBY)
+        class FooClass
+        end
+
+        class FooClass::BarClass
+              ^^^^^^^^^^^^^^^^^^ Use nested module/class definitions instead of compact style.
+        end
+      RUBY
+
+      expect_correction(<<~RUBY)
+        class FooClass
+        end
+
+        class FooClass
+          class BarClass
+          end
+        end
+      RUBY
+    end
+
+    it 'registers an offense for not nested classes when namespace is defined as a module' do
+      expect_offense(<<~RUBY)
+        module FooClass
+        end
+
+        class FooClass::BarClass
+              ^^^^^^^^^^^^^^^^^^ Use nested module/class definitions instead of compact style.
+        end
+      RUBY
+
+      expect_correction(<<~RUBY)
+        module FooClass
+        end
+
+        module FooClass
+          class BarClass
+          end
+        end
+      RUBY
+    end
+
     it 'registers an offense for not nested classes with explicit superclass' do
       expect_offense(<<~RUBY)
         class FooClass::BarClass < Super