`scoping` should respect current class and STI constraint (#29199)

A relation includes `klass`, so it can not be used as it is if current
class is different from `current_scope.klass`. It should be created new
relation by current class to respect the klass and STI constraint.

Fixes #17603.
Fixes #23576.

activerecord/lib/active_record/scoping/named.rb

@@ -24,8 +24,14 @@ module ClassMethods
         # You can define a scope that applies to all finders using
         # {default_scope}[rdoc-ref:Scoping::Default::ClassMethods#default_scope].
         def all
+          current_scope = self.current_scope
+
           if current_scope
-            current_scope.clone
+            if self == current_scope.klass
+              current_scope.clone
+            else
+              relation.merge!(current_scope)
+            end
           else
             default_scoped
           end

activerecord/test/cases/scoping/relation_scoping_test.rb

@@ -240,6 +240,20 @@ def test_scoping_is_correctly_restored
     assert_nil SpecialComment.current_scope
   end
 
+  def test_scoping_respects_current_class
+    Comment.unscoped do
+      assert_equal "a comment...", Comment.all.what_are_you
+      assert_equal "a special comment...", SpecialComment.all.what_are_you
+    end
+  end
+
+  def test_scoping_respects_sti_constraint
+    Comment.unscoped do
+      assert_equal comments(:greetings), Comment.find(1)
+      assert_raises(ActiveRecord::RecordNotFound) { SpecialComment.find(1) }
+    end
+  end
+
   def test_circular_joins_with_scoping_does_not_crash
     posts = Post.joins(comments: :post).scoping do
       Post.first(10)

activerecord/test/models/comment.rb

@@ -60,6 +60,10 @@ def to_s
 
 class SpecialComment < Comment
   default_scope { where(deleted_at: nil) }
+
+  def self.what_are_you
+    "a special comment..."
+  end
 end
 
 class SubSpecialComment < SpecialComment