Skip the STI condition when evaluating a default scope

Given a default_scope on a parent of the current class, where that
parent is not the base class, the parent's STI condition would become
attached to the evaluated default scope, and then override the child's
own STI condition.

Instead, we can treat the STI condition as though it is a default scope,
and skip it in this situation: the scope will be merged into the base
relation, which already contains the correct STI condition.

Fixes #22426.

activerecord/lib/active_record/core.rb

@@ -275,7 +275,7 @@ def cached_find_by_statement(key, &block) # :nodoc:
       def relation # :nodoc:
         relation = Relation.create(self, arel_table, predicate_builder)
 
-        if finder_needs_type_condition?
+        if finder_needs_type_condition? && !ignore_default_scope?
           relation.where(type_condition).create_with(inheritance_column.to_sym => sti_name)
         else
           relation

activerecord/lib/active_record/scoping/default.rb

@@ -122,11 +122,11 @@ def build_default_scope(base_rel = nil) # :nodoc:
         end
 
         def ignore_default_scope? # :nodoc:
-          ScopeRegistry.value_for(:ignore_default_scope, self)
+          ScopeRegistry.value_for(:ignore_default_scope, base_class)
         end
 
         def ignore_default_scope=(ignore) # :nodoc:
-          ScopeRegistry.set_value_for(:ignore_default_scope, self, ignore)
+          ScopeRegistry.set_value_for(:ignore_default_scope, base_class, ignore)
         end
 
         # The ignore_default_scope flag is used to prevent an infinite recursion

activerecord/test/cases/scoping/default_scoping_test.rb

@@ -459,4 +459,18 @@ def test_with_abstract_class_where_clause_should_not_be_duplicated
     scope = Bus.all
     assert_equal scope.where_clause.ast.children.length, 1
   end
+
+  def test_sti_conditions_are_not_carried_in_default_scope
+    ConditionalStiPost.create! body: ''
+    SubConditionalStiPost.create! body: ''
+    SubConditionalStiPost.create! title: 'Hello world', body: ''
+
+    assert_equal 2, ConditionalStiPost.count
+    assert_equal 2, ConditionalStiPost.all.to_a.size
+    assert_equal 3, ConditionalStiPost.unscope(where: :title).to_a.size
+
+    assert_equal 1, SubConditionalStiPost.count
+    assert_equal 1, SubConditionalStiPost.all.to_a.size
+    assert_equal 2, SubConditionalStiPost.unscope(where: :title).to_a.size
+  end
 end

activerecord/test/models/post.rb

@@ -263,3 +263,10 @@ class PostWithCommentWithDefaultScopeReferencesAssociation < ActiveRecord::Base
 class SerializedPost < ActiveRecord::Base
   serialize :title
 end
+
+class ConditionalStiPost < Post
+  default_scope { where(title: 'Untitled') }
+end
+
+class SubConditionalStiPost < ConditionalStiPost
+end