Prevent duplicating where clauses when model is extended from an abstract class

Fixes #19528

fix for mysql2 test

better test

activerecord/lib/active_record/scoping/default.rb

@@ -100,6 +100,7 @@ module ActiveRecord
         end
 
         def build_default_scope(base_rel = relation) # :nodoc:
+          return if abstract_class?
           if !Base.is_a?(method(:default_scope).owner)
             # The user has defined their own default scope method, so call that
             evaluate_default_scope { default_scope }

activerecord/test/cases/scoping/default_scoping_test.rb

@@ -3,6 +3,7 @@ require 'models/post'
 require 'models/comment'
 require 'models/developer'
 require 'models/computer'
+require 'models/vehicle'
 
 class DefaultScopingTest < ActiveRecord::TestCase
   fixtures :developers, :posts, :comments
@@ -441,4 +442,9 @@ class DefaultScopingTest < ActiveRecord::TestCase
     assert_equal 1, scope.where_clause.ast.children.length
     assert_equal Developer.where(name: "David"), scope
   end
+
+  def test_with_abstract_class_where_clause_should_not_be_duplicated
+    scope = Bus.all
+    assert_equal scope.where_clause.ast.children.length, 1
+  end
 end

activerecord/test/models/vehicle.rb

@@ -0,0 +1,7 @@
+class Vehicle < ActiveRecord::Base
+  self.abstract_class = true
+  default_scope -> { where("tires_count IS NOT NULL") }
+end
+
+class Bus < Vehicle
+end