[5.8] fix MorphTo Relation ignores parent $timestamp when touching

src/Illuminate/Database/Eloquent/Model.php

@@ -294,6 +294,10 @@ public static function isIgnoringTouch($class = null)
     {
         $class = $class ?: static::class;
 
+        if (! get_class_vars($class)['timestamps'] || ! $class::UPDATED_AT) {
+            return true;
+        }
+
         foreach (static::$ignoreOnTouch as $ignoredClass) {
             if ($class === $ignoredClass || is_subclass_of($class, $ignoredClass)) {
                 return true;

tests/Database/DatabaseEloquentModelTest.php

@@ -1896,6 +1896,27 @@ protected function addMockConnection($model)
             return new BaseBuilder($connection, $grammar, $processor);
         });
     }
+
+    public function testTouchingModelWithTimestamps()
+    {
+        $this->assertFalse(
+            Model::isIgnoringTouch(Model::class)
+        );
+    }
+
+    public function testNotTouchingModelWithUpdatedAtNull()
+    {
+        $this->assertTrue(
+            Model::isIgnoringTouch(EloquentModelWithUpdatedAtNull::class)
+        );
+    }
+
+    public function testNotTouchingModelWithoutTimestamps()
+    {
+        $this->assertTrue(
+            Model::isIgnoringTouch(EloquentModelWithoutTimestamps::class)
+        );
+    }
 }
 
 class EloquentTestObserverStub
@@ -2303,3 +2324,15 @@ class EloquentModelEventObjectStub extends Model
         'saving' => EloquentModelSavingEventStub::class,
     ];
 }
+
+class EloquentModelWithoutTimestamps extends Model
+{
+    protected $table = 'stub';
+    public $timestamps = false;
+}
+
+class EloquentModelWithUpdatedAtNull extends Model
+{
+    protected $table = 'stub';
+    const UPDATED_AT = null;
+}