Force sortBy() to maintain original order of items with identical values (#21214)

Author: damiani

src/Illuminate/Support/Collection.php

@@ -1348,7 +1348,7 @@ public function sortBy($callback, $options = SORT_REGULAR, $descending = false)
         // function which we were given. Then, we will sort the returned values and
         // and grab the corresponding values for the sorted keys from this array.
         foreach ($this->items as $key => $value) {
-            $results[$key] = $callback($value, $key);
+            $results[$key] = [$callback($value, $key), $key];
         }
 
         $descending ? arsort($results, $options)

tests/Support/SupportCollectionTest.php

@@ -817,6 +817,15 @@ public function testSortByAlwaysReturnsAssoc()
         $this->assertEquals([1 => 'dayle', 0 => 'taylor'], $data->all());
     }
 
+    public function testSortByMaintainsOriginalOrderOfItemsWithIdenticalValues()
+    {
+        $data = new Collection([['name' => 'taylor'], ['name' => 'dayle'], ['name' => 'dayle']]);
+        $data = $data->sortBy('name');
+
+        $this->assertEquals([['name' => 'dayle'], ['name' => 'dayle'], ['name' => 'taylor']], array_values($data->all()));
+        $this->assertEquals([1, 2, 0], $data->keys()->all());
+    }
+
     public function testReverse()
     {
         $data = new Collection(['zaeed', 'alan']);