Issue #3491304: Clone field does not work

Closes #3491304

src/Element/EntityFieldSelect.php

@@ -93,7 +93,8 @@ public static function processEntityFieldSelect(&$element, FormStateInterface $f
       $default_value = array_merge($default_value, $element['#default_value']);
     }
 
-    $values += $default_value;
+    $values = isset($values) && is_array($values) ? $values : [];
+    $values = array_merge($values, $default_value);
     $element['#prefix'] = '<div id="' . EntityFieldSelect::getElementHTMLId($element, $complete_form) . '">';
     $element['#suffix'] = '</div>';
     if ($element['#title_display'] != 'invisible') {

[Alberto Paderno]

$values += $default_value; gives an array that is different from the array returned from array_merge($values, $default_value);. It can be verified with the following code.

$a1 = ["a" => 1, "b" => 2];
$a2 = ["a" => 3, "c" => 4];
$a1 = array_merge($a1, $a2);
print_r($a1);

$a1 = ["a" => 1, "b" => 2];
$a2 = ["a" => 3, "c" => 4];
$a1 += $a2;
print_r($a1);

[Alberto Paderno]

Furthermore, the error reported in the issue is Unsupported operand types: string + array, which means $values is a string. The code used in this merge request would use an empty array instead of a string, which probably would cause issues later.