Issue #3491304: Clone field does not work

Closes #3491304

changed title from created mr of the patch to Issue #3491304: Clone field does not work

$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);

requested changes

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.