Can't confirm when using recursive = true in merge():

defined('_JEXEC') or die;
use Joomla\Registry\Registry;

$obj1 = new stdClass;
$obj1->a = 'a';
$obj1->b = 'b';
$obj1->c = 'c';

$arr1 = ['d', 'e', 'f'];

$registry1 = new Registry;
$registry2 = new Registry;

$registry1->set('val1', 1);
$registry1->set('val2', 2);
$registry1->set('obj1', $obj1);
$registry1->set('arr1', $arr1);
$registry2->set('val3', 3);

$registry2->merge($registry1, true);

echo '  <pre>' . print_r($registry2, true) . '</pre>';exit;

Output:

Joomla\Registry\Registry Object
(
    [data:protected] => stdClass Object
        (
            [val3] => 3
            [val1] => 1
            [val2] => 2
            [obj1] => stdClass Object
                (
                    [a] => a
                    [b] => b
                    [c] => c
                )

            [arr1] => Array
                (
                    [0] => d
                    [1] => e
                    [2] => f
                )

        )

    [initialized:protected] => 1
    [separator] => .
)

@horvathcsabazoltan Can you check and report back if it works for you when doing as advised in the previous comment? Thanks in advance.

It's almost perfect. But all classes are converted to stdClass.

defined('_JEXEC') or die;
use Joomla\Registry\Registry;

class FooBar {
    public $property1 = 'property-1-value';
    public $property2 = 'property-2-value';
}

$registry = new Registry();
$registry->set('associative-array-item', ['key1' => 'value1', 'key2' => 'value2']);
$registry->set('object-item', (object)['property1' => 'value1', 'property2' => 'value2']);
$registry->set('class-item', new FooBar);

echo '<pre>Before merge<br>';
print_r($registry);

$merge1 = new Registry(['merge' => '$recursive = false']);
$merge1->merge($registry);
$merge2 = new Registry(['merge' => '$recursive = true']);
$merge2->merge($registry, true);

echo 'Merge results:<br>';
print_r($merge1);
print_r($merge2);

echo '</pre>';

Output:

Before merge
Joomla\Registry\Registry Object
(
    [data:protected] => stdClass Object
        (
            [associative-array-item] => Array
                (
                    [key1] => value1
                    [key2] => value2
                )

            [object-item] => stdClass Object
                (
                    [property1] => value1
                    [property2] => value2
                )

            [class-item] => FooBar Object
                (
                    [property1] => property-1-value
                    [property2] => property-2-value
                )

        )

    [initialized:protected] => 
    [separator] => .
)
Merge results:
Joomla\Registry\Registry Object
(
    [data:protected] => stdClass Object
        (
            [merge] => $recursive = false
            [associative-array-item] => Array
                (
                    [key1] => value1
                    [key2] => value2
                )

            [object-item] => Array
                (
                    [property1] => value1
                    [property2] => value2
                )

            [class-item] => Array
                (
                    [property1] => property-1-value
                    [property2] => property-2-value
                )

        )

    [initialized:protected] => 1
    [separator] => .
)
Joomla\Registry\Registry Object
(
    [data:protected] => stdClass Object
        (
            [merge] => $recursive = true
            [associative-array-item] => stdClass Object
                (
                    [key1] => value1
                    [key2] => value2
                )

            [object-item] => stdClass Object
                (
                    [property1] => value1
                    [property2] => value2
                )

            [class-item] => stdClass Object
                (
                    [property1] => property-1-value
                    [property2] => property-2-value
                )

        )

    [initialized:protected] => 1
    [separator] => .
)

Another strange thing: associative arrays are converted to stdClass if $recursive = true

defined('_JEXEC') or die;
use Joomla\Registry\Registry;

class FooBar {
    public $property1 = 'property-1-value';
    public $property2 = 'property-2-value';
    public $arrayprop = ['key1' => 'value1', 'key2' => 'value2'];
}

$registry = new Registry();
$registry->set('associative-array-item', ['key1' => 'value1', 'key2' => 'value2']);
$registry->set('class-item', new FooBar);

echo '<pre>Before merge<br>';
print_r($registry);

$merge2 = new Registry(['merge' => '$recursive = true']);
$merge2->merge($registry, true);

echo 'Merge results:<br>';
print_r($merge2);
echo '</pre>';

Output:

Before merge
Joomla\Registry\Registry Object
(
    [data:protected] => stdClass Object
        (
            [associative-array-item] => Array
                (
                    [key1] => value1
                    [key2] => value2
                )

            [class-item] => FooBar Object
                (
                    [property1] => property-1-value
                    [property2] => property-2-value
                    [arrayprop] => Array
                        (
                            [key1] => value1
                            [key2] => value2
                        )

                )

        )

    [initialized:protected] => 
    [separator] => .
)
Merge results:
Joomla\Registry\Registry Object
(
    [data:protected] => stdClass Object
        (
            [merge] => $recursive = true
            [associative-array-item] => stdClass Object
                (
                    [key1] => value1
                    [key2] => value2
                )

            [class-item] => stdClass Object
                (
                    [property1] => property-1-value
                    [property2] => property-2-value
                    [arrayprop] => stdClass Object
                        (
                            [key1] => value1
                            [key2] => value2
                        )

                )

        )

    [initialized:protected] => 1
    [separator] => .
)

All I can say is: "That is by design." if I inspect the Registry class and and merge(), bindData() .

I think the right place for the issue/discussion is https://github.com/joomla-framework/registry

Joomla 3 uses version 1.6 at the moment https://github.com/joomla-framework/registry/blob/1.6.3/src/Registry.php . Joomla 4 the current 2 beta https://github.com/joomla-framework/registry/blob/2.0.0-beta/src/Registry.php . But there I don't see a change concerning this issue here.

@ReLater: Thank you, I submitted a new issue where you said: joomla-framework/registry#57

closing here then