P.S.: I also think the error message is wrong. According to

https://wiki.php.net/rfc/magic-methods-signature

The return type for __debugInfo is ?array, so null should also be valid

I get similar results whether Xdebug is enabled or not, on PHP 8.0.21, 8.1.9, and 7.4.29.

Without Xdebug:

$ php -n tests/base/bug02100.phpt

Warning: Uncaught Exception: X in /home/derick/dev/php/derickr-xdebug/tests/base/bug02100.phpt:6
Stack trace:
#0 [internal function]: test->__debugInfo()
#1 /home/derick/dev/php/derickr-xdebug/tests/base/bug02100.phpt(12): var_dump(Object(test))
#2 {main}
  thrown in /home/derick/dev/php/derickr-xdebug/tests/base/bug02100.phpt on line 6

Fatal error: __debuginfo() must return an array in /home/derick/dev/php/derickr-xdebug/tests/base/bug02100.phpt on line 12

With Xdebug:

$ php tests/base/bug02100.phpt

Warning: Uncaught Exception: X in /home/derick/dev/php/derickr-xdebug/tests/base/bug02100.phpt:6
Stack trace:
#0 [internal function]: test->__debugInfo()
#1 /home/derick/dev/php/derickr-xdebug/tests/base/bug02100.phpt(12): var_dump(Object(test))
#2 {main}
  thrown in /home/derick/dev/php/derickr-xdebug/tests/base/bug02100.phpt on line 6

Call Stack:
    0.0003     384672   1. {main}() /home/derick/dev/php/derickr-xdebug/tests/base/bug02100.phpt:0
    0.0011     384752   2. var_dump($value = class test {  }) /home/derick/dev/php/derickr-xdebug/tests/base/bug02100.phpt:12

Fatal error: __debuginfo() must return an array in /home/derick/dev/php/derickr-xdebug/tests/base/bug02100.phpt on line 12

Call Stack:
    0.0003     384672   1. {main}() /home/derick/dev/php/derickr-xdebug/tests/base/bug02100.phpt:0
    0.0011     384752   2. var_dump($value = class test {  }) /home/derick/dev/php/derickr-xdebug/tests/base/bug02100.phpt:12

The fatal error comes from PHP itself, so I can't control the output, and I also think it's the right result, as you're not returning an array at all, because you're not returning anything from __debugInfo(), as the Exception short-circuits the return. I don't think there is a bug here.

Hm, strange. On Linux, I also see the behavior you describe, but on macOS (PHP from Homebrew) it behaved as I described in the OP. Here is a modified testcase that only fails when XDebug is enabled:

<?php
class test
{
    public function throw(test $object)
    {
        throw new Exception('X');
    }

    public function __debugInfo()
    {
        throw new Exception('X');
        return [];
    }
}

try {
    $t1 = new test;
    $t1->throw(new test);
} catch (Exception $e) {
    echo 'All is well';
}

This might be related: https://github.com/php/php-src/issues/7922

I can reproduce it with your new case, and I have a "hack" PR that disables using __debugInfo() while an exception is already in flight: https://github.com/xdebug/xdebug/pull/846

https://github.com/xdebug/xdebug/pull/846