Description:
------------
Dynamically passing an invalid value to a typehinted function argument which is also passed by reference while a custom exception-throwing error handler is active will cause PHP to hang.

Interesting observations:

- doesn't happen in PHP versions prior to 7.1.0
- removing the namespace prevents the bug from happening
- the error handler must throw an exception for the hang to occur
- the call must be dynamic or a fatal error is produced instead
- the kind of typehint doesn't seem to matter (it happens with array, a class, a scalar typehint etc.)

Test script:
---------------
<?php

namespace Foo;

set_error_handler(function () {
    throw new \Exception();
});

call_user_func(function (array &$ref) {}, 'not_an_array_variable');


Expected result:
----------------
Fatal error: Uncaught Exception in script.php on line 6

Exception:  in script.php on line 6

Call Stack:
    0.0010     412712   1. {main}() script.php:0
    0.0010     413352   2. call_user_func:{script.php:9}() script.php:9
    0.0010     416144   3. Foo\{closure}() script.php:9


Actual result:
--------------
The script doesn't terminate. PHP hangs indefinitely.