Deprecation errors are inconsistently triggered for underscore class aliases #18285

Description

The following code:

<?php

namespace Foo {
    class __ {}
}

namespace Bar {
    class _ {} // Deprecated
}

namespace {
    class __ {}
    
    class_alias('Foo\__', 'Foo\_'); // Is this meant to be deprecated?
    class_alias('__', '_'); // Deprecated
}

namespace Baz {
    use Foo\_;
}

namespace Qux {
    use Foo\__ as _; // Is this meant to be deprecated?
}

Resulted in this output:

Deprecated: Using "_" as a class name is deprecated since 8.4 in /in/ZZMJs on line 8

Deprecated: Using "_" as a class alias is deprecated since 8.4 in /in/ZZMJs on line 15

But I expected this output instead:

Deprecated: Using "_" as a class name is deprecated since 8.4 in /in/ZZMJs on line 8

Deprecated: Using "_" as a class alias is deprecated since 8.4 in /in/ZZMJs on line 14

Deprecated: Using "_" as a class alias is deprecated since 8.4 in /in/ZZMJs on line 15

Deprecated: Using "_" as a class alias is deprecated since 8.4 in /in/ZZMJs on line 23

I encountered this issue while trying to migrate a large closed source project to PHP 8.4. The namespaced _ class was used as a wrapper for the gettext _ function (eg new _("string")), to delay performing the translation of cached content until the http response is stringified. The class had ~35k instantiations in source control and more instances in compressed + serialized database records. This made class aliases a potentially appealing solution, but I understand from @Girgias that these not emitting deprecation errors is an oversight.

PHP Version

8.4.1 - 8.4.6 on 3v4l

Operating System

No response