Fatal error during autoloading of complex inheritance chain with LSP checks since PHP 8.1.0

Description

The following code:

<?php

$depth = 0;

spl_autoload_register(function ($class) use (&$depth) {
    $depth++;
    $indent = str_repeat('| ', $depth);
    echo $indent."$depth: $class\n";

    if ($class == 'IFace') {
        interface IFace {}
    } elseif ($class == 'IFaceImplementor') {
        class IFaceImplementor implements IFace {}
    } elseif ($class == 'InterfaceOfMainParent') {
        interface InterfaceOfMainParent
        {
            public function methodForSecondaryLspCheck(): IFace;
        }
    } elseif ($class == 'MainParent') {
        abstract class MainParent implements InterfaceOfMainParent
        {
            public function methodForSecondaryLspCheck(): IFaceImplementor {}
        }
    } elseif ($class == 'Intermediate') {
        abstract class Intermediate extends MainParent {}
    } elseif ($class == 'Child1') {
        class Child1 extends Intermediate {}
    } elseif ($class == 'Child2') {
        class Child2 extends Intermediate {}
    } elseif ($class == 'EntrypointParent') {
        abstract class EntrypointParent
        {
            abstract public function methodForLspCheck1(): MainParent;
            abstract public function methodForLspCheck2(): MainParent;
        }
    } elseif ($class == 'Entrypoint') {
        class Entrypoint extends EntrypointParent
        {
            public function methodForLspCheck1(): Child1 {}
            public function methodForLspCheck2(): Child2 {}
        }
    }

    $depth--;
});

class_exists(Entrypoint::class);

Resulted in this output:

| 1: Entrypoint
| | 2: EntrypointParent
| | 2: Child1
| | | 3: Intermediate
| | | | 4: MainParent
| | | | | 5: InterfaceOfMainParent
| | | | | 5: Child2

Fatal error: During inheritance of MainParent, while autoloading Child2: Uncaught Error: Class "Intermediate" not found in php-wasm run script:29
Stack trace:
#0 php-wasm run script(20): {closure}('Child2')
#1 php-wasm run script(25): {closure}('MainParent')
#2 php-wasm run script(27): {closure}('Intermediate')
#3 php-wasm run script(37): {closure}('Child1')
#4 [internal function]: {closure}('Entrypoint')
#5 php-wasm run script(47): class_exists('Entrypoint')
#6 {main} in php-wasm run script on line 20

But I expected this output instead:

| 1: Entrypoint
| | 2: EntrypointParent
| | 2: Child1
| | | 3: Intermediate
| | | | 4: MainParent
| | | | | 5: InterfaceOfMainParent
| | | | | 5: IFaceImplementor
| | | | | | 6: IFace
| | 2: Child2

• Full reproduction repository: https://github.com/janedbal/php-bug-complex-inheritance-chain-autoloading
• 3v4l version: https://3v4l.org/G5fFe
• Broken since: PHP 8.1.0

PHP Version

PHP 8.5.0RC1 (cli) (built: Oct  8 2025 22:41:41) (NTS)
Copyright (c) The PHP Group
Built by https://github.com/docker-library/php
Zend Engine v4.5.0RC1, Copyright (c) Zend Technologies
    with Zend OPcache v8.5.0RC1, Copyright (c), by Zend Technologies

Operating System

No response

Start with the reproduction repository and the class_exists(Entrypoint::class) entry point, then compare the autoload trace on PHP 8.1.0 and newer versions. Investigate the inheritance and LSP-check sequence shown in the issue. Done means the reproduction completes without the fatal error and produces the expected autoload order.