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Date: Thu, 22 Jul 2021 10:31:35 +0200
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Subject: Method compatibility checks and order in which files are included
From: sebastian@php.net (Sebastian Bergmann)

We noticed that the PHARs of PHPUnit stopped working with PHP 8.1 [1]. 
Before [2], a commit pushed today, PHPUnit loaded all sourcecode files 
packaged in the PHAR on startup (to work around problems that (hopefully) 
no longer exist because PHPUnit's PHAR is scoped using PHP-Scoper 
nowadays). This loading of all sourcecode files lead to the error reported 
in [1]. The "static include list" used for this is generated by PHPAB [3] 
which performs a topological sort based "class implements interface", 
"class uses trait", "class extends class", etc. dependencies. This 
topological sort does not, however, consider type declarations for 
properties as well parameters and return values as dependencies.

What follows is a minimal, reproducing example:

.
├── autoload.php
├── Collection.php
└── CollectionIterator.php

0 directories, 3 files

This is the contents of autoload.php:

<?php declare(strict_types=1);
require __DIR__ . '/Collection.php';
require __DIR__ . '/CollectionIterator.php';

This is the contents of Collection.php:

<?php declare(strict_types=1);
class Collection implements IteratorAggregate
{
     public function getIterator(): CollectionIterator
     {
         return new CollectionIterator($this);
     }
}

And this is the contents of CollectionIterator.php:

<?php declare(strict_types=1);
class CollectionIterator implements Iterator
{
     public function __construct(Collection $collection)
     {
     }

     public function current(): mixed
     {
     }

     public function next(): void
     {
     }

     public function key(): int
     {
     }

     public function valid(): bool
     {
     }

     public function rewind(): void
     {
     }
}

When I run autoload.php then I get

Fatal error: Could not check compatibility between 
Collection::getIterator(): CollectionIterator
and IteratorAggregate::getIterator(): Traversable, because class 
CollectionIterator is not
available in /home/sb/example/Collection.php on line 4

I am using PHP 8.1-dev (590af4678bbdbb9422bfcc568a0b8d2d1a6f74f5). The 
error does not occur with PHP 8.0.8.

I assume the following happens:

* The compiler works on Collection.php
* The compiler sees that Collection implements IteratorAggregate
* The compiler sees that Collection::getIterator() returns CollectionIterator
* The compiler performs the method compatiblity check
* The method compatiblity check cannot be performed because 
CollectionIterator has not been compiled yet

When I change autoload.php like so

<?php declare(strict_types=1);
require __DIR__ . '/CollectionIterator.php';
require __DIR__ . '/Collection.php';

then the method compatiblity check can be performed (and succeeds without 
error).

After this long introduction (sorry!), I can finally ask my question: can 
there be situations where this alternate order of requiring the source 
files would also fail? Is there a potential for cyclic dependencies 
between two sourcefiles that can only be resolved when autoloading is 
used? I am "scared" that using "static include list" may become unfeasible 
when more compile-time checks such as the method compatibility check are 
added.

-- 
[1] 
https://github.com/sebastianbergmann/phpunit/issues/4740#issuecomment-884667189
[2] 
https://github.com/sebastianbergmann/phpunit/commit/4b5756d8b093d5ae74801586e621703508ec4be1
[3] https://github.com/theseer/Autoload