silverstripe-framework: ClassManifest picks up classes that won't exists because of `class_exists` statements

Affected Version

^4.0

Description

Sometimes you have classes that will only conditionally exists if another class exists. This is helpful when you’re implementing a feature that will only be available if another module is installed.

Only problem is that because ClassManifest uses static code analysis to decide what classes exists it won’t pick up if there’s a statement like this at the start of the file.

if (!class_exists(RegistryPage::class)) {
    return;
}

Steps to Reproduce

Create a file with this code.

<?php

use SilverStripe\Core\ClassInfo;
use SilverStripe\Dev\BuildTask;
use SilverStripe\ORM\DataObject;

class TestBuild extends BuildTask {

    public function run($request)
    {
        $subClasses = ClassInfo::subclassesFor(DataObject::class);

        if (in_array('Test', $subClasses)) {
            echo "Test is returned by subclassesFor\n";
        } else {
            echo "Test is NOT returned by subclassesFor\n";
        }

    }

}

if (!class_exists(NonExistant::class)) {
    return;
}

class Test extends DataObject {

}

Expected outcome: When you run sake dev/tasks/TestBuild, “Test is NOT returned by subclassesFor” should be outputted. Actual outcome: When you run sake dev/tasks/TestBuild, “Test is returned by subclassesFor” is outputted.