Loading deferred service providers with a non-deferred service provider

app.php

<?php

return [
  'providers' => [
    'App\Modules\Foo\FooModuleServiceProvider'
  ],
];

AppServiceProvider.php

<?php namespace App\Modules\Foo\Providers;

use Illuminate\Support\ServiceProvider;

class AppServiceProvider extends ServiceProvider {
  protected $defer = true;
  
  public function register() {
    $this->app->bind(FooThingInterface::class, FooThing::class);
  }
}

FooModuleServiceProvider.php

<?php namespace App\Modules\Foo;

use Illuminate\Support\ServiceProvider;

class FooModuleServiceProvider extends ServiceProvider {
  protected $defer = false;
  
  public function register() {
    $this->app->register('App\Modules\Foo\Providers\RouteServiceProvider'); // Not deferred since routes are always needed
    $this->app->register('App\Modules\Foo\Providers\AppServiceProvider'); // Deferred since you don't need all these bindings until you need them
  }
}

The problem with this setup is that Illuminate\Support\ServiceProvider::isDeferred is only checked when building the manifest file in Illuminate\Foundation\ProviderRepository, so registering a deferred service provider anywhere else but the app.providers array just loads it normally.