Symfony version(s) affected: tested with 3.4 and 4.3

Description

If I define a synthetic service in my service.yaml config file where binds are already defined, I get the error

Invalid service "my_synthetic_service": class "" does not exist.

How to reproduce

1. Create a new app:

symfony new my-app

1. Add the service definition and the bind in config/services.yaml

services:
    _defaults:
        # ...
        bind:
            $someBind: someValue

    # ...

    my_synthetic_service:
        synthetic: true
        public: true

1. Run any console command

./bin/console debug:container my_synthetic_service

1. Get the full error

Unused binding "$someBind" in service "my_synthetic_service".
Could be related to:

• Invalid service "my_synthetic_service": class "" does not exist.

This error is a little more understandable than the first one, but I have to try finding the minimal failing exemple to get this error

1. Add some service that use the bind

<?php
namespace App\Service;
class SomeService
{
    public function __construct(string $someBind) {  }
}

1. Re-run the console command
2. Get the minimal error

Invalid service "my_synthetic_service": class "" does not exist.

This error is way worse to debug the real problem