The psr/container interfaces have been updated with type declarations. The lack of those is what kept us from adding property type declarations to the get() and has() methods of our own ContainerInterface.

A small BC break is that we have never prevented calling code from passing null as the service ID. Even without strict types, this will cause a TypeError after my changes. I already had to update AutowirePass because of that.

On the other hand, it was neither documented that we allow null here nor did the container do anything useful (has(null) always resulted in false and get(null) always returned null).