Scope mismatch can lead to subtle bugs

CDI allows injection of a non-proxyable object created by a provider into higher level contextes. This can lead to subtle bugs, see the following example, the first username that accesses the service is returned for other users:

@ApplicationScoped
public class ServiceClass {

    @Inject
    @UserName
    private String userName;

}

@RequestScoped
public class UserNameProvider {

    @Inject
    private HttpServletRequest request;

    @Produces
    @UserName
    public String userName() {
        return request.getUserPrincipal().getName();
    }

}

CDI should fail to start when it detects such a situation. Do note that this bug does not require direct injection (Service->userName), it can occur transitively as well (Service->User->userName).