Given the new features with anonymous services and service defaults, I was assuming the following syntax would work:

services:
    _defaults:
        factory:        [ "@doctrine", getRepository ]

    MyProject\UserRepository:
        arguments:      [ "AppBundle:User" ]

This errors with: "Please add the class to service "_defaults" even if it is constructed by a factory since we might need to add method calls based on compile-time checks."

Changing the defaults to:

    _defaults:
        class:          Doctrine\ORM\EntityRepository
        factory:        [ "@doctrine", getRepository ]

Appears to work at first, but doesn't, as this actually causes a service called _defaults to be created, and the other services in the file to ignore the defaults altogether.