Yeah, it might needs a bit of work to finish it, I wasn't really familiar with the DI components internal implementation before starting this PR so I might have missed some things.

Concerning thegetConstructor method, would it be fine to just returnnull in case of a factory expression since as far as I can see it is used to get the constructor arguments which doesn't apply in the case of an expression?

Concerning BC breaks, the only one I see is concerning the typing of the factory property in case someone was writing a compiler pass that doesn't expect to receive an expression object from it.

I've decided to go with returning theReflectionFunction of an empty closure inAbstractRecursivePass::getConstructor since an expression can be seen as an anonymous function without any argument.

Thinking about all this, what you want to achieve can be done by writing:

$services->set(MyServiceInterface::class)    ->factory('current')    ->arg([expr("env('MY_ENV_VAR') ? service('some_service') : service('some_other_service')")])

If this works already, this proves we don't need to change any interfaces. Instead, we might want to improve the way this can be expressed.

While working on the feature I thought of a similar workaround with a user defined identity function but it still feels like a hack and having a supported way of doing without any hacks would be better for documentation and discoverability.

It took me a while to figure out an approach that could work, and here it is:#45512
Please have a look!

Closing in favor of#45512