I was looking for a way to save a definition declaration when one uses a FQCN reference:

services:
    my_service:
        arguments: [ '@App\Foo' ]

and not have to declare App\Foo.

One way to achieve this is to couple it with autowiring: if the @App\Foo ref is invalid, AutowirePass can create the target service. This was done in #21859 and further enhanced in #21873 (which adds the requirement that this can happen only in autowired holder definitions.)

Then I wondered: what would one expect with _defaults and _instanceof?
eg

services:
    _instanceof:
        App\FooInterface:
            tags: [ 'foo_tag' ]
    my_service:
        arguments: [ '@App\Foo' ]

Should the App\Foo definition get the foo_tag? Might be a yes, especially if we teach that:

services:
    my_service:
        arguments: [ '@App\Foo' ]

is equivalent to:

services:
    App\Foo : ~
    my_service:
        arguments: [ '@App\Foo' ]

you still follow me?

This is what this PR achieves - and for this, it needs to work at the loader level.

The next step is working with @App\FooInterface: to me, since this cannot be a definition, it has to play well with autowiring, which should be applicable (opt-in-only as usual) to find the correct service implenenting App\FooInterface. OK?

But then, let's says we have class Bar extends Foo {}, and this config:

services:
    _defaults:
        autowire: true
    App\Bar: ~
    my_service:
        arguments: [ '@App\Foo' ]

What should @App\Foo be resolved to? App\Bar as autowiring would do with the corresponding type hint? Or an implicit new App\Foo definition as the previous equivalence principle would lead to?

That's ambiguous to me.