Since #30257 we can inject complex tagged collections easily.

The only thing left before PHP gets generics is compile-time checking of the collection item type.

The syntax I propose:

services:
    foo_service:
        class: Foo
        tags:
            - foo

    bar:
        class: Bar
        arguments:
            - !tagged
                tag: foo
                # new attribute
                instanceof: FooInterface

Note that having the foo tag does not imply being an instance of FooInterface.