I just created a service definition file like this SCCEE:

services:
    _defaults:
        tags:           [ { name: kernel.event_subscriber } ]

    some.event.subscriber:
        calls:
            - [setLogger, ["@logger"]]
        tags:           [ { name: monolog.logger, channel: specific_channel } ]

And then spent half an hour debugging why my events weren't being processed.

The reasoning is sound of course - I provided a default, and then I provided a real value overwriting the default. It's however not intuitive and imho highly confusing that this doesn't work like this out of the box.

I see 2 solutions to this:

1. Merge tags by default based on name. Seems to make sense but it may go wrong in edge cases I'm not seeing.
2. If a 'mergeable' property like tags is provided both in default/instanceof and definition, require setting a property in the like merge_tags explicitly to true or false, and bail out if it isn't set.

Comments?