[neonsunset]

Only if you go out of your way to author a method with (Guid someGuid = default) argument. I've never seen it happen with Guids, if someone gives you default(Guid) - they did it on purpose, it's no different to explicitly setting `0` to an integer-typed UserID property.

If supplying Guid is optional, you just make it Guid?.

To be fair, I don't think offering default(T) by default (ha) is the best choice for structs. In F#, you have to explicitly do `Unchecked.defaultOf` and otherwise it will just not let you have your way - it is watertight. I much prefer this approach even if it can be less convenient at times.