In reply to Jenny C:
> Given that they are members of shengan so they're is no passport control on the borders with most of their neighbours, how will they know if you crossed into the country by an official route or not?
That's the point of the rumoured law, they wouldn't need to know, if you are a non-EU citizen or without a Schengen visa, and you are in the country, and there is no record of yourself having entered the country, because you've not declared your presence upon entrance at a border post, then that it itself would constitute an offence.
I guess the idea of this is to incite asylum seekers to declare themselves when they enter.
As for British tourists after Brexit willing to travel across the EU for less than 90 days, I suspect there will be an ESTA type system and you would have to declare in advance which countries you which to visit.
Longer than 90 days of course and they'll need to qualify for some sort of visa but that's likely to be limited to on country.