Funny this comes up now. Just two weeks ago I had a discussion on chessvariants.com with Greg Strong in relation to his program ChessV and the detectability of illegal moves. And as an example of a variant where such a thing would be virtually impossible to detect without search, I mentioned a (hypothetical) variant that would considering check illegal unless it would eventually result in a checkmate. I.e. once you give a check, you would have to keep checking every move.
This seems a bit far-fetched, but is actually similar to the situation in Paco Shako. Where it would be illegal to trigger a release (by jumping into an embracing pair) if you cannot eventually find a way to terminate the resulting chain of releases.
Every now and then I see a suggestion: Eliminate check except when it is mate.
I saw the suggestion here from @FabledMagikarp:
www.chess.com/article/view/no-castling-chess-kramnik-alphazero#comment-45300440
and here from @ZhenyaChaynikov:
www.chess.com/forum/view/chess960-chess-variants/letrsquos-invent-some-very-weird-pieces?page=24#comment-44731154
First, it must be made clear what is meant by this. At first sight, I can see two interpretations:
1.
The King is not harmed by check (in other words: immune to capture) and can ignore it, except when it's mate. The King can move into the reach of an enemy piece without consequence.
2.
For the opponent, it is illegal to give check, except when it's mate. It is not yet clear what to do with a King moving into the reach of an enemy piece. Is that also illegal? Or is it the opponent who must 'resolve the illegal check' coming from his own piece?
First of all, any person designing such a variant, should rethink the definition of check and mate. Currently, the definition of mate refers to the definition of check ("you are mate when you are in check and not able to resolve the check"). So when we change the meaning of "check" we will have to look at the definition of mate again.
Both of these interpretation have something odd:
Check is avoidable King capture, mate is unavoidable King capture. It is a bit counter-intuitive, or contradictory, to call unavoidable King capture a win, and at the same time reward avoidable King capture with absolute King safety. This is also why I don't favour this suggestion.