Sorry I should have commented here (see the note above).

First, Adam is right, the current synced version is "distance" specific and that is plainly wrong (and not what I intended to propose). As I said in a previous note, I'm working on a generic one.

Bruno, it's a IMHO mandatory step in the direction of run-time specified geometries. But I intended to stick with the compile time checking with my current code (I'm working on runtime algo in an other branch whose state is far from becoming a PR !)

I think that we could offer both options (and so two interfaces) and that without requiring a compile macro. But that means removing the MPL_ASSERT from not_implemented.

I can postpone the integration of a 'global' is_implemented<> to a later stage (move it as an implementation detail of variant for distance). It sounds like it needs more thinking, no ?