Hi,

Sorry for the late reaction on this. I basically agree with what was discussed so far, just to specifically answer the original questions:

1. Testing if not_implemented_tag is a base class of the dispatcher is
not (yet) reliable. For simple algorithms like distance it works but for
algorithms that depend on others it won't. Concrete example : intersects
calls extreme_points. extreme_points is not implemented for anything
else than rings, polygons and boxes. Yet this limitation is not
"reported" to the intersects dispatcher.

As discussed with Adam, this needs to be fixed, and the policy needs to be more closely followed. It's not important only for what you're trying to do: the support status is wrong if this rule is broken. This would be the first thing to do.

2. The dispatcher is an implementation detail (not a public interface)
so should we rely on it ?

Same as Adam said, not to be used by external users but definitely something we can rely on within the library. We have always made sure to have very consistency interface here, and it was made fully consistent when support status was implemented.

3. The filtering should happend after the variant resolution that means
rewriting all the variant code in the runtime extension. Unless a hook
is implemented to augment this code with "runtime filtering".

That would be another stage in the multistage dispatching I've put in place, or maybe directly done within the variant resolution stage.

4. Maybe an extension is not a good idea and runtime support should be a
core functionality of the library. I think so because it seems to
require an extra requirements : the algorithms must report compile time
errors in a specified way when invoked with not supported types not just
rise an MPL assertion.

To me it's completely obvious that this cannot be an extension (and shouldn't, conceptually). It's probably impossible to implement it in a non-intrusive way. It has to be implemented as a core feature. Unfortunately I'm busy finishing variants work, only 1 or 2 algos left but I really don't have much spare time for doing any work currently.

Regards

Bruno