[Karellen]

> I suppose it would break any currently compliant C++ compiler

Maybe not?

I'm pretty sure that "trap" is an allowed behaviour for NaNs in some parts of the standard already, if properly documented. I don't see why unspecified/implementation-defined behaviour for shifts couldn't be allowed to do the same, on appropriate targets?

But also, the new behaviour should only be required of compilers in the to-be-written `--std=c++3X` (or whatever) mode. Any compilers conforming to an earlier version of the standard can already do whatever they're currently doing.