Bo Persson via Std-Proposals:
> The main(?) problem being that if one function added a new possible
> exception, every function calling it - directly or indirectly - would
> have to be modified as well.
>
> Will we not get the same effect with this new feature?

I'm pretty sure definition checking was in C++0x with full blown
concepts as Alex Stepanov wanted. But it was taken out and C++20 got
only Concepts "Lite". So I assume there were some difficulties
implementing it.