On Sat, 4 Jan 2020 at 18:52, Lyberta via Std-Proposals
<std-proposals_at_[hidden]> wrote:
>
> 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.

As far as I understand things, implementability is not the problem of
definition checking.
The code-compatibility and scalability-of-compatibility, as Bo hints,
is. To perhaps oversimplify,
C++0x concepts strived for full definition checking with incomplete
facilities to opt out of it,
and C++20 concepts punt full definition checking to a possible,
perhaps even hypothetical
future, to get the arguably more important parts into our hands.