Re: [boost] Re: is_convertible corner case

From: Peter Dimov (pdimov_at_[hidden])
Date: 2003-01-24 18:45:37

From: "David Abrahams" <dave_at_[hidden]>
> "Paul Mensonides" <pmenso57_at_[hidden]> writes:
>
> > I do, however, agree that we need more support from the language for
> > generic programming and some type of standardized API into the
> > compiler's type system.  And I definitely think that "undefined
> > behavior" is unreasonable when the situation is easily diagnosable
> > and not platform specific.
>
> I tend to agree on a "moral/aesthetic" level, but on a practical level
> we have to tread carefully.  The question, "can we just have an
> operator which produces a compile-time constant value saying whether
> its operand is a valid expression?" has come up a few times in the
> committee.  Each time, the implementors looked at their codebases and
> said "oooh, that's really hard to do."  I think the short form of the
> reason is that C++ compilers generally don't have the ability to
> recover from errors reliably.  That may explain why your 2nd, 3rd,
> 4th... diagnostic messages tend to be useless gibberish ;-)

But how does this apply to is_convertible<X, int> when a private X::operator
int()? Or are you discussing something else?

I see no reason to make that undefined behavior. It's either "false", "true"
(Comeau says true BTW), "unspecified", or "ill formed, no diagnostic
required" - in order of preference.