Re: [boost] RFC Expression Validity Asserts

Subject: Re: [boost] RFC Expression Validity Asserts
From: TONGARI J (tongari95_at_[hidden])
Date: 2015-01-22 21:49:03

2015-01-23 10:11 GMT+08:00 Matt Calabrese <rivorus_at_[hidden]>:

> On Thu, Jan 22, 2015 at 5:24 PM, TONGARI J <tongari95_at_[hidden]> wrote:
>
> > 2015-01-23 8:26 GMT+08:00 Matt Calabrese <rivorus_at_[hidden]>:
> >
> > > On Thu, Jan 22, 2015 at 4:12 PM, Matt Calabrese <rivorus_at_[hidden]>
> > wrote:
> > >
> > > > On Thu, Jan 22, 2015 at 4:09 PM, Matt Calabrese <rivorus_at_[hidden]>
> > > wrote:
> > > >
> > > >> For instance, you can use it to easily do:
> > > >>
> > > >> template <class T>
> > > >> struct has_preincrement_operator
> > > >>     : std::integral_constant<bool, IS_VALID_EXPR((T) a, ++a)> {};
> > > >>
> > > >>
> > > >
> > > Hmm, actually this use doesn't seem to work, though I'm not sure
> why... I
> > > should have tested it in the browser before posting the sample. I'm
> > pretty
> > > sure it fails due to a compile-bug but I'm not entirely certain.
> > >
> >
> > I'm sure that lambda cannot be used in unevaluated context.
>
>
> Right, but the magic of this trick is that it's not actually using the
> lambda in an unevaluated context (at least that's my understanding, and
> I've searched the standard for all instances of the term unevaluated)! A
> lambda itself never actually appears in a
> typeid/decltype/sizeof/alignof/noexcept. I do the tricks indirectly, using
> the lambdas just as a means to introduce declarations inside a nested scope
> of the expression and to create nested templates via generic lambdas for
> SFINAE. No lambda itself is actually invoked or directly used in an
> unevaluated context. I deduce information that the lambdas contain
> indirectly.
>
> Anyway, what is interesting is that the following *does* compile, as I
> thought it would:
>
> //////////
> using T = std::integral_constant<bool, IS_VALID_EXPR((int) a, ++a)>;
> //////////
>
> The above uses the lambda internally in the same way as the previous email,
> but without failure. I just pass the result of the macro invocation as a
> compile-time bool template argument, with the main difference being that I
> use "int" here instead of a dependent type. I don't immediately see why
> that difference should cause failure. As far as I can tell, they should
> both compile, or if I'm wrong somewhere, it seems that NEITHER should
> compile.
>

You're right, I misunderstood the situation.

Your code doesn't work for clang, it seems to be more strict than g++:
 "error: a lambda expression may not appear inside of a constant expression"
I suspect it's also a bug, since a hand-written functor does compile.