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Subject: Re: [boost] [optional] optional(Expr&&) is insufficiently constrained
From: Paul Fultz II (pfultz2_at_[hidden])
Date: 2016-02-18 19:53:02

On Thursday, February 18, 2016 at 6:19:54 PM UTC-6, Andrzej Krzemienski
> 2016-02-18 23:03 GMT+01:00 Stephan T. Lavavej <s..._at_[hidden]
> <javascript:>>:
> > Hi,
> >
> > A user reported a bug (VSO#191303/Connect#2351203, see [1]) using
> > Boost.Optional with MSVC 2015 Update 2. The original test case involved
> a
> > minor bug in VC's tuple, which I'll look into fixing in the future
> > (specifically, N-tuples recursively construct (N - 1)-tuples, in a way
> > that's visible to overload resolution). However, there is also a bug in
> > boost::optional.
> >
> Hi,
> I have created a ticket abut this problem:
> Frankly, I do not know how to implement it in a portable way (considering
> older compilers) without type trait boost::is_constructible. I am
> currently
> blocked on it:
> As an aside, the "prevent_binding" stuff should probably be expressed as a
> > SFINAE constraint, not a static_assert.
> >
> About this remark, I am not sure. I understand that you want the
> constructors to be SFINAE-friendly, but at the same time I would loose the
> opportunity to convey a text message to the user-programmers, as to what
> the problem is. Now I do it with the static_assert.

You can add a message to the end of boolean clause in `enable_if` in C++11,
like this:

template<class T, class=
    std::enable_if<(boost:is_constructible<T>() && "Constructor

So when the compiler notes that the constructor was disabled by `enable_if`,
it will point to the boolean clause which contains your message. At least on
clang, it shows a preview of the line where the diagnostic occured, so the
message is shown as well. I don't how other compilers work.

One issue with this approach is that it requires a fairly compliant C++11
compiler(allowing literal strings in template parameters). Gcc 4.9 or
does not support this. Clang does. I don't about other compilers.


> I wish I had the feature "delete for a reason"
> Optional::Optional(R&&r) = delete("because it would cause...");

Well there is no need for another new feature, just better diagnostics from
current compilers.

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