Re: [boost] [optional] optional(Expr&&) is insufficiently constrained

On Thursday, February 18, 2016 at 6:19:54 PM UTC-6, Andrzej Krzemienski wrote:

2016-02-18 23:03 GMT+01:00 Stephan T. Lavavej javascript:>:

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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: https://svn.boost.org/trac/boost/ticket/12002 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: https://svn.boost.org/trac/boost/ticket/12003.

As an aside, the "prevent_binding" stuff should probably be expressed as a

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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::type

optional(T&&); 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 earlier 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. Paul