Re: [boost] the Boost policy for protecting against "too perfect forwarding"

Subject: Re: [boost] the Boost policy for protecting against "too perfect forwarding"
From: Andrey Semashev (andrey.semashev_at_[hidden])
Date: 2014-04-25 11:17:12

On Friday 25 April 2014 16:48:03 Andrzej Krzemienski wrote:
> Hi,
> By "too perfect forwarding" I mean the issue described in either of the
> following articles:
> http://akrzemi1.wordpress.com/2013/10/10/too-perfect-forwarding/
> http://flamingdangerzone.com/cxx11/2012/06/05/is_related.html
> http://ericniebler.com/2013/08/07/universal-references-and-the-copy-construc
> to/
> 
> In short, a perfect-forwarding constructor can inadvertently become a
> better match than a copy constructor when we want to make a copy.
> 
> There are two ways for protecting against it:
> 1. enable_if -- to disable the unwanted overloads
> 2. Add additional constructor overloads.
> 
> (1) appears cleaner, but some compilers might not support SFINAE.
> 
> My question is if there is a consensus in Boost how to approach this
> problem. Is Boost supposed to support compilers w/o SFINAE? (It looks so,
> because we have macro BOOST_NO_SFINAE.
> 
> I can see Boost.Any just uses SFINAE for this purpose without any check.
> 
> Are there any compilers supported by Boost that have rvalue references but
> no SFINAE?

I think you can safely rely on SFINAE with any recent compiler. 
BOOST_NO_SFINAE was used with ancient compilers and there even was initiative 
of removing it, if I'm not mistaken.