Re: [boost] [Proto] Errors due to narrowing conversions in matches.hpp

Subject: Re: [boost] [Proto] Errors due to narrowing conversions in matches.hpp
From: Eric Niebler (eric_at_[hidden])
Date: 2012-04-21 21:47:49

Peter, can you comment?

On 4/21/2012 1:14 PM, Michel Morin wrote:
> Some tests of Boost.Phoenix fail on clang trunk in a C++11 mode
> due to narrowing conversions of non-type template arguments,
> which are prohibited in C++11.

Really?! Wow, that's a breaking change I hadn't heard of before.

> Specifically, the narrowing conversions occur at Line 354 and 363
> in boost/proto/matches.hpp.
> They can be simply resolved using `static_cast<bool>(...)`.

I can certainly add the static_cast, but it looks like this may in fact
be a "bug" in boost::is_placeholder, which is defined as:

template< class T > struct is_placeholder
{
    enum _vt { value = 0 };
};

Ideally, this should be a valid MPL Integral Constant of type bool, like:

template< class T >
struct is_placeholder
  : mpl::false_
{};

If that is not portable enough, it could be:

template< class T > struct is_placeholder
{
    BOOST_STATIC_CONSTANT(bool, value = false);
};

Peter, do you prefer one or the other, or is there some reason you'd
like to stick with the original formulation?

-- 
Eric Niebler
BoostPro Computing
http://www.boostpro.com