Re: [boost] [mpl] Problem with VC++14 and boost::mpl::size_t

Subject: Re: [boost] [mpl] Problem with VC++14 and boost::mpl::size_t
From: Edward Diener (eldiener_at_[hidden])
Date: 2015-07-26 10:20:42

On 7/25/2015 2:48 AM, Matt Calabrese wrote:
> On Fri, Jul 24, 2015 at 7:41 PM, Edward Diener <eldiener_at_[hidden]>
> wrote:
>
>> pointing to the line after it encounters 'struct size_t' as far as I can
>> make out ( you need to preprocess the size_t.cpp test, comment out the
>> #line directives and run the test against the preprocessed result to see
>> what the compiler is objecting to ). It therefore fails a number of mpl
>> tests which all prior versions of VC++ pass.
>>
>> It flags this error in 5 mpl tests which use the boost::mpl::size_t
>> integral wrapper, among which the easiest to analyze is the size_t.cpp test
>> itself.
>>
>> Anybody have any idea what is going on here with the compiler, or whether
>> it can be fixed somehow in VC++14 for the mpl code ?
>>
>
> I don't have the compiler, but my guess is it's probably actually confused
> on the line:
>
> typedef size_t type;
>
> A simple workaround to try could be to change the typedef to:
>
> typedef mpl::size_t<N> type;
>
> If that fix works, I'd be surprised if such a change would break any other
> compilers. This is all just a guess, though.
>

My latest investigation, using the mpl::size_t code directly in a test, 
shows that:

// size_t_bug.cpp

#include <cstddef>
namespace boost { namespace mpl {
struct integral_c_tag { static const int value = 0; };
template< std::size_t N >
struct size_t
{
     static const std::size_t value = N;
     typedef size_t type;
     typedef std::size_t value_type;
     typedef integral_c_tag tag;
     typedef boost::mpl::size_t< static_cast<std::size_t>((value + 1)) > 
next;
     typedef boost::mpl::size_t< static_cast<std::size_t>((value - 1)) > 
prior;
     operator std::size_t() const { return 
static_cast<std::size_t>(this->value); }
};
template< std::size_t N >
std::size_t const boost::mpl::size_t< N >::value;
}}

#include <boost/core/lightweight_test.hpp>
int main()
{
     return 0;
}
using namespace boost;
using namespace mpl;

fails under VC++14 with:

size_t_bug.cpp(26): error C2872: 'size_t': ambiguous symbol
predefined C++ types (compiler internal)(16): note: could be 'unsigned 
int size_t'
size_t_bug.cpp(6): note: or       'boost::mpl::size_t'

but if I comment out the include of lightweight test or if I comment out 
the using directive, the source compiles without error. How using 
directives which occur after all code can affect the compilation is 
beyond me.

Anybody who would like to pitch in and try to determine what is causing 
VC++14 to fail with the above code, which is essentially why it is 
failing with the MPL tests involving boost::mpl::size_t, please feel 
free to do so.

Needless to say VC++12 and below, as well as gcc and clang, compile the 
test without error.