[cfe-dev] Error when testing clang with VC++ RTL and Boost MPL

[Edward Diener]

On 11/13/2013 2:59 PM, Jordan Rose wrote:
>
> On Nov 13, 2013, at 11:47 , Edward Diener
> <eldlistmailingz at tropicsoft.com
> <mailto:eldlistmailingz at tropicsoft.com>> wrote:
>
>> On 11/13/2013 9:46 AM, Edward Diener wrote:
>>> I am seeing this error when testing Boost MPL with clang using the VC++
>>> RTL:
>>>
>>> "bitwise.cpp(40,25) :  error: non-type template argument evaluates to
>>> 4294967295, which cannot be narrowed to type 'long' [-Wc++11-narrowing]
>>> MPL_ASSERT_RELATION( (bitor_<_0,_ffffffff>::value), ==, 0xffffffff );"
>>>
>>> The typedefs are:
>>>
>>> typedef integral_c<unsigned int, 0> _0;
>>> typedef integral_c<unsigned int, 0xffffffff> _ffffffff;
>>>
>>> The bitor_ in the Boost MPL is evaluating constants at compile time,
>>> doing a bitwise or ('|').
>>>
>>> Why does clang think that 0xffffffff is a 'long' when used in the
>>> comparison ? According to the C++ standard the type of 0xffffffff is the
>>> first of int, unsigned int, long, unsigned long, long long, unsigned
>>> long long in which its value can fit ( section 2.14.2 ). Is this a clang
>>> bug ?
>>
>> Sorry, the problem is not as I had assumed above. It is actually
>> because the MPL_ASSERT_RELATION macro devolves down to a template
>> class where the values are of type 'long'. Is there something in C++11
>> which says that implicit conversion of a value from an 'unsigned int'
>> to a 'long' is illegal ? That is what clang is telling me but I can
>> find no such restriction in the C++11 standard regarding this.
>
> I think it’s more likely that on Windows ‘long’ is the same size as
> ‘int’, and so you have an overflow error. You need to fix the
> template…which I guess means submitting a patch to Boost.

I tried a patch of the test code where I set:

typedef integral_c<int, 0xffffffff> _ffffffff;

but clang still complains that:

bitwise.cpp(23,24) :  error: non-type template argument evaluates to 
4294967295, which cannot be narrowed to type 'int' [-Wc++11-narrowing]
typedef integral_c<int, 0xffffffff> _ffffffff;

This I do not understand. Why does 0xffffffff evaluate to 4294967295 
rather than -1 ? Where in the C++ standard does it say that hex literals 
are unsigned values by default ?