DeviceN support for N=2 broke in Boost 1.72

[dmwilk]

Existing code using any of the boost::gil::dev2n32f types (defined in typedefs.hpp) fails because of explicit static assertion check. I would expect if gil is going to continue defining these types that they continue to compile; explicit exclusion of support for channels=2 is surprising given the number of useful formats this represents (2D flow vectors, grayscale images with alpha, duotones)

My system uses DeviceN=2 extensively & refactoring is a nontrivial endeavor

C++ Minimal Working Example

int main
{
	boost::gil::dev2n32f_pixel_t	this_wont_compile;    // defined in typedefs.hpp
	boost::gil::dev3n32f_pixel_t	this_does_compile;

	boost::gil::image<boost::gil::dev2n32f_pixel_t, false>		nope;
	boost::gil::image<boost::gil::dev3n32f_pixel_t, false>		yep;
        return 0;
}

failure is in a static_assert expressing non-implementation so should be same in all compilers

[striezel]

failure is in a static_assert expressing non-implementation so should be same in all compilers

Possibly yes, but in my opinion it would have been a good idea to include the message here nonetheless, so people might get a better idea where to look when trying to fix this error.

Using the provided minimal working example (MWE)

#include <boost/gil.hpp>

int main()
{
	boost::gil::dev2n32f_pixel_t	this_wont_compile;    // defined in typedefs.hpp
	boost::gil::dev3n32f_pixel_t	this_does_compile;

	boost::gil::image<boost::gil::dev2n32f_pixel_t, false>		nope;
	boost::gil::image<boost::gil::dev3n32f_pixel_t, false>		yep;
        return 0;
}

I get the following error in GCC 11.2.0 using the current master from boostorg/boost@38e705a:

$ g++ main.cpp 
In file included from /usr/include/boost/gil/typedefs.hpp:13,
                 from /usr/include/boost/gil/channel_algorithm.hpp:13,
                 from /usr/include/boost/gil/color_convert.hpp:11,
                 from /usr/include/boost/gil/image_view_factory.hpp:11,
                 from /usr/include/boost/gil/algorithm.hpp:15,
                 from /usr/include/boost/gil.hpp:12,
                 from main.cpp:1:
/usr/include/boost/gil/device_n.hpp: In instantiation of 'struct boost::gil::devicen_t<2>':
/usr/include/boost/gil/device_n.hpp:56:8:   required from 'struct boost::gil::devicen_layout_t<2>'
/usr/include/boost/gil/pixel.hpp:99:8:   required from 'struct boost::gil::pixel<boost::gil::scoped_channel_value<float, boost::gil::float_point_zero<float>, boost::gil::float_point_one<float> >, boost::gil::devicen_layout_t<2> >'
main.cpp:5:31:   required from here
/usr/include/boost/gil/device_n.hpp:46:16: error: static assertion failed: invalid number of DeviceN color components
   46 |         N == 1 || (3 <= N && N <= 5),
      |         ~~~~~~~^~~~~~~~~~~~~~~~~~~~~
/usr/include/boost/gil/device_n.hpp:46:16: note: '((2 == 1) || ((3 <= 2) && (2 <= 5)))' evaluates to false

On the other hand, it compiles with Boost 1.71 just fine. Compiling with 1.72 fails, error message is the same.
The static_assert was introduced in commit 5611bd5 of PR #274.

I'll try to fix this.

[striezel]

@dmwilk:
The changes in #654 fix your issue. At least your minimal working example is now indeed a working example again. :)

[mloskot]

@dmwilk Thanks for reporting and thank you @striezel for the fix!