travis-ci build error

include/gfx_color_cpp14.hpp: In instantiation of ‘constexpr const gfx::pixel<gfx::channel_traits<gfx::channel_name::L, 1ul, 0u, 1u, 0u, 1u> > gfx::color<gfx::pixel<gfx::channel_traits<gfx::channel_name::L, 1ul, 0u, 1u, 0u, 1u> > >::black’:
test/test_draw_targets.hpp:59:5:   required from here
include/gfx_color_cpp14.hpp:18:88:   in constexpr expansion of ‘gfx::convert<gfx::pixel<gfx::channel_traits<gfx::channel_name::R, 21ul, 0u, 2097151u, 0u, 2097151u>, gfx::channel_traits<gfx::channel_name::G, 22ul, 0u, 4194303u, 0u, 4194303u>, gfx::channel_traits<gfx::channel_name::B, 21ul, 0u, 2097151u, 0u, 2097151u> >, gfx::pixel<gfx::channel_traits<gfx::channel_name::L, 1ul, 0u, 1u, 0u, 1u> > >(gfx::pixel<gfx::channel_traits<gfx::channel_name::R, 21ul, 0u, 2097151u, 0u, 2097151u>, gfx::channel_traits<gfx::channel_name::G, 22ul, 0u, 4194303u, 0u, 4194303u>, gfx::channel_traits<gfx::channel_name::B, 21ul, 0u, 2097151u, 0u, 2097151u> >(1, 0.0, 0.0, 0.0), 0u)’
include/gfx_pixel.hpp:1384:40: error: ‘constexpr gfx::gfx_result gfx::convert(PixelTypeLhs, PixelTypeRhs*, const PixelTypeRhs*) [with PixelTypeLhs = gfx::pixel<gfx::channel_traits<gfx::channel_name::R, 21ul, 0u, 2097151u, 0u, 2097151u>, gfx::channel_traits<gfx::channel_name::G, 22ul, 0u, 4194303u, 0u, 4194303u>, gfx::channel_traits<gfx::channel_name::B, 21ul, 0u, 2097151u, 0u, 2097151u> >; PixelTypeRhs = gfx::pixel<gfx::channel_traits<gfx::channel_name::L, 1ul, 0u, 1u, 0u, 1u> >]’ called in a constant expression
         if(gfx_result::success!=convert(lhs,&result,background)) {
                                        ^
include/gfx_color_cpp14.hpp: In instantiation of ‘constexpr const gfx::pixel<gfx::channel_traits<gfx::channel_name::L, 8ul, 0u, 255u, 0u, 255u> > gfx::color<gfx::pixel<gfx::channel_traits<gfx::channel_name::L, 8ul, 0u, 255u, 0u, 255u> > >::gray’:
test/test_draw_targets.hpp:191:33:   required from here
include/gfx_color_cpp14.hpp:61:87:   in constexpr expansion of ‘gfx::convert<gfx::pixel<gfx::channel_traits<gfx::channel_name::R, 21ul, 0u, 2097151u, 0u, 2097151u>, gfx::channel_traits<gfx::channel_name::G, 22ul, 0u, 4194303u, 0u, 4194303u>, gfx::channel_traits<gfx::channel_name::B, 21ul, 0u, 2097151u, 0u, 2097151u> >, gfx::pixel<gfx::channel_traits<gfx::channel_name::L, 8ul, 0u, 255u, 0u, 255u> > >(gfx::pixel<gfx::channel_traits<gfx::channel_name::R, 21ul, 0u, 2097151u, 0u, 2097151u>, gfx::channel_traits<gfx::channel_name::G, 22ul, 0u, 4194303u, 0u, 4194303u>, gfx::channel_traits<gfx::channel_name::B, 21ul, 0u, 2097151u, 0u, 2097151u> >(1, 5.0196078431372504e-1, 5.0196078431372504e-1, 5.0196078431372504e-1), 0u)’
include/gfx_pixel.hpp:1384:40: error: ‘constexpr gfx::gfx_result gfx::convert(PixelTypeLhs, PixelTypeRhs*, const PixelTypeRhs*) [with PixelTypeLhs = gfx::pixel<gfx::channel_traits<gfx::channel_name::R, 21ul, 0u, 2097151u, 0u, 2097151u>, gfx::channel_traits<gfx::channel_name::G, 22ul, 0u, 4194303u, 0u, 4194303u>, gfx::channel_traits<gfx::channel_name::B, 21ul, 0u, 2097151u, 0u, 2097151u> >; PixelTypeRhs = gfx::pixel<gfx::channel_traits<gfx::channel_name::L, 8ul, 0u, 255u, 0u, 255u> >]’ called in a constant expression
include/gfx_pixel.hpp:766:40: note: ‘constexpr gfx::gfx_result gfx::convert(PixelTypeLhs, PixelTypeRhs*, const PixelTypeRhs*) [with PixelTypeLhs = gfx::pixel<gfx::channel_traits<gfx::channel_name::R, 21ul, 0u, 2097151u, 0u, 2097151u>, gfx::channel_traits<gfx::channel_name::G, 22ul, 0u, 4194303u, 0u, 4194303u>, gfx::channel_traits<gfx::channel_name::B, 21ul, 0u, 2097151u, 0u, 2097151u> >; PixelTypeRhs = gfx::pixel<gfx::channel_traits<gfx::channel_name::L, 8ul, 0u, 255u, 0u, 255u> >]’ is not usable as a constexpr function because:
     constexpr static inline gfx_result convert(PixelTypeLhs source,PixelTypeRhs* result,const PixelTypeRhs* background=nullptr) {