Fixed numpy_image not compiling when C++14 is enabled.

dlib/python/numpy_image.h

@@ -13,6 +13,7 @@
 #include <pybind11/pybind11.h>
 #include <dlib/image_transforms/assign_image.h>
 #include <stdint.h>
+#include <type_traits>
 
 namespace py = pybind11;
 
@@ -356,18 +357,28 @@ namespace pybind11
         {
             using basic_pixel_type = typename dlib::pixel_traits<pixel_type>::basic_pixel_type;
 
-            static PYBIND11_DESCR name() {
-                constexpr size_t channels = dlib::pixel_traits<pixel_type>::num;
-                if (channels == 1)
+            template <size_t channels> 
+            static PYBIND11_DESCR getname(typename std::enable_if<channels==1,int>::type) {
                 return _("numpy.ndarray[(rows,cols),") + npy_format_descriptor<basic_pixel_type>::name() + _("]");
-                else if (channels == 2)
+            };
+            template <size_t channels> 
+            static PYBIND11_DESCR getname(typename std::enable_if<channels!=1,int>::type) {
+                if (channels == 2)
                     return _("numpy.ndarray[(rows,cols,2),") + npy_format_descriptor<basic_pixel_type>::name() + _("]");
                 else if (channels == 3)
                     return _("numpy.ndarray[(rows,cols,3),") + npy_format_descriptor<basic_pixel_type>::name() + _("]");
                 else if (channels == 4)
                     return _("numpy.ndarray[(rows,cols,4),") + npy_format_descriptor<basic_pixel_type>::name() + _("]");
-                else
-                    DLIB_CASSERT(false,"unsupported pixel type");
+            };
+
+            static PYBIND11_DESCR name() {
+                constexpr size_t channels = dlib::pixel_traits<pixel_type>::num;
+                // The reason we have to call getname() in this wonky way is because
+                // pybind11 uses a type that records the length of the returned string in
+                // the type.  So we have to do this overloading to make the return type
+                // from name() consistent.  In C++17 this would be a lot cleaner with
+                // constexpr if, but can't use C++17 yet because of lack of wide support  :(
+                return getname<channels>(0);
             }
         };