Switch away from typenums for numpy descriptors

include/pybind11/numpy.h

@@ -91,9 +91,7 @@ public:
 
     template <typename Type> array(size_t size, const Type *ptr) {
         API& api = lookup_api();
-        PyObject *descr = api.PyArray_DescrFromType_(detail::npy_format_descriptor<Type>::typenum());
-        if (descr == nullptr)
-            pybind11_fail("NumPy: unsupported buffer format!");
+        PyObject *descr = detail::npy_format_descriptor<Type>::descr();
         Py_intptr_t shape = (Py_intptr_t) size;
         object tmp = object(api.PyArray_NewFromDescr_(
             api.PyArray_Type_, descr, 1, &shape, nullptr, (void *) ptr, 0, nullptr), false);
@@ -128,6 +126,8 @@ protected:
         static API api = API::lookup();
         return api;
     }
+
+    template <typename T, typename SFINAE> friend struct detail::npy_format_descriptor;
 };
 
 template <typename T, int ExtraFlags = array::forcecast> class array_t : public array {
@@ -140,7 +140,7 @@ public:
         if (ptr == nullptr)
             return nullptr;
         API &api = lookup_api();
-        PyObject *descr = api.PyArray_DescrFromType_(detail::npy_format_descriptor<T>::typenum());
+        PyObject *descr = detail::npy_format_descriptor<T>::descr();
         PyObject *result = api.PyArray_FromAny_(ptr, descr, 0, 0, API::NPY_ENSURE_ARRAY_ | ExtraFlags, nullptr);
         if (!result)
             PyErr_Clear();
@@ -158,6 +158,10 @@ private:
         array::API::NPY_INT_,  array::API::NPY_UINT_,  array::API::NPY_LONGLONG_, array::API::NPY_ULONGLONG_ };
 public:
     static int typenum() { return values[detail::log2(sizeof(T)) * 2 + (std::is_unsigned<T>::value ? 1 : 0)]; }
+    static PyObject* descr() {
+        if (auto obj = array::lookup_api().PyArray_DescrFromType_(typenum())) return obj;
+        else pybind11_fail("Unsupported buffer format!");
+    }
     template <typename T2 = T, typename std::enable_if<std::is_signed<T2>::value, int>::type = 0>
     static PYBIND11_DESCR name() { return _("int") + _<sizeof(T)*8>(); }
     template <typename T2 = T, typename std::enable_if<!std::is_signed<T2>::value, int>::type = 0>
@@ -167,7 +171,11 @@ template <typename T> constexpr const int npy_format_descriptor<
     T, typename std::enable_if<std::is_integral<T>::value>::type>::values[8];
 
 #define DECL_FMT(Type, NumPyName, Name) template<> struct npy_format_descriptor<Type> { \
-    static int typenum() { return array::API::NumPyName; }         \
+    static int typenum() { return array::API::NumPyName; } \
+    static PyObject* descr() { \
+        if (auto obj = array::lookup_api().PyArray_DescrFromType_(typenum())) return obj; \
+        else pybind11_fail("Unsupported buffer format!"); \
+    } \
     static PYBIND11_DESCR name() { return _(Name); } }
 DECL_FMT(float, NPY_FLOAT_, "float32");
 DECL_FMT(double, NPY_DOUBLE_, "float64");