minor ndarray tweak to improve an error message

include/nanobind/ndarray.h

@@ -410,21 +410,26 @@ template <typename... Args> class ndarray {
 private:
     template <typename... Ts>
     NB_INLINE int64_t byte_offset(Ts... indices) const {
-        static_assert(
-            !std::is_same_v<Scalar, void>,
+        constexpr bool has_scalar = !std::is_same_v<Scalar, void>,
+                       has_shape = !std::is_same_v<typename Info::shape_type, void>;
+
+        static_assert(has_scalar,
             "To use nb::ndarray::operator(), you must add a scalar type "
             "annotation (e.g. 'float') to the ndarray template parameters.");
-        static_assert(
-            !std::is_same_v<Scalar, void>,
+        static_assert(has_shape,
             "To use nb::ndarray::operator(), you must add a nb::shape<> "
             "annotation to the ndarray template parameters.");
-        static_assert(sizeof...(Ts) == Info::shape_type::size,
-                      "nb::ndarray::operator(): invalid number of arguments");
-        size_t counter = 0;
-        int64_t index = 0;
-        ((index += int64_t(indices) * m_dltensor.strides[counter++]), ...);
-
-        return (int64_t) m_dltensor.byte_offset + index * sizeof(typename Info::scalar_type);
+        if constexpr (has_scalar && has_shape) {
+            static_assert(sizeof...(Ts) == Info::shape_type::size,
+                          "nb::ndarray::operator(): invalid number of arguments");
+            size_t counter = 0;
+            int64_t index = 0;
+            ((index += int64_t(indices) * m_dltensor.strides[counter++]), ...);
+
+            return (int64_t) m_dltensor.byte_offset + index * sizeof(typename Info::scalar_type);
+        } else {
+            return 0;
+        }
     }
 
     detail::ndarray_handle *m_handle = nullptr;