array: allow index narrowing

src/array.hpp

@@ -410,7 +410,7 @@ public:
             static_assert(sizeof...(idxs) <= Shape::ndim,
                           "number of indices must be smaller than the number of dimensions");
 
-            const std::array<size_t, Shape::ndim> m{idxs...};
+            const std::array<size_t, Shape::ndim> m{static_cast<size_t>(idxs)...};
             size_t idx = 0;
 
             for (size_t i = 0; i < sizeof...(idxs); ++i) {