Document drop index ordering and reset reused slice offsets

docs/source/view.rst

@@ -30,6 +30,12 @@ Slices can be specified in the following ways:
 - :cpp:func:`xt::keep(i0, i1, i2, ...) <xt::keep>` a slice selecting non-contiguous indices to keep on the underlying expression
 - :cpp:func:`xt::drop(i0, i1, i2, ...) <xt::drop>` a slice selecting non-contiguous indices to drop on the underlying expression
 
+For ``xt::drop``, indices must be in increasing order after resolving negative indices
+relative to the sliced axis. For an axis of size 6, ``xt::drop(0, -1)`` gives the ordered
+positions 0 and 5, whereas ``xt::drop(-1, 0)`` does not satisfy this requirement.
+The library does not sort or check the order. When reusing a slice with a different
+axis size, its resolved indices must still satisfy this requirement.
+
 .. code::
 
     #include <vector>

include/xtensor/views/xslice.hpp

@@ -478,6 +478,9 @@ namespace xt
      * xt::view(a, xt::drop(0, 2); // => {{3, 4, 5}}
      * @endcode
      *
+     * The indices must be in increasing order after resolving negative indices
+     * relative to the sliced axis. The library does not sort or check their order.
+     *
      * @param indices The container of indices to drop
      * @return instance of xdrop_slice
      */
@@ -1506,6 +1509,7 @@ namespace xt
             m_indices[i] = m_raw_indices[i] < 0 ? static_cast<size_type>(shape) + m_raw_indices[i]
                                                 : m_raw_indices[i];
         }
+        m_inc.clear();
         size_type cum = size_type(0);
         size_type prev_cum = cum;
         for (std::size_t i = 0; i < sz; ++i)

test/test_xview.cpp

@@ -1259,6 +1259,15 @@ namespace xt
         EXPECT_EQ(a(1, 1, 1), 456);
     }
 
+    TEST(xview, drop_normalize_reuse)
+    {
+        auto slice = drop(-2, -1);
+        slice.normalize(4);
+        EXPECT_EQ(slice(1), 1);
+        slice.normalize(6);
+        EXPECT_EQ(slice(2), 2);
+    }
+
     TEST(xview, drop_negative)
     {
         xtensor<double, 3, layout_type::row_major> a = {