BUG: Fix assert in nditer buffer setup

When using a buffered iteration, there was a new assert to check
that strides were set up nicely to ensure that we are not missing
dimension coalescing.

However, when the strides are set up to track an index, then they
were still set up with a 0 for length 1.
I _think_ that is just unnecessary (i.e. the assert is correct to
point it out).  But, let's do this first for backporting at least.

Signed-off-by: Sebastian Berg <[email protected]>

Author: seberg

numpy/_core/src/multiarray/nditer_constr.c

@@ -2077,8 +2077,14 @@ npyiter_find_buffering_setup(NpyIter *iter, npy_intp buffersize)
         npy_intp *strides = NAD_STRIDES(axisdata);
 
         for (int iop = 0; iop < nop; iop++) {
-            /* Check that we set things up nicely (if shape is ever 1) */
-            assert((axisdata->shape == 1) ? (prev_strides[iop] == strides[iop]) : 1);
+            /*
+             * Check that we set things up nicely so strides coalesc.  Except
+             * for index operands, which currently disrupts coalescing.
+             * NOTE(seberg): presumably `npyiter_compute_index_strides` should
+             * not set the strides to 0, but this was safer for backporting.
+             */
+            assert((axisdata->shape != 1) || (prev_strides[iop] == strides[iop])
+                   || (op_itflags[iop] & (NPY_ITER_C_INDEX|NPY_ITER_F_INDEX)));
 
             if (op_single_stride_dims[iop] == idim) {
                 /*  Best case: the strides still collapse for this operand. */

numpy/lib/tests/test_index_tricks.py

@@ -199,6 +199,11 @@ def test_empty_array_unravel(self):
         with assert_raises(ValueError):
             np.unravel_index([1], (2, 1, 0))
 
+    def test_regression_size_1_index(self):
+        # actually tests the nditer size one index tracking
+        # regression test for gh-29690
+        np.unravel_index(np.array([[1, 0, 1, 0]], dtype=np.uint32), (4,))
+
 class TestGrid:
     def test_basic(self):
         a = mgrid[-1:1:10j]