Merge pull request numpy#29780 from seberg/issue-29690

BUG: Fix assert in nditer buffer setup

Author: mattip

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]