BUG: Fix arr.flat.index for large or big-endian machines

The type read when exposing was previously int, but has to be
intp.  this would only be visible for >2**31 elements, but is
also visible on big-endian machines.

Closes gh-19153

numpy/core/src/multiarray/iterators.c

@@ -1063,13 +1063,15 @@ static PyMemberDef iter_members[] = {
         T_OBJECT,
         offsetof(PyArrayIterObject, ao),
         READONLY, NULL},
-    {"index",
-        T_INT,
-        offsetof(PyArrayIterObject, index),
-        READONLY, NULL},
     {NULL, 0, 0, 0, NULL},
 };
 
+static PyObject *
+iter_index_get(PyArrayIterObject *self)
+{
+    return PyArray_PyIntFromIntp(self->index);
+}
+
 static PyObject *
 iter_coords_get(PyArrayIterObject *self)
 {
@@ -1096,10 +1098,12 @@ iter_coords_get(PyArrayIterObject *self)
 }
 
 static PyGetSetDef iter_getsets[] = {
+    {"index",
+        (getter)iter_index_get,
+        NULL, NULL, NULL},
     {"coords",
         (getter)iter_coords_get,
-        NULL,
-        NULL, NULL},
+        NULL, NULL, NULL},
     {NULL, NULL, NULL, NULL, NULL},
 };
 

numpy/core/tests/test_multiarray.py

@@ -5364,6 +5364,17 @@ def test_refcount(self):
             assert_(abs(sys.getrefcount(ind) - rc_ind) < 50)
             assert_(abs(sys.getrefcount(indtype) - rc_indtype) < 50)
 
+    def test_index_getset(self):
+        it = np.arange(10).reshape(2, 1, 5).flat
+        with pytest.raises(AttributeError):
+            it.index = 10
+
+        for _ in it:
+            pass
+        # Check the value of `.index` is updated correctly (see also gh-19153)
+        # If the type was incorrect, this would show up on big-endian machines
+        assert it.index == it.base.size
+
 
 class TestResize: