BUG: VOID_nonzero could sometimes mutate alignment flag

This fixes that invocations of `VOID_nonzero` could flip the
alignment flag on the original array even though the original
array is not modified.

numpy/core/src/multiarray/arraytypes.c.src

@@ -2759,10 +2759,10 @@ VOID_nonzero (char *ip, PyArrayObject *ap)
             dummy_fields.descr = new;
             if ((new->alignment > 1) && !__ALIGNED(ip + offset,
                         new->alignment)) {
-                PyArray_CLEARFLAGS(ap, NPY_ARRAY_ALIGNED);
+                PyArray_CLEARFLAGS(dummy_arr, NPY_ARRAY_ALIGNED);
             }
             else {
-                PyArray_ENABLEFLAGS(ap, NPY_ARRAY_ALIGNED);
+                PyArray_ENABLEFLAGS(dummy_arr, NPY_ARRAY_ALIGNED);
             }
             if (new->f->nonzero(ip+offset, dummy_arr)) {
                 nonz = NPY_TRUE;

numpy/core/tests/test_numeric.py

@@ -1497,6 +1497,18 @@ def __bool__(self):
         a = np.array([[False], [TrueThenFalse()]])
         assert_raises(RuntimeError, np.nonzero, a)
 
+    def test_nonzero_sideffects_structured_void(self):
+        # Checks that structured void does not mutate alignment flag of
+        # original array.
+        arr = np.zeros(5, dtype="i1,i8,i8")  # `ones` may short-circuit
+        assert arr.flags.aligned  # structs are considered "aligned"
+        assert not arr["f2"].flags.aligned
+        # make sure that nonzero/count_nonzero do not flip the flag:
+        np.nonzero(arr)
+        assert arr.flags.aligned
+        np.count_nonzero(arr)
+        assert arr.flags.aligned
+
     def test_nonzero_exception_safe(self):
         # gh-13930