BUG: Fix cast safety and comparisons for zero sized voids

numpy/core/src/multiarray/arrayobject.c

@@ -41,6 +41,7 @@ maintainer email:  oliphant.travis@ieee.org
 #include "arraytypes.h"
 #include "scalartypes.h"
 #include "arrayobject.h"
+#include "convert_datatype.h"
 #include "conversion_utils.h"
 #include "ctors.h"
 #include "dtypemeta.h"
@@ -1390,9 +1391,13 @@ array_richcompare(PyArrayObject *self, PyObject *other, int cmp_op)
                 return Py_NotImplemented;
             }
 
-            _res = PyArray_CanCastTypeTo(PyArray_DESCR(self),
-                                         PyArray_DESCR(array_other),
-                                         NPY_EQUIV_CASTING);
+            _res = PyArray_CheckCastSafety(
+                    NPY_EQUIV_CASTING,
+                    PyArray_DESCR(self), PyArray_DESCR(array_other), NULL);
+            if (_res < 0) {
+                PyErr_Clear();
+                _res = 0;
+            }
             if (_res == 0) {
                 /* 2015-05-07, 1.10 */
                 Py_DECREF(array_other);
@@ -1441,9 +1446,13 @@ array_richcompare(PyArrayObject *self, PyObject *other, int cmp_op)
                 return Py_NotImplemented;
             }
 
-            _res = PyArray_CanCastTypeTo(PyArray_DESCR(self),
-                                         PyArray_DESCR(array_other),
-                                         NPY_EQUIV_CASTING);
+            _res = PyArray_CheckCastSafety(
+                    NPY_EQUIV_CASTING,
+                    PyArray_DESCR(self), PyArray_DESCR(array_other), NULL);
+            if (_res < 0) {
+                PyErr_Clear();
+                _res = 0;
+            }
             if (_res == 0) {
                 /* 2015-05-07, 1.10 */
                 Py_DECREF(array_other);

numpy/core/src/multiarray/convert_datatype.c

@@ -457,7 +457,7 @@ PyArray_GetCastSafety(
  *        is ignored).
  * @return 0 for an invalid cast, 1 for a valid and -1 for an error.
  */
-static int
+NPY_NO_EXPORT int
 PyArray_CheckCastSafety(NPY_CASTING casting,
         PyArray_Descr *from, PyArray_Descr *to, PyArray_DTypeMeta *to_dtype)
 {
@@ -2841,6 +2841,10 @@ cast_to_void_dtype_class(
     loop_descrs[1]->elsize = given_descrs[0]->elsize;
     Py_INCREF(given_descrs[0]);
     loop_descrs[0] = given_descrs[0];
+    if (loop_descrs[0]->type_num == NPY_VOID &&
+            loop_descrs[0]->subarray == NULL && loop_descrs[1]->names == NULL) {
+        return NPY_NO_CASTING | _NPY_CAST_IS_VIEW;
+    }
     return NPY_SAFE_CASTING | _NPY_CAST_IS_VIEW;
 }
 

numpy/core/src/multiarray/convert_datatype.h

@@ -71,6 +71,10 @@ NPY_NO_EXPORT NPY_CASTING
 PyArray_GetCastSafety(
         PyArray_Descr *from, PyArray_Descr *to, PyArray_DTypeMeta *to_dtype);
 
+NPY_NO_EXPORT int
+PyArray_CheckCastSafety(NPY_CASTING casting,
+        PyArray_Descr *from, PyArray_Descr *to, PyArray_DTypeMeta *to_dtype);
+
 NPY_NO_EXPORT NPY_CASTING
 legacy_same_dtype_resolve_descriptors(
         PyArrayMethodObject *self,

numpy/core/tests/test_casting_unittests.py

@@ -147,6 +147,9 @@ def test_to_void(self):
         assert not np.can_cast("U1", "V1")
         # Structured to unstructured is just like any other:
         assert np.can_cast("d,i", "V", casting="same_kind")
+        # Unstructured void to unstructured is actually no cast at all:
+        assert np.can_cast("V3", "V", casting="no")
+        assert np.can_cast("V0", "V", casting="no")
 
 
 class TestCasting:

numpy/core/tests/test_numeric.py

@@ -1724,6 +1724,22 @@ def test_array_equiv(self):
         assert_(not res)
         assert_(type(res) is bool)
 
+    @pytest.mark.parametrize("dtype", ["V0", "V3", "V10"])
+    def test_compare_unstructured_voids(self, dtype):
+        zeros = np.zeros(3, dtype=dtype)
+
+        assert_array_equal(zeros, zeros)
+        assert not (zeros != zeros).any()
+
+        if dtype == "V0":
+            # Can't test != of actually different data
+            return
+
+        nonzeros = np.array([b"1", b"2", b"3"], dtype=dtype)
+
+        assert not (zeros == nonzeros).any()
+        assert (zeros != nonzeros).all()
+
 
 def assert_array_strict_equal(x, y):
     assert_array_equal(x, y)