BUG: Ensure nditer always adds necessary casts (and tiny simplification)

Author: seberg

numpy/_core/src/multiarray/mapping.c

@@ -2035,7 +2035,6 @@ array_assign_subscript(PyArrayObject *self, PyObject *ind, PyObject *op)
         goto fail;
     }
 
-    int allocated_array = 0;
     if (tmp_arr == NULL) {
         /* Fill extra op, need to swap first */
         tmp_arr = mit->extra_op;
@@ -2049,7 +2048,11 @@ array_assign_subscript(PyArrayObject *self, PyObject *ind, PyObject *op)
         if (PyArray_CopyObject(tmp_arr, op) < 0) {
              goto fail;
         }
-        allocated_array = 1;
+        /*
+         * In this branch we copy directly from a newly allocated array which
+         * may have a new descr:
+         */
+        descr = PyArray_DESCR(tmp_arr);
     }
 
     if (PyArray_MapIterCheckIndices(mit) < 0) {
@@ -2097,8 +2100,7 @@ array_assign_subscript(PyArrayObject *self, PyObject *ind, PyObject *op)
         // for non-REFCHK user DTypes. See gh-27057 for the prior discussion about this.
         if (PyArray_GetDTypeTransferFunction(
                 1, itemsize, itemsize,
-                allocated_array ? PyArray_DESCR(mit->extra_op) : PyArray_DESCR(self),
-                PyArray_DESCR(self),
+                descr, PyArray_DESCR(self),
                 0, &cast_info, &transfer_flags) != NPY_SUCCEED) {
             goto fail;
         }

numpy/_core/src/multiarray/nditer_constr.c

@@ -1315,8 +1315,10 @@ npyiter_check_casting(int nop, PyArrayObject **op,
         printf("\n");
 #endif
         /* If the types aren't equivalent, a cast is necessary */
-        if (op[iop] != NULL && !PyArray_EquivTypes(PyArray_DESCR(op[iop]),
-                                                     op_dtype[iop])) {
+        npy_intp view_offset = NPY_MIN_INTP;
+        if (op[iop] != NULL && !(PyArray_SafeCast(
+                    PyArray_DESCR(op[iop]), op_dtype[iop], &view_offset,
+                    NPY_NO_CASTING, 1) && view_offset == 0)) {
             /* Check read (op -> temp) casting */
             if ((op_itflags[iop] & NPY_OP_ITFLAG_READ) &&
                         !PyArray_CanCastArrayTo(op[iop],