BUG: Ensure traverse info copy works also if in-place (used by nditer)

Also removes a comment that was always fishy: we should have used this
function, and we did, except for a funny little in-place bug!

Author: seberg

numpy/_core/src/multiarray/dtype_traversal.c

@@ -398,13 +398,6 @@ subarray_traverse_data_free(NpyAuxData *data)
 }
 
 
-/*
- * We seem to be neither using nor exposing this right now, so leave it NULL.
- * (The implementation below should be functional.)
- */
-#define subarray_traverse_data_clone NULL
-
-#ifndef subarray_traverse_data_clone
 /* traverse data copy function */
 static NpyAuxData *
 subarray_traverse_data_clone(NpyAuxData *data)
@@ -426,7 +419,6 @@ subarray_traverse_data_clone(NpyAuxData *data)
 
     return (NpyAuxData *)newdata;
 }
-#endif
 
 
 static int
@@ -469,7 +461,7 @@ get_subarray_traverse_func(
 
     auxdata->count = size;
     auxdata->base.free = &subarray_traverse_data_free;
-    auxdata->base.clone = subarray_traverse_data_clone;
+    auxdata->base.clone = &subarray_traverse_data_clone;
 
     if (get_traverse_func(
             traverse_context, dtype, aligned,

numpy/_core/src/multiarray/dtype_traversal.h

@@ -69,18 +69,22 @@ static inline int
 NPY_traverse_info_copy(
         NPY_traverse_info *traverse_info, NPY_traverse_info *original)
 {
-    traverse_info->func = NULL;
+    /* Note that original may be identical to traverse_info! */
     if (original->func == NULL) {
         /* Allow copying also of unused clear info */
+        traverse_info->func = NULL;
         return 0;
     }
-    traverse_info->auxdata = NULL;
     if (original->auxdata != NULL) {
         traverse_info->auxdata = NPY_AUXDATA_CLONE(original->auxdata);
         if (traverse_info->auxdata == NULL) {
+            traverse_info->func = NULL;
             return -1;
         }
     }
+    else {
+        traverse_info->auxdata = NULL;
+    }
     Py_INCREF(original->descr);
     traverse_info->descr = original->descr;
     traverse_info->func = original->func;