Merge pull request #20793 from seberg/evil-reducelike-no-value-based

BUG: Fix that reducelikes honour out always (and live int he future)

1 files changed, 18 insertions, 0 deletions

diff --git a/numpy/core/src/umath/ufunc_object.c b/numpy/core/src/umath/ufunc_object.c
index 415ff0f07..2674c85d9 100644
--- a/numpy/core/src/umath/ufunc_object.c
+++ b/numpy/core/src/umath/ufunc_object.c
@@ -2720,6 +2720,21 @@ reducelike_promote_and_resolve(PyUFuncObject *ufunc,
      * is NULL) so we pass `arr` instead in that case.
      */
     PyArrayObject *ops[3] = {out ? out : arr, arr, out};
+
+    /*
+     * TODO: This is a dangerous hack, that works by relying on the GIL, it is
+     *       terrible, terrifying, and trusts that nobody does crazy stuff
+     *       in their type-resolvers.
+     *       By mutating the `out` dimension, we ensure that reduce-likes
+     *       live in a future without value-based promotion even when legacy
+     *       promotion has to be used.
+     */
+    npy_bool evil_ndim_mutating_hack = NPY_FALSE;
+    if (out != NULL && PyArray_NDIM(out) == 0 && PyArray_NDIM(arr) != 0) {
+        evil_ndim_mutating_hack = NPY_TRUE;
+        ((PyArrayObject_fields *)out)->nd = 1;
+    }
+
     /*
      * TODO: If `out` is not provided, arguably `initial` could define
      *       the first DType (and maybe also the out one), that way
@@ -2740,6 +2755,9 @@ reducelike_promote_and_resolve(PyUFuncObject *ufunc,
 
     PyArrayMethodObject *ufuncimpl = promote_and_get_ufuncimpl(ufunc,
             ops, signature, operation_DTypes, NPY_FALSE, NPY_TRUE, NPY_TRUE);
+    if (evil_ndim_mutating_hack) {
+        ((PyArrayObject_fields *)out)->nd = 0;
+    }
     /* DTypes may currently get filled in fallbacks and XDECREF for error: */
     Py_XDECREF(operation_DTypes[0]);
     Py_XDECREF(operation_DTypes[1]);