From 975a94623b1c84979ef9bfe620745299f420315a Mon Sep 17 00:00:00 2001
From: Sebastian Berg <sebastian@sipsolutions.net>
Date: Tue, 4 Jan 2022 14:53:23 -0800
Subject: BUG: Relax dtype identity check in reductions

In some cases, e.g. ensure-native-byte-order will return not the
default, but a copy of the descriptor.

This (and maybe metadata) makes it somewhat annoying to ensure
exact identity between descriptors for reduce "operands" as
returned by the resolve-descirptors method of the ArrayMethod.

To avoid this problem, we check for no-casting (which implies
viewable with `offset == 0`) rather than strict identity.

Unfortunately, this means that descriptor resolution must be slightly
more careful, but in general this should all be perfectly well
defined.

Closes gh-20699
---
 numpy/core/src/umath/ufunc_object.c |  8 ++++++--
 numpy/core/tests/test_ufunc.py      | 11 +++++++++++
 2 files changed, 17 insertions(+), 2 deletions(-)

diff --git a/numpy/core/src/umath/ufunc_object.c b/numpy/core/src/umath/ufunc_object.c
index 52b354353..83e18a363 100644
--- a/numpy/core/src/umath/ufunc_object.c
+++ b/numpy/core/src/umath/ufunc_object.c
@@ -2764,9 +2764,13 @@ reducelike_promote_and_resolve(PyUFuncObject *ufunc,
      * The first operand and output should be the same array, so they should
      * be identical.  The second argument can be different for reductions,
      * but is checked to be identical for accumulate and reduceat.
+     * Ideally, the type-resolver ensures that all are identical, but we do
+     * not enforce this here strictly.  Otherwise correct handling of
+     * byte-order changes (or metadata) requires a lot of care; see gh-20699.
      */
-    if (out_descrs[0] != out_descrs[2] || (
-            enforce_uniform_args && out_descrs[0] != out_descrs[1])) {
+    if (!PyArray_EquivTypes(out_descrs[0], out_descrs[2]) || (
+            enforce_uniform_args && !PyArray_EquivTypes(
+                    out_descrs[0], out_descrs[1]))) {
         PyErr_Format(PyExc_TypeError,
                 "the resolved dtypes are not compatible with %s.%s. "
                 "Resolved (%R, %R, %R)",
diff --git a/numpy/core/tests/test_ufunc.py b/numpy/core/tests/test_ufunc.py
index 76e4cdcfd..9a9d46da0 100644
--- a/numpy/core/tests/test_ufunc.py
+++ b/numpy/core/tests/test_ufunc.py
@@ -2148,6 +2148,17 @@ class TestUfunc:
             # It would be safe, but not equiv casting:
             ufunc(a, c, out=out, casting="equiv")
 
+    def test_reducelike_byteorder_resolution(self):
+        # See gh-20699, byte-order changes need some extra care in the type
+        # resolution to make the following succeed:
+        arr_be = np.arange(10, dtype=">i8")
+        arr_le = np.arange(10, dtype="<i8")
+
+        assert np.add.reduce(arr_be) == np.add.reduce(arr_le)
+        assert_array_equal(np.add.accumulate(arr_be), np.add.accumulate(arr_le))
+        assert_array_equal(
+            np.add.reduceat(arr_be, [1]), np.add.reduceat(arr_le, [1]))
+
     def test_reducelike_out_promotes(self):
         # Check that the out argument to reductions is considered for
         # promotion.  See also gh-20455.
-- 
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