From 42d3104fa72d4c461c7817646501bdb4b9eb7e30 Mon Sep 17 00:00:00 2001
From: Marten van Kerkwijk <mhvk@astro.utoronto.ca>
Date: Sat, 7 Oct 2023 13:18:30 +0200
Subject: [PATCH] Ensure mask calculation is correct also for multi-op ufunc
 with scalars

---
 astropy/utils/masked/core.py                 |  4 +++-
 astropy/utils/masked/tests/test_functions.py | 22 ++++++++++++++++++++
 docs/changes/utils/15450.bugfix.rst          |  2 ++
 3 files changed, 27 insertions(+), 1 deletion(-)
 create mode 100644 docs/changes/utils/15450.bugfix.rst

diff --git a/astropy/utils/masked/core.py b/astropy/utils/masked/core.py
index aa16e51354a..7cf2edcf62b 100644
--- a/astropy/utils/masked/core.py
+++ b/astropy/utils/masked/core.py
@@ -697,7 +697,9 @@ def _combine_masks(self, masks, out=None, where=True, copy=True):
                 np.copyto(out, masks[0], where=where)
                 return out
 
-        out = np.logical_or(masks[0], masks[1], out=out, where=where)
+        # [...] at the end to ensure we have an array, not a scalar, and
+        # thus can be used for in-place changes in the loop.
+        out = np.logical_or(masks[0], masks[1], out=out, where=where)[...]
         for mask in masks[2:]:
             np.logical_or(out, mask, out=out, where=where)
         return out
diff --git a/astropy/utils/masked/tests/test_functions.py b/astropy/utils/masked/tests/test_functions.py
index 9bdc3ccba67..9ace08958f6 100644
--- a/astropy/utils/masked/tests/test_functions.py
+++ b/astropy/utils/masked/tests/test_functions.py
@@ -5,6 +5,7 @@
 coverage.  Complete coverage of all numpy functions is done
 with less detailed tests in test_function_helpers.
 """
+import erfa.ufunc as erfa_ufunc
 import numpy as np
 import pytest
 from numpy.testing import assert_array_equal
@@ -151,6 +152,27 @@ def test_3op_ufunc(self):
         assert_array_equal(ma_mb.unmasked, expected_data)
         assert_array_equal(ma_mb.mask, expected_mask)
 
+    def test_multi_op_ufunc(self):
+        mask = [True, False, False]
+        iy = Masked([2000, 2001, 2002], mask=mask)
+        im = Masked([1, 2, 3], mask=mask)
+        idy = Masked([10, 20, 25], mask=mask)
+        ihr = Masked([11, 12, 13], mask=[False, False, True])
+        imn = np.array([50, 51, 52])
+        isc = np.array([12.5, 13.6, 14.7])
+        result = erfa_ufunc.dtf2d("utc", iy, im, idy, ihr, imn, isc)
+        # Also test scalar
+        result0 = erfa_ufunc.dtf2d("utc", iy[0], im[0], idy[0], ihr[0], imn[0], isc[0])
+        expected = erfa_ufunc.dtf2d(
+            "utc", iy.unmasked, im.unmasked, idy.unmasked, ihr.unmasked, imn, isc
+        )
+        expected_mask = np.array([True, False, True])
+        for res, res0, exp in zip(result, result0, expected):
+            assert_array_equal(res.unmasked, exp)
+            assert_array_equal(res.mask, expected_mask)
+            assert res0.unmasked == exp[0]
+            assert res0.mask == expected_mask[0]
+
     @pytest.mark.parametrize("axis", (0, 1, None))
     def test_add_reduce(self, axis):
         ma_reduce = np.add.reduce(self.ma, axis=axis)
diff --git a/docs/changes/utils/15450.bugfix.rst b/docs/changes/utils/15450.bugfix.rst
new file mode 100644
index 00000000000..07e2e9f7b73
--- /dev/null
+++ b/docs/changes/utils/15450.bugfix.rst
@@ -0,0 +1,2 @@
+Ufuncs with more than 2 operands (such as ``erfa.dtf2d``) now work
+also if all inputs are scalars and more than two inputs have masks.