BUG: Fix segmentation fault (#21436)

numpy/core/src/umath/ufunc_object.c

@@ -5884,6 +5884,13 @@ ufunc_at(PyUFuncObject *ufunc, PyObject *args)
 
     NPY_BEGIN_THREADS_DEF;
 
+    if (ufunc->core_enabled) {
+        PyErr_Format(PyExc_TypeError,
+            "%s.at does not support ufunc with non-trivial signature: %s has signature %s.",
+            ufunc->name, ufunc->name, ufunc->core_signature);
+        return NULL;
+    }
+
     if (ufunc->nin > 2) {
         PyErr_SetString(PyExc_ValueError,
             "Only unary and binary ufuncs supported at this time");

numpy/core/tests/test_ufunc.py

@@ -389,7 +389,7 @@ def test_signature8(self):
         assert_equal(ixs, (0, 0, 0, 1, 2))
         assert_equal(flags, (self.can_ignore, self.size_inferred, 0))
         assert_equal(sizes, (3, -1, 9))
-    
+
     def test_signature9(self):
         enabled, num_dims, ixs, flags, sizes = umt.test_signature(
             1, 1, "(  3)  -> ( )")
@@ -2002,6 +2002,20 @@ def test_inplace_fancy_indexing(self):
         # Test multiple output ufuncs raise error, gh-5665
         assert_raises(ValueError, np.modf.at, np.arange(10), [1])
 
+        # Test maximum
+        a = np.array([1, 2, 3])
+        np.maximum.at(a, [0], 0)
+        assert_equal(np.array([1, 2, 3]), a)
+
+    def test_at_not_none_signature(self):
+        # Test ufuncs with non-trivial signature raise a TypeError
+        a = np.ones((2, 2, 2))
+        b = np.ones((1, 2, 2))
+        assert_raises(TypeError, np.matmul.at, a, [0], b)
+
+        a = np.array([[[1, 2], [3, 4]]])
+        assert_raises(TypeError, np.linalg._umath_linalg.det.at, a, [0])
+
     def test_reduce_arguments(self):
         f = np.add.reduce
         d = np.ones((5,2), dtype=int)