TST: Add test for reduceat/accumulate output shape mismatch

At least the reduceat path seems to have been untested before.
The slight change in code layout (added assert) is just to make
the code slightly easier to read.  Since otherwise it looks like
there is an additional `else` branch when `out` is given.

Author: seberg

numpy/core/src/umath/ufunc_object.c

@@ -3524,8 +3524,12 @@ PyUFunc_Reduceat(PyUFuncObject *ufunc, PyArrayObject *arr, PyArrayObject *ind,
             Py_INCREF(out);
         }
     }
-    /* Allocate the output for when there's no outer iterator */
-    else if (out == NULL) {
+    else {
+        /*
+         * Allocate the output for when there's no outer iterator, we always
+         * use the outer_iteration path when `out` is passed.
+         */
+        assert(out == NULL);
         Py_INCREF(descrs[0]);
         op[0] = out = (PyArrayObject *)PyArray_NewFromDescr(
                                     &PyArray_Type, descrs[0],

numpy/core/tests/test_ufunc.py

@@ -2155,6 +2155,22 @@ def test_reduce_noncontig_output(self):
         assert_equal(y_base[1,:], y_base_copy[1,:])
         assert_equal(y_base[3,:], y_base_copy[3,:])
 
+    @pytest.mark.parametrize("with_cast", [True, False])
+    def test_reduceat_and_accumulate_out_shape_mismatch(self, with_cast):
+        # Should raise an error mentioning "shape" or "size"
+        arr = np.arange(5)
+        out = np.arange(3)  # definitely wrong shape
+        if with_cast:
+            # If a cast is necessary on the output, we can be sure to use
+            # the generic NpyIter (non-fast) path.
+            out = out.astype(np.float64)
+
+        with pytest.raises(ValueError, match="(shape|size)"):
+            np.add.reduceat(arr, [0, 3], out=out)
+
+        with pytest.raises(ValueError, match="(shape|size)"):
+            np.add.accumulate(arr, out=out)
+
     @pytest.mark.parametrize('out_shape',
                              [(), (1,), (3,), (1, 1), (1, 3), (4, 3)])
     @pytest.mark.parametrize('keepdims', [True, False])