Bug: Fix fill violating read-only flag. (numpy#22959)

PyArray_FillWithScalar checks if destination is writeable before attempting to fill it.  A relevant test is added as a method of TestRegression

Closes numpygh-22922

Co-authored-by: Sebastian Berg <sebastian@sipsolutions.net>

numpy/core/src/multiarray/convert.c

@@ -359,6 +359,11 @@ PyArray_ToString(PyArrayObject *self, NPY_ORDER order)
 NPY_NO_EXPORT int
 PyArray_FillWithScalar(PyArrayObject *arr, PyObject *obj)
 {
+
+    if (PyArray_FailUnlessWriteable(arr, "assignment destination") < 0) {
+        return -1;
+    }
+
     /*
      * If we knew that the output array has at least one element, we would
      * not actually need a helping buffer, we always null it, just in case.

numpy/core/tests/test_multiarray.py

@@ -403,6 +403,13 @@ def test_fill_struct_array(self):
         assert_array_equal(x['a'], [3.5, 3.5])
         assert_array_equal(x['b'], [-2, -2])
 
+    def test_fill_readonly(self):
+        # gh-22922
+        a = np.zeros(11)
+        a.setflags(write=False)
+        with pytest.raises(ValueError, match=".*read-only"):
+            a.fill(0)
+
 
 class TestArrayConstruction:
     def test_array(self):