BUG: Fix incorrect return type in reduce without initial value

The return type should not be integer of course (why doesn't C warn
about this?).

Closes gh-20921

numpy/core/src/umath/reduction.c

@@ -68,7 +68,7 @@ count_axes(int ndim, const npy_bool *axis_flags)
  * Returns -1 if an error occurred, and otherwise the reduce arrays size,
  * which is the number of elements already initialized.
  */
-NPY_NO_EXPORT int
+static npy_intp
 PyArray_CopyInitialReduceValues(
                     PyArrayObject *result, PyArrayObject *operand,
                     const npy_bool *axis_flags, const char *funcname,

numpy/core/tests/test_ufunc.py

@@ -14,6 +14,7 @@
     assert_almost_equal, assert_array_almost_equal, assert_no_warnings,
     assert_allclose, HAS_REFCOUNT, suppress_warnings
     )
+from numpy.testing._private.utils import requires_memory
 from numpy.compat import pickle
 
 
@@ -1555,6 +1556,17 @@ def check_identityless_reduction(self, a):
                                     [[0, 1, 1], [1, 1, 1]])
         assert_equal(np.minimum.reduce(a, axis=()), a)
 
+    @requires_memory(6 * 1024**3)
+    def test_identityless_reduction_huge_array(self):
+        # Regression test for gh-20921 (copying identity incorrectly failed)
+        arr = np.zeros((2, 2**31), 'uint8')
+        arr[:, 0] = [1, 3]
+        arr[:, -1] = [4, 1]
+        res = np.maximum.reduce(arr, axis=0)
+        del arr
+        assert res[0] == 3
+        assert res[-1] == 4
+
     def test_identityless_reduction_corder(self):
         a = np.empty((2, 3, 4), order='C')
         self.check_identityless_reduction(a)