BUG: The reduction output must not cause the input to be broadcast

This was previously checked for, but during the refactor using
NPY_ITER_REDUCTION_AXIS to allocate the result in the iterator
instead of manually was lost here.
Einsum has a similar issue, but this is not modified here.

2 files changed, 15 insertions, 1 deletions

diff --git a/numpy/core/src/umath/reduction.c b/numpy/core/src/umath/reduction.c
index bcfbdd954..4037a4757 100644
--- a/numpy/core/src/umath/reduction.c
+++ b/numpy/core/src/umath/reduction.c
@@ -241,7 +241,8 @@ PyUFunc_ReduceWrapper(PyArrayObject *operand, PyArrayObject *out,
                   NPY_ITER_ALLOCATE |
                   NPY_ITER_NO_SUBTYPE;
     op_flags[1] = NPY_ITER_READONLY |
-                  NPY_ITER_ALIGNED;
+                  NPY_ITER_ALIGNED |
+                  NPY_ITER_NO_BROADCAST;
 
     if (wheremask != NULL) {
         op[2] = wheremask;
diff --git a/numpy/core/tests/test_ufunc.py b/numpy/core/tests/test_ufunc.py
index f7126f135..d8f407c97 100644
--- a/numpy/core/tests/test_ufunc.py
+++ b/numpy/core/tests/test_ufunc.py
@@ -1922,6 +1922,19 @@ class TestUfunc:
             assert_(check is out)
             assert_array_equal(check, correct_out)
 
+    def test_reduce_output_does_not_broadcast_input(self):
+        # Test that the output shape cannot broadcast an input dimension
+        # (it never can add dimensions, but it might expand an existing one)
+        a = np.ones((1, 10))
+        out_correct = (np.empty((1, 1)))
+        out_incorrect = np.empty((3, 1))
+        np.add.reduce(a, axis=-1, out=out_correct, keepdims=True)
+        np.add.reduce(a, axis=-1, out=out_correct[:, 0], keepdims=False)
+        with assert_raises(ValueError):
+            np.add.reduce(a, axis=-1, out=out_incorrect, keepdims=True)
+        with assert_raises(ValueError):
+            np.add.reduce(a, axis=-1, out=out_incorrect[:, 0], keepdims=False)
+
     def test_reduce_output_subclass_ok(self):
         class MyArr(np.ndarray):
             pass