BUG: don't override original errors when casting inside np.dot() fails

This makes things harder to debug.

2 files changed, 13 insertions, 1 deletions

diff --git a/numpy/core/src/multiarray/multiarraymodule.c b/numpy/core/src/multiarray/multiarraymodule.c
index 9e42c115c..22909ae1a 100644
--- a/numpy/core/src/multiarray/multiarraymodule.c
+++ b/numpy/core/src/multiarray/multiarraymodule.c
@@ -919,7 +919,10 @@ PyArray_MatrixProduct2(PyObject *op1, PyObject *op2, PyArrayObject* out)
     typenum = PyArray_ObjectType(op2, typenum);
     typec = PyArray_DescrFromType(typenum);
     if (typec == NULL) {
-        PyErr_SetString(PyExc_TypeError, "Cannot find a common data type.");
+        if (!PyErr_Occurred()) {
+            PyErr_SetString(PyExc_TypeError,
+                            "Cannot find a common data type.");
+        }
         return NULL;
     }
 
diff --git a/numpy/core/tests/test_multiarray.py b/numpy/core/tests/test_multiarray.py
index d1b306ef0..68fb6acf7 100644
--- a/numpy/core/tests/test_multiarray.py
+++ b/numpy/core/tests/test_multiarray.py
@@ -2693,6 +2693,15 @@ class TestMethods(object):
         assert_raises(TypeError, np.dot, c, A)
         assert_raises(TypeError, np.dot, A, c)
 
+    def test_dot_casting_fails(self):
+
+        class A(object):
+            def __array__(self, *args, **kwargs):
+                raise NotImplementedError
+
+        # Don't override the error from calling __array__()
+        assert_raises(NotImplementedError, np.dot, A(), A())
+
     def test_dot_out_mem_overlap(self):
         np.random.seed(1)