Add missing downcast (#1330)

Summary:
Pull Request resolved: https://github.com/facebookresearch/faiss/pull/1330

add missing downcast for Index (and test for github sync)

See github issue https://github.com/facebookresearch/faiss/issues/1278

Reviewed By: beauby

Differential Revision: D23053182

fbshipit-source-id: 1ce4c248342332ce632ecfd5074affa3ddf55b66

faiss/python/swigfaiss.swig

@@ -579,6 +579,7 @@ struct AsyncIndexSearchC {
     DOWNCAST ( IndexIVFFlat )
     DOWNCAST ( IndexIVF )
     DOWNCAST ( IndexFlat )
+    DOWNCAST ( IndexRefineFlat )
     DOWNCAST ( IndexPQ )
     DOWNCAST ( IndexScalarQuantizer )
     DOWNCAST ( IndexLSH )

tests/test_build_blocks.py

@@ -615,6 +615,18 @@ class TestSWIGWrap(unittest.TestCase):
 
         [index.id_map.at(int(i)) for i in range(index.ntotal)]
 
+    def test_downcast_Refine(self):
+
+        index = faiss.IndexRefineFlat(
+            faiss.IndexScalarQuantizer(10, faiss.ScalarQuantizer.QT_8bit)
+        )
+
+        # serialize and deserialize
+        index2 = faiss.deserialize_index(
+            faiss.serialize_index(index)
+        )
+
+        assert isinstance(index2, faiss.IndexRefineFlat)
 
 if __name__ == '__main__':
     unittest.main()