new more complet fix. Now we check that we don't downcast at this place too.

theano/tensor/blas.py

@@ -1113,23 +1113,15 @@ def _gemm_from_factored_list(lst):
     """Returns None, or a list to replace node.outputs
     """
 
-    # Make every pair in list have matching dtypes
-    def is_pair(sM):
-        try:
-            s, M = sM
-            return True
-        except Exception:
-            return False
-
     lst2 = []
     # Remove the tuple that can't be casted correctly.
     # This can happen when we try to cast a complex to a real
     for sM in lst:
-        if is_pair(sM):
-            try:
-                lst2.append(T.cast(sM[0],sM[1].type.dtype), sM[1])
-            except TypeError:
-                pass
+        if len(sM) == 2:
+            sm0, sm1 = sM
+            sm0 = T.as_tensor_variable(sm0)
+            if theano.scalar.upcast(sm0.dtype, sm1.dtype) == sm1.dtype:
+                lst2.append((T.cast(sm0, sm1.dtype), sM[1]))
     lst = lst2
 
     # Try every pair in the sM_list, trying to turn it into a gemm operation