make void* the generic spanning type of all pointer types

Cython/Compiler/PyrexTypes.py

@@ -3733,9 +3733,6 @@ def independent_spanning_type(type1, type2):
         return py_object_type
     span_type = _spanning_type(type1, type2)
     if span_type is None:
-        if type1.is_ptr and type2.is_ptr:
-            # incompatible pointers, void* will do as a result
-            return c_void_ptr_type
         return error_type
     return span_type
 
@@ -3777,6 +3774,9 @@ def _spanning_type(type1, type2):
             # external types are unsafe, so we use PyObject instead
             return py_object_type
         return type2
+    elif type1.is_ptr and type2.is_ptr:
+        # incompatible pointers, void* will do as a result
+        return c_void_ptr_type
     else:
         return None