MPI_DOUBLE_COMPLEX
From experiments with dumpi it looks like using MPI_DOUBLE_COMPLEX results in some user-defined MPI types, at least with some combinations of compiler and MPI (eg. gcc 9.2, + openmpi 2.1.6 on JLSE). A test using MPI_DOUBLE and 2x number of elements seems to work and remove the appearance of user-defined MPI types. We may consider this kind of switch since user-defined MPI types can cause headaches.