ACCUMULATE, unlike REDUCE, can use with derived
datatypes with predefinied operations, with some
restrictions outlined in MPI-3:11.3.4. The derived
datatype must be composed entierly from one predefined
datatype (so you can do all the construction you want,
but at the bottom, you can only use one datatype, say,
MPI_INT).
Refs. open-mpi/ompi#6275
Signed-off-by: Gilles Gouaillardet <gilles@rist.or.jp>
(back-ported from commit open-mpi/ompi@bc1cab5498)
f76c81a758