Since dtype_file_extent_fn is likely to be written by an expert,
I'd prefer not having any restrictions, so that
MPI_File_get_type_extent can work as it's supposed to.
However, I think it is sufficient for dtype_file_extent_fn to work
only for basic datatypes. Even etype is not needed, because if etype
is derived, the MPI-IO implementation anyhow needs to work in terms of
the basic datatypes within the etype. The extent of the etype can be
calculated from that. Similarly, basic datatypes are sufficient for
MPI_File_get_type_extent to figure out the rest.
Therefore, I think we should require that dtype_file_extent_fn works
for all basic datatypes only.
Rajeev