Vector2, Vector3 and Matrix3 define their own MPITrait::type functions that return an MPI_Datatype for them. It is created the first time the function is called and put into a static variable. It is therefore destroyed after the main() function exits, which is usually after MPI_Finalize has been called. Freeing the MPI_Datatype is therefore not possible and mpi::Datatype cannot be used.
Reported by @stewart.