These are the packages that would be merged, in order:

Calculating dependencies ... done!
Dependency resolution took 0.89 s (backtrack: 0/20).


The following USE changes are necessary to proceed:
 (see "package.use" in the portage(5) man page for more details)
# required by sci-physics/rivet-4.1.3::gentoo[highfive]
# required by =sci-physics/rivet-4.1.3 (argument)
>=sci-libs/hdf5-2.2.0 cxx

!!! The ebuild selected to satisfy "sci-libs/hdf5[cxx]" has unmet requirements.
- sci-libs/hdf5-2.2.0::gentoo USE="hl threads zlib -cxx -debug -fortran -mpi -szip -test -unsupported"

  The following REQUIRED_USE flag constraints are unsatisfied:
    !unsupported? ( threads? ( !cxx !hl ) )

  The above constraints are a subset of the following complete expression:
    !unsupported? ( at-most-one-of ( cxx mpi ) threads? ( !cxx !mpi !fortran !hl ) )

(dependency required by "sci-physics/rivet-4.1.3::gentoo[highfive]" [ebuild])
(dependency required by "=sci-physics/rivet-4.1.3" [argument])