$ spack spec ecp-proxy-apps
Input spec
--------------------------------
ecp-proxy-apps
Concretized
--------------------------------
==> Error: An unsatisfiable variant constraint has been detected for spec:
hdf5~mpi
^zlib@1.2.11%gcc@5.4.0+optimize+pic+shared arch=linux-gentoo2-x86_64
while trying to concretize the partial spec:
py-h5py+mpi
^openmpi@3.1.0%gcc@5.4.0~cuda fabrics= ~java~memchecker~pmi schedulers= ~sqlite3~thread_multiple+vt arch=linux-gentoo2-x86_64
^hwloc@:1.999
^pkgconf@1.4.0%gcc@5.4.0 arch=linux-gentoo2-x86_64
^numactl
^autoconf@2.69%gcc@5.4.0 arch=linux-gentoo2-x86_64
^m4@1.4.18%gcc@5.4.0+sigsegv arch=linux-gentoo2-x86_64
^libsigsegv
^perl@5.26.2%gcc@5.4.0+cpanm+shared arch=linux-gentoo2-x86_64
^gdbm@1.14.1%gcc@5.4.0 arch=linux-gentoo2-x86_64
^readline@7.0%gcc@5.4.0 arch=linux-gentoo2-x86_64
^ncurses@6.0%gcc@5.4.0~symlinks~termlib arch=linux-gentoo2-x86_64
^automake@1.16.1%gcc@5.4.0 arch=linux-gentoo2-x86_64
^libtool@2.4.6%gcc@5.4.0 arch=linux-gentoo2-x86_64
^zlib@1.2.11%gcc@5.4.0+optimize+pic+shared arch=linux-gentoo2-x86_64
^py-cython@0.28.1%gcc@5.4.0 arch=linux-gentoo2-x86_64
^python@2.7.14%gcc@5.4.0+dbm~optimizations+pic+pythoncmd+shared~tk~ucs4 arch=linux-gentoo2-x86_64
^bzip2@1.0.6%gcc@5.4.0+shared arch=linux-gentoo2-x86_64
^openssl@1.0.2n%gcc@5.4.0+systemcerts arch=linux-gentoo2-x86_64
^sqlite@3.22.0%gcc@5.4.0 arch=linux-gentoo2-x86_64
^py-mpi4py
^py-setuptools@39.0.1%gcc@5.4.0 arch=linux-gentoo2-x86_64
^py-numpy@1.14.2%gcc@5.4.0+blas+lapack arch=linux-gentoo2-x86_64
^openblas@0.2.20%gcc@5.4.0 cpu_target= ~ilp64+pic+shared threads=none ~virtual_machine arch=linux-gentoo2-x86_64
^py-pkgconfig
^py-six@1.11.0%gcc@5.4.0 arch=linux-gentoo2-x86_64
py-h5py requires hdf5 variant +mpi, but spec asked for ~mpi
Thanks to @spakin for reporting.