Todd Gamblin 520b71e89b concretizer: handle virtual spec constraints better ...

Specs like hdf5 ^mpi were unsatisfiable because we added a requierment
for `node("mpi").`.  This can't be resolved because "mpi" is not a
package.

- [x] Introduce `virtual_node()`, which says *some* provider must be in
      the DAG.

2020-11-17 10:04:13 -08:00

..

spack

concretizer: handle virtual spec constraints better

2020-11-17 10:04:13 -08:00