A Spack environment which contains root specs with custom versions (i.e. versions not declared in a version directive in the corresponding package.py) cannot be concretized together.

Steps to reproduce the issue

The following environment:

spack:
  specs:
  - hdf5@atom-broadwell
  - zlib
  concretization: together
  config:
    concretizer: clingo

gives an error during concretization.

Error Message

The error message is:

$ spack -e . concretize
==> The following constraints are unsatisfiable:
  condition(220)
  dependency_condition(220,"concretizationroot","hdf5")
  dependency_type(220,"build")
  imposed_constraint(220,"version_satisfies","hdf5","atom-broadwell")
  root("concretizationroot")
==> Error: concretizationroot does not satisfy unknown

Information on your system

• Spack: 0.16.1-2510-43c74f2f17
• Python: 3.6.9
• Platform: linux-ubuntu18.04-broadwell
• Concretizer: clingo

Additional information

• I have run spack debug report and reported the version of Spack/Python/Platform
• I have searched the issues of this repo and believe this is not a duplicate
• I have run the failing commands in debug mode and reported the output