Spec matrix: concretization failures when matrixing MPI dependency about spack

[koysean]

This spack.yaml reproduces the issue with no additional config besides a compiler. I've configured a dummy pocl external in there, which works fine to demonstrate the concretization behavior.

spack:
  view: false
  concretizer:
    unify: when_possible
  packages:
    pocl:
      externals:
        - spec: [email protected]
          prefix: /usr
      buildable: false
    opencl:
      require: 'pocl'
  specs:
    - matrix:
      - [gettext, double-batched-fft-library, amrex]
      - ['^openmpi']
      - ['%gcc']

[alalazo]

Would you be able to submit a minimal reproducer of your issue? That would be extremely helpful.

[koysean]

btw I forgot to link this earlier but #42655 is the PR that is responsible for the difference between v0.21 and v0.22