Failed to concretize spec: unsupported variant 'mpi' for package 'hdf5' – Concretizer component

Goal: reduce the learning curve for newcomers by allowing the concretizer to automatically accept an external MPI implementation when a package’s mpi variant is requested but not provided by the package itself.

Uncertainty: Spack’s concretizer currently treats the mpi variant as a hard constraint; it can be configured to relax variant constraints when a compatible external dependency is detected, but this behavior is not the default and may differ between releases, raising questions about reproducibility and user expectations.

Should the concretizer automatically relax variant constraints when an external MPI implementation satisfies the variant?
What safety criteria should govern such relaxation to preserve build reproducibility?
How would enabling this behavior by default affect existing workflows and the principle of exact spec reproduction?