#6494 removed compiler packages from the invariant to eliminate non-explicit use of ocaml-system, but doing so we forgot about the cases such as the ones advertised in the compiler announcements where the only available package is tagged with avoid-version.

$ opam switch create 5.4 5.4.0~alpha1
[ERROR] No compiler matching `5.4.0~alpha1' found, use `opam switch list-available' to see what is available, or use `--packages' to select packages explicitly.

I think the wanted fix for this is to only remove the avoid-version packages from the formula when at least one non-avoid-version package with that version is available. If there are only avoid-version packages available then we could either:

• add them all to the formula
• or error with a good error message asking to be more precise if there are more than one