propellor 5.9.0 released with [[!toggle text="these changes"]]
[[!toggleable text="""
   * Added custom type error messages when Properties don't combine due to
     conflicting MetaTypes.
   * Added custom type error messages for ensureProperty and tightenTargets.
   * Note that those changes made ghc 8.0.1 in a few cases unable to infer
     types when ensureProperty or tightenTargets is used, while later ghc
     versions had no difficulty. If this affects building your properties,
     adding a type annotation to the code will work around the problem.
   * Added custom type error messages displayed when type inference
     fails when using ensureProperty and tightenTargets, that suggest
     adding a type annotation.
   * Use the type-errors library to detect when the type checker gets stuck
     unable to reduce type-level operations on MetaTypes, and avoid
     displaying massive error messages.
   * But, since type-errors is a new library not available in eg Debian
     yet, added a WithTypeErrors build flag. When the library is not
     available, cabal will automatically disable that build flag,
     and it will build without the type-errors library.
   * EnsurePropertyAllowed, TightenTargetsAllowed, and CheckCombinable
     types have changed to Constraint.
     (API change)
   * Try harder to avoid displaying an excessive amount of type error
     messages when many properties have been combined in a props list.
   * Libvirt.installed: install libvirt-daemon-system
     Thanks, David Bremner"""]]