Add a warning when a deprecated package is used in the build plan about cabal

[coot]

Ideally there would be a flag --allow-deprecated (and a cabal.project option allow-deprecated), similar to --allow-{newer,older}, which explicitly allows cabal to use deprecated package version in the build plan. Packages which are deprecated are deprecated for a reason, and it's rather surprising when they are picked by cabal constraint solver.

This actually gives control to the end user, and to a package maintainer. Maintainers no longer will need to use base < 0 trick on hackage, which seems like a hack around missing --allow-deprecated option.

[philderbeast]

This is for deprecated versions isn't it and not for whole package deprecations?

Deprecating a version adds a range which excludes just that version. All of this information is collected in the preferred-versions file that's included in the index tarball. [maintainers: edit]

If all the available versions of a package are non-preferred or deprecated, cabal-install will treat this the same as if none of them are. This feature doesn't affect whether or not to install a package, only for selecting versions after a given package has decided to be installed. Entire-package deprecation is also available, but it's separate from preferred versions.
SOURCE: https://hackage.haskell.org/package/binary/preferred