There have been some major changes to Stack's internals since version 1.5.1 in order to update the package and extra-dep syntax, support extensible custom snapshots, and to support version 2.0 of the Cabal library. This blog post has a detailed write-up of the package, extra-dep, and custom snapshot changes: https://www.fpcomplete.com/blog/2017/07/stacks-new-extensible-snapshots
We'd appreciate extra testing before we cut a release candidate for version 1.6 to make sure we catch any edge cases in your projects before the release. We've been using the new version internally so it should be relatively stable, but issues you catch in the pre-release can be fixed before we release v1.6 whereas otherwise they might have to wait until the next major release. To make this a bit easier, we've build pre-release binaries for Linux and macOS, which you can download here: https://github.com/commercialhaskell/stack/releases/tag/v22.214.171.12471022
. If you are on another platform, run `stack upgrade --git` to build a stack executable from the master branch.
If you do run into any trouble, please open an issue: https://github.com/commercialhaskell/stack/issues