Test GHC 8.2.1-rc1 with Stack. https://mail.haskell.org/pipermail/ghc-devs/2017-April/014039.html

stack.yaml

compiler: ghc-8.2.0.20170404
compiler-check: match-exact
resolver: lts-8.8
setup-info:
  ghc:
    linux64:
      8.2.0.20170404:
        url: https://downloads.haskell.org/~ghc/8.2.1-rc1/ghc-8.2.0.20170404-x86_64-deb8-linux.tar.xz
        content-length: 138846620
        sha1: 3fd303b5753b095ce020131405dcf43199e6b447
    macosx:
      8.2.0.20170404:
        url: https://downloads.haskell.org/~ghc/8.2.1-rc1/ghc-8.2.0.20170404-x86_64-apple-darwin.tar.xz
        content-length: 124010408
        sha1: 301de7f5b6ed87ade2182ed2552df2ef39b98b03
    # The Windows build is broken :(
    # https://www.reddit.com/r/haskell/comments/64he68/announce_ghc_821_release_candidate_1/dg2b5ei/
    windows64:
      8.2.0.20170404:
        url: https://downloads.haskell.org/~ghc/8.2.1-rc1/ghc-8.2.0.20170404-x86_64-unknown-mingw32.tar.xz
        content-length: 173924808
        sha1: 5cfd11230ff95423a7b19104232247e3fab0dba3

Not a 404 here, maybe you tried during the downtime?

PSA: also need a key for linux64-nopie