WIP: Enable -Werror in CI

[bgamari]

As noted in #644, this will need changes in haskell-ci to pass.

[RyanGlScott]

An alternative approach to this would be to add local-ghc-options: -Werror to cabal.haskell-ci and then run haskell-ci regenerate. This will ensure that CI is run with -Werror enabled without actually putting -Werror into the .cabal file itself (which cabal check doesn't like).

[treeowl]

Is this going to go somewhere?

[sjakobi]

An alternative approach to this would be to add local-ghc-options: -Werror to cabal.haskell-ci and then run haskell-ci regenerate. This will ensure that CI is run with -Werror enabled without actually putting -Werror into the .cabal file itself (which cabal check doesn't like).

That sounds much better to me.

[sjakobi]

As pointed out in #740 (comment), we already build the library with -Werror when GHC >= 8.6.

That seems good enough for me.

I'll tentatively close this, but please re-open if you think there's more to do here.