support GHC 9.0

[amesgen]

The code changes are due to simplified subsumption

TODO:

• lots of dependencies are not yet updated on hackage
• spec test suite of servant-http-streams fails
• fix servant-quickcheck to work with http-client >= 0.7

---

I use this cabal.project.local: https://gist.github.com/amesgen/f9cd921b6cd8b39cd5f64f06c3bcf0d2

[akhesaCaro]

Hi amesgen,

We changed the CI, could you rebase your branch to see if it passes the CI checks?

[amesgen]

Added a (temporary) commit to also test GHC 9.0.1.

[amesgen]

Uhhh, CI for GHC 9.0.1 passes except for this doctest error (EDIT see next comment):

src/Servant/API/ContentTypes.hs:123: failure in expression `:{
instance Accept HTML where
    contentType _ = "text" // "html" /: ("charset", "utf-8")
:}'
expected: 
 but got: ghc: panic! (the 'impossible' happened)
          ^
            (GHC version 9.0.1:
          \tlinkSomeBCOs: no break array
          
          Please report this as a GHC bug:  https://www.haskell.org/ghc/reportabug

Maybe it is a good idea to migrate to cabal-docspec.

[amesgen]

Ah, this is due to https://gitlab.haskell.org/ghc/ghc/-/issues/19460

GHC 9.0.1 has another serious bug (https://mail.haskell.org/pipermail/haskell-cafe/2021-March/133540.html), so let's wait for GHC 9.0.2, as this PR can't be merged either way as several dependencies are not yet updated.

[maksbotan]

Hi!

Sorry I didn't look for existing PRs and made pretty much the same thing in #1409. Will wait for 9.0.2 with bugfixes.

[amesgen]

Now depends on haskell-servant/servant-quickcheck#78 for http-client-0.7.* support.

[amesgen]

Superseded by #1409