Fix GHC 9.2 build

[guibou]

Close #1513.

GHC 9.2 needs explicit kind signature here, I don't really understand
why.

This kind signature is correct and not too restritive, because HasLink
is technically defined class HasLink endpoint which means that it is
infered as k -> Constraint. In the instance signature, we have
HasLink ((arr :: a -> b) :> sub), so here the k is the same kind as
the one of :> which is not polykinded.

Note: build tested with GHC 9.2 and the following workflow:

$ nix-shell ./ --argstr compiler 'ghc921' --arg pkgs 'import <nixpkgs> {}' -A servant

[nix-shell:~/srcs/servant]$ nix-shell -p cabal-install

[nix-shell:~/srcs/servant]$ cabal build servant
Up to date

I had to bump a few bounds:

diff --git a/cabal.project b/cabal.project
index ac4a4e3b..e2573fa9 100644
--- a/cabal.project
+++ b/cabal.project
@@ -57,7 +57,7 @@ constraints:
   foundation >=0.0.14,
   memory <0.14.12 || >0.14.12
 
-constraints: base-compat ^>=0.11
+constraints: base-compat ^>=0.12
 constraints: semigroups ^>=0.19
 
 -- allow-newer: sqlite-simple-0.4.16.0:semigroups
@@ -74,3 +74,5 @@ allow-newer: servant-js:servant
 
 -- ghc 9
 allow-newer: tdigest:base
+
+allow-newer: *
diff --git a/servant/servant.cabal b/servant/servant.cabal
index f5f973bb..14193e18 100644
--- a/servant/servant.cabal
+++ b/servant/servant.cabal
@@ -80,7 +80,7 @@ library
   --
   -- note: mtl lower bound is so low because of GHC-7.8
   build-depends:
-      base                   >= 4.9      && < 4.16
+      base                   >= 4.9      && < 4.17
     , bytestring             >= 0.10.8.1 && < 0.12
     , constraints            >= 0.2
     , mtl                    >= 2.2.2    && < 2.3
@@ -98,7 +98,7 @@ library
   -- Other dependencies: Lower bound around what is in the latest Stackage LTS.
   -- Here can be exceptions if we really need features from the newer versions.
   build-depends:
-      base-compat            >= 0.10.5   && < 0.12
+      base-compat            >= 0.12   && < 0.13
     , aeson                  >= 1.4.1.0  && < 3
     , attoparsec             >= 0.13.2.2 && < 0.15
     , bifunctors             >= 5.5.3    && < 5.6

(I have no idea what I'm doing there).

This patchs works in production and should be relatively safe considering that it only changes error messages, it should have no impact on running programs.

[guibou]

/cc @teto because you care as much as me about GHC 9.2 support ;)

[gdeest]

Looks fine to me.

Note that your change was already part of #1482 ; we will probably have to wait for the rest of the ecosystem to support 9.2 before we can enable CI.

[guibou]

Thank you for merging, and sorry for the noise, I did not saw the initial MR.