Make quickcheck-classes usage optional (again)

quickcheck-classes (actually one of its deps, aeson) conflicts with
template-haskell in GHC 9.10 as of 2024-05-13

This partially reverts commit a16541b.

tests/Properties/Stack.hs

@@ -1,5 +1,9 @@
+{-# LANGUAGE CPP #-}
 {-# LANGUAGE ScopedTypeVariables #-}
+
+#ifdef VERSION_quickcheck_classes
 {-# LANGUAGE GeneralizedNewtypeDeriving #-}
+#endif
 
 module Properties.Stack where
 
@@ -11,11 +15,13 @@ import qualified XMonad.StackSet as S (filter)
 
 import Data.Maybe
 
+#ifdef VERSION_quickcheck_classes
 import Data.Proxy
 import Test.QuickCheck.Classes (
     Laws (lawsTypeclass, lawsProperties), Proxy1 (Proxy1),
     foldableLaws, traversableLaws,
     )
+#endif
 
 
 -- The list returned by index should be the same length as the actual
@@ -59,6 +65,7 @@ prop_differentiate xs =
     where _ = xs :: [Int]
 
 
+#ifdef VERSION_quickcheck_classes
 -- Check type class laws of 'Data.Foldable.Foldable' and 'Data.Traversable.Traversable'.
 newtype TestStack a = TestStack (Stack a)
     deriving (Eq, Read, Show, Foldable, Functor)
@@ -75,3 +82,6 @@ prop_laws_Stack = format (foldableLaws p) <> format (traversableLaws p)
     p = Proxy :: Proxy TestStack
     format laws = [ ("Stack: " <> lawsTypeclass laws <> ": " <> name, prop)
                   | (name, prop) <- lawsProperties laws ]
+#else
+prop_laws_Stack = []
+#endif

xmonad.cabal

@@ -54,6 +54,8 @@ flag pedantic
   default:     False
   manual:      True
 
+flag quickcheck-classes
+
 library
   exposed-modules: XMonad
                    XMonad.Config
@@ -123,12 +125,15 @@ test-suite properties
   hs-source-dirs: tests
   build-depends:  base
                 , QuickCheck >= 2
-                , quickcheck-classes >= 0.4.3
                 , X11
                 , containers
                 , xmonad
   default-language: Haskell2010
 
+  if flag(quickcheck-classes)
+    -- quickcheck-classes or its deps conflict with template-haskell in GHC 9.10 as of 2024-05-13
+    build-depends: quickcheck-classes >= 0.4.3
+
   if impl(ghc > 9.8)
     ghc-options:   -Wno-x-partial