Deal with breaking change in QuickCheck

Author: MaximilianAlgehed

src/Constrained/Core.hs

@@ -1,3 +1,4 @@
+{-# LANGUAGE CPP #-}
 {-# LANGUAGE FlexibleInstances #-}
 {-# LANGUAGE GADTs #-}
 {-# LANGUAGE ImportQualifiedPost #-}
@@ -135,7 +136,9 @@ instance Typeable c => Show (Evidence c) where
 unionWithMaybe :: (a -> a -> a) -> Maybe a -> Maybe a -> Maybe a
 unionWithMaybe f ma ma' = (f <$> ma <*> ma') <|> ma <|> ma'
 
+#if !MIN_VERSION_QuickCheck(2, 17, 0)
 instance Arbitrary a => Arbitrary (NonEmpty a) where
   arbitrary = do
     NonEmpty xs <- arbitrary
     pure (NE.fromList xs)
+#endif