Build all local packages with -Werror

- Satisfy -Wmissing-signatures in test-runtime-deps
- Satisfy -Wx-partial in HackageBenchmark
- Satisfy -Wunused-imports in QuickCheck.Instances.Cabal
- Use partial pattern for filtering in list comprehension
- Don't error on deprecated import

Cabal-QuickCheck/src/Test/QuickCheck/Instances/Cabal.hs

@@ -3,7 +3,9 @@
 {-# OPTIONS_GHC -fno-warn-orphans #-}
 module Test.QuickCheck.Instances.Cabal () where
 
+#if !MIN_VERSION_base(4,18,0)
 import Control.Applicative        (liftA2)
+#endif
 import Data.Bits                  (shiftR)
 import Data.Char                  (isAlphaNum, isDigit, toLower)
 import Data.List                  (intercalate, (\\))

Cabal-tests/tests/CheckTests.hs

@@ -3,7 +3,6 @@ module Main
     ) where
 
 import Test.Tasty
-import Test.Tasty.ExpectedFailure
 import Test.Tasty.Golden.Advanced (goldenTest)
 
 import Data.Algorithm.Diff                    (PolyDiff (..), getGroupedDiff)

Cabal-tests/tests/UnitTests/Distribution/PackageDescription/Check.hs

@@ -1,3 +1,6 @@
+-- For the deprecated import of Distribution.Compat.Prelude.Internal
+{-# OPTIONS_GHC -Wwarn=deprecations #-}
+
 module UnitTests.Distribution.PackageDescription.Check (tests) where
 
 import Distribution.Compat.Prelude.Internal

cabal-testsuite/static/Main.hs

@@ -1,3 +1,4 @@
 module Main where
 
+main :: IO ()
 main = return ()

solver-benchmarks/HackageBenchmark.hs

@@ -1,4 +1,3 @@
-{-# OPTIONS_GHC -fno-warn-orphans #-}
 {-# LANGUAGE RecordWildCards #-}
 {-# LANGUAGE TupleSections #-}
 {-# LANGUAGE ScopedTypeVariables #-}
@@ -19,7 +18,6 @@ import Control.Monad (forM, replicateM, unless, when)
 import qualified Data.ByteString as BS
 import Data.List (nub, unzip4)
 import Data.Maybe (isJust, catMaybes)
-import Data.Monoid ((<>))
 import Data.String (fromString)
 import Data.Function ((&))
 import Data.Time (NominalDiffTime, diffUTCTime, getCurrentTime)
@@ -187,7 +185,7 @@ hackageBenchmarkMain = do
           then do
             putStrLn $ "Obtaining the package list (using " ++ argCabal1 ++ ") ..."
             list <- readProcess argCabal1 ["list", "--simple-output"] ""
-            return $ nub [mkPackageName $ head (words line) | line <- lines list]
+            return $ nub [mkPackageName n | n : _ <- words <$> lines list]
           else do
             putStrLn "Using given package list ..."
             return argPackages
@@ -345,7 +343,8 @@ isExpectedResult Unknown        = False
 -- should be the same. If they aren't the same, we returns Unknown.
 combineTrialResults :: [CabalResult] -> CabalResult
 combineTrialResults rs
-  | allEqual rs                          = head rs
+  | r:_ <- rs
+  , allEqual rs                          = r
   | allEqual [r | r <- rs, r /= Timeout] = Timeout
   | otherwise                            = Unknown
   where