Use ReaderT instead of DerivingVia because of ghc 8.4

Cabal/src/Distribution/Simple/Build/Monad.hs

@@ -1,8 +1,9 @@
-{-# LANGUAGE DerivingVia #-}
+{-# LANGUAGE GeneralizedNewtypeDeriving #-}
 {-# LANGUAGE NamedFieldPuns #-}
+{-# LANGUAGE PatternSynonyms #-}
 
 module Distribution.Simple.Build.Monad
-  ( BuildM (..)
+  ( BuildM (BuildM)
   , runBuildM
   , PreBuildComponentInputs (..)
 
@@ -49,8 +50,16 @@ data PreBuildComponentInputs = PreBuildComponentInputs
   }
 
 -- | Computations carried out in the context of building a component (e.g. @'buildAllExtraSources'@)
-newtype BuildM a = BuildM (PreBuildComponentInputs -> IO a)
-  deriving (Functor, Applicative, Monad, MonadReader PreBuildComponentInputs, MonadIO) via ReaderT PreBuildComponentInputs IO
+newtype BuildM a = BuildM' (ReaderT PreBuildComponentInputs IO a)
+  deriving (Functor, Applicative, Monad, MonadReader PreBuildComponentInputs, MonadIO)
+
+-- Ideally we'd use deriving via ReaderT PreBuildComponentInputs IO, but ghc 8.4 doesn't support it.
+
+-- | Construct a t'BuildM' action from an IO function on 'PreBuildComponentInputs'.
+pattern BuildM :: (PreBuildComponentInputs -> IO a) -> BuildM a
+pattern BuildM f = BuildM' (ReaderT f)
+
+{-# COMPLETE BuildM #-}
 
 -- | Run a 'BuildM' action, i.e. a computation in the context of building a component.
 runBuildM :: BuildingWhat -> LocalBuildInfo -> TargetInfo -> BuildM a -> IO a

Cabal/src/Distribution/Simple/GHC/Build.hs

@@ -1,5 +1,3 @@
-{-# LANGUAGE BlockArguments #-}
-
 module Distribution.Simple.GHC.Build where
 
 import Distribution.Compat.Prelude
@@ -83,7 +81,7 @@ build numJobs pkg_descr = do
             targetDir_absolute </> (targetDirName ++ "-tmp")
         | otherwise = error "GHC.build: targetDir is empty"
 
-  liftIO do
+  liftIO $ do
     createDirectoryIfMissingVerbose verbosity True targetDir_absolute
     createDirectoryIfMissingVerbose verbosity True buildTargetDir_absolute
 

Cabal/src/Distribution/Simple/GHC/Build/ExtraSources.hs

@@ -1,4 +1,3 @@
-{-# LANGUAGE BlockArguments #-}
 {-# LANGUAGE DisambiguateRecordFields #-}
 {-# LANGUAGE LambdaCase #-}
 {-# LANGUAGE NamedFieldPuns #-}
@@ -136,7 +135,7 @@ buildExtraSources
   -> BuildM [FilePath]
   -- ^ Returns the list of extra sources that were built
 buildExtraSources description componentSourceGhcOptions wantDyn viewSources ghcProg buildTargetDir =
-  BuildM \PreBuildComponentInputs{buildingWhat, localBuildInfo = lbi, targetInfo} ->
+  BuildM $ \PreBuildComponentInputs{buildingWhat, localBuildInfo = lbi, targetInfo} ->
     let
       bi = componentBuildInfo (targetComponent targetInfo)
       verbosity = buildingWhatVerbosity buildingWhat

Cabal/src/Distribution/Simple/GHC/Build/Link.hs

@@ -88,11 +88,9 @@ linkOrLoadComponent ghcProg pkg_descr extraSources (buildTargetDir, targetDir) (
             PD.ldOptions bi
               ++ [ "-static"
                  | withFullyStaticExe lbi -- ROMES:TODO: wb withStaticLib??
-                 -- ROMES:TODO: wouldn't this be best handled by re-using the build opt for the way being linked?
                  ]
               -- Pass extra `ld-options` given
               -- through to GHC's linker.
-              -- ROMES:TODO: Why not for the dyn linker opts too?
               ++ maybe
                 []
                 programOverrideArgs
@@ -247,7 +245,8 @@ linkLibrary buildTargetDir cleanedExtraLibDirs pkg_descr verbosity runGhcProg li
     -- ROMES:TODO: I'm fairly certain that, just like the executable, we can keep just the
     -- module input list, and point to the right sources dir (as is already
     -- done), and GHC will pick up the right suffix (p_ for profile, dyn_ when
-    -- -shared...). That would mean linking the lib would be just like the executable, and we could more easily merge the two.
+    -- -shared...).
+    -- That would mean linking the lib would be just like the executable, and we could more easily merge the two.
     --
     -- Right now, instead, we pass the path to each object file.
     ghcBaseLinkArgs =
@@ -377,14 +376,12 @@ linkLibrary buildTargetDir cleanedExtraLibDirs pkg_descr verbosity runGhcProg li
 
   -- ROMES: Why exactly branch on staticObjectFiles, rather than any other build
   -- kind that we might have wanted instead?
+  -- This would be simpler by not adding every object to the invocation, and
+  -- rather using module names.
   unless (null staticObjectFiles) $ do
     info verbosity (show (ghcOptPackages (Internal.componentGhcOptions verbosity lbi libBi clbi buildTargetDir)))
     traverse_ linkWay wantedWays
 
--- ROMES:TODO: Have to be careful about target dir vs build dir here! They are
--- not the same on exes, and we don't want to conflate them for now, even
--- though we want to drop the distinction eventually.
-
 -- | Link the executable resulting from building this component, be it an
 -- executable, test, or benchmark component.
 linkExecutable