Require GHC 8.8 as the minimum compiler for bootstrapping

This allows us to remove several bits of CPP that are either always
true or no longer reachable. As an added bonus, we no longer need to
worry about importing `Control.Monad.Fail.fail` qualified to avoid
clashing with `Control.Monad.fail`, since the latter is now the same
as the former.

21 files changed, 28 insertions, 81 deletions

diff --git a/.gitlab-ci.yml b/.gitlab-ci.yml
index 6506e2789c..e830ea1565 100644
--- a/.gitlab-ci.yml
+++ b/.gitlab-ci.yml
@@ -2,7 +2,7 @@ variables:
   GIT_SSL_NO_VERIFY: "1"
 
   # Commit of ghc/ci-images repository from which to pull Docker images
-  DOCKER_REV: 408eff66aef6ca2b44446c694c5a56d6ca0460cc
+  DOCKER_REV: 3f731f5d37a156e7ebe10cd32656946083baaf4a
 
   # Sequential version number capturing the versions of all tools fetched by
   # .gitlab/ci.sh.
@@ -390,7 +390,7 @@ validate-x86_64-darwin:
   tags:
     - x86_64-darwin
   variables:
-    GHC_VERSION: 8.6.5
+    GHC_VERSION: 8.8.3
     CABAL_INSTALL_VERSION: 3.0.0.0
     BIN_DIST_PREP_TAR_COMP: "ghc-x86_64-apple-darwin.tar.xz"
     MACOSX_DEPLOYMENT_TARGET: "10.7"
@@ -419,7 +419,7 @@ validate-x86_64-darwin:
   tags:
     - x86_64-darwin
   variables:
-    GHC_VERSION: 8.6.3
+    GHC_VERSION: 8.8.3
     MACOSX_DEPLOYMENT_TARGET: "10.7"
     ac_cv_func_clock_gettime: "no"
     LANG: "en_US.UTF-8"
diff --git a/compiler/GHC/Core/Lint.hs b/compiler/GHC/Core/Lint.hs
index 86c7ebdeea..3aeb05700c 100644
--- a/compiler/GHC/Core/Lint.hs
+++ b/compiler/GHC/Core/Lint.hs
@@ -70,7 +70,6 @@ import GHC.Types.Demand ( splitStrictSig, isBotDiv )
 import GHC.Driver.Types
 import GHC.Driver.Session
 import Control.Monad
-import qualified Control.Monad.Fail as MonadFail
 import MonadUtils
 import Data.Foldable      ( toList )
 import Data.List.NonEmpty ( NonEmpty )
@@ -2249,16 +2248,13 @@ instance Applicative LintM where
       (<*>) = ap
 
 instance Monad LintM where
-#if !MIN_VERSION_base(4,13,0)
-  fail = MonadFail.fail
-#endif
   m >>= k  = LintM (\ env errs ->
                        let (res, errs') = unLintM m env errs in
                          case res of
                            Just r -> unLintM (k r) env errs'
                            Nothing -> (Nothing, errs'))
 
-instance MonadFail.MonadFail LintM where
+instance MonadFail LintM where
     fail err = failWithL (text err)
 
 instance HasDynFlags LintM where
diff --git a/compiler/GHC/Core/Make.hs b/compiler/GHC/Core/Make.hs
index 599b28a22e..8ae58c5948 100644
--- a/compiler/GHC/Core/Make.hs
+++ b/compiler/GHC/Core/Make.hs
@@ -85,7 +85,6 @@ import Util
 import Data.List
 
 import Data.Char        ( ord )
-import Control.Monad.Fail as MonadFail ( MonadFail )
 
 infixl 4 `mkCoreApp`, `mkCoreApps`
 
@@ -640,7 +639,7 @@ mkFoldrExpr elt_ty result_ty c n list = do
            `App` list)
 
 -- | Make a 'build' expression applied to a locally-bound worker function
-mkBuildExpr :: (MonadFail.MonadFail m, MonadThings m, MonadUnique m)
+mkBuildExpr :: (MonadFail m, MonadThings m, MonadUnique m)
             => Type                                     -- ^ Type of list elements to be built
             -> ((Id, Type) -> (Id, Type) -> m CoreExpr) -- ^ Function that, given information about the 'Id's
                                                         -- of the binders for the build worker function, returns
diff --git a/compiler/GHC/Core/Op/ConstantFold.hs b/compiler/GHC/Core/Op/ConstantFold.hs
index 9b897f8efd..710d9ac75b 100644
--- a/compiler/GHC/Core/Op/ConstantFold.hs
+++ b/compiler/GHC/Core/Op/ConstantFold.hs
@@ -61,7 +61,6 @@ import GHC.Core.Coercion   (mkUnbranchedAxInstCo,mkSymCo,Role(..))
 import Control.Applicative ( Alternative(..) )
 
 import Control.Monad
-import qualified Control.Monad.Fail as MonadFail
 import Data.Bits as Bits
 import qualified Data.ByteString as BS
 import Data.Int
@@ -796,11 +795,7 @@ instance Monad RuleM where
                 Nothing -> Nothing
                 Just r  -> runRuleM (g r) env iu fn args
 
-#if !MIN_VERSION_base(4,13,0)
-  fail = MonadFail.fail
-#endif
-
-instance MonadFail.MonadFail RuleM where
+instance MonadFail RuleM where
     fail _ = mzero
 
 instance Alternative RuleM where
diff --git a/compiler/GHC/Core/Op/Specialise.hs b/compiler/GHC/Core/Op/Specialise.hs
index b43bc90ef1..0a09d818f9 100644
--- a/compiler/GHC/Core/Op/Specialise.hs
+++ b/compiler/GHC/Core/Op/Specialise.hs
@@ -50,7 +50,6 @@ import GHC.Types.Unique.DFM
 import GHC.Core.TyCo.Rep (TyCoBinder (..))
 
 import Control.Monad
-import qualified Control.Monad.Fail as MonadFail
 
 {-
 ************************************************************************
@@ -2551,11 +2550,8 @@ instance Monad SpecM where
                                case f y of
                                    SpecM z ->
                                        z
-#if !MIN_VERSION_base(4,13,0)
-    fail = MonadFail.fail
-#endif
 
-instance MonadFail.MonadFail SpecM where
+instance MonadFail SpecM where
    fail str = SpecM $ error str
 
 instance MonadUnique SpecM where
diff --git a/compiler/GHC/Core/Unify.hs b/compiler/GHC/Core/Unify.hs
index 99c206472c..72b62ab9cd 100644
--- a/compiler/GHC/Core/Unify.hs
+++ b/compiler/GHC/Core/Unify.hs
@@ -46,7 +46,6 @@ import GHC.Types.Unique.FM
 import GHC.Types.Unique.Set
 
 import Control.Monad
-import qualified Control.Monad.Fail as MonadFail
 import Control.Applicative hiding ( empty )
 import qualified Control.Applicative
 
@@ -1244,9 +1243,6 @@ instance Applicative UM where
       (<*>)  = ap
 
 instance Monad UM where
-#if !MIN_VERSION_base(4,13,0)
-  fail     = MonadFail.fail
-#endif
   m >>= k  = UM (\state ->
                   do { (state', v) <- unUM m state
                      ; unUM (k v) state' })
@@ -1260,7 +1256,7 @@ instance Alternative UM where
 
 instance MonadPlus UM
 
-instance MonadFail.MonadFail UM where
+instance MonadFail UM where
     fail _   = UM (\_ -> SurelyApart) -- failed pattern match
 
 initUM :: TvSubstEnv  -- subst to extend
diff --git a/compiler/GHC/Types/Unique/Supply.hs b/compiler/GHC/Types/Unique/Supply.hs
index 56c85efcce..403b88917e 100644
--- a/compiler/GHC/Types/Unique/Supply.hs
+++ b/compiler/GHC/Types/Unique/Supply.hs
@@ -44,7 +44,6 @@ import MonadUtils
 import Control.Monad
 import Data.Bits
 import Data.Char
-import Control.Monad.Fail as Fail
 
 #include "Unique.h"
 
@@ -156,7 +155,7 @@ instance Applicative UniqSM where
     (*>) = thenUs_
 
 -- TODO: try to get rid of this instance
-instance Fail.MonadFail UniqSM where
+instance MonadFail UniqSM where
     fail = panic
 
 -- | Run the 'UniqSM' action, returning the final 'UniqSupply'
diff --git a/compiler/main/SysTools/Process.hs b/compiler/main/SysTools/Process.hs
index a95d9c958a..eda4b29bc0 100644
--- a/compiler/main/SysTools/Process.hs
+++ b/compiler/main/SysTools/Process.hs
@@ -36,14 +36,10 @@ import FileCleanup
 -- @process >= 1.6.8.0@).
 enableProcessJobs :: CreateProcess -> CreateProcess
 #if defined(MIN_VERSION_process)
-#if MIN_VERSION_process(1,6,8)
 enableProcessJobs opts = opts { use_process_jobs = True }
 #else
 enableProcessJobs opts = opts
 #endif
-#else
-enableProcessJobs opts = opts
-#endif
 
 -- Similar to System.Process.readCreateProcessWithExitCode, but stderr is
 -- inherited from the parent process, and output to stderr is not captured.
diff --git a/compiler/typecheck/TcRnTypes.hs b/compiler/typecheck/TcRnTypes.hs
index ab4d783143..aa3c0412ee 100644
--- a/compiler/typecheck/TcRnTypes.hs
+++ b/compiler/typecheck/TcRnTypes.hs
@@ -124,7 +124,6 @@ import PrelNames ( isUnboundName )
 import GHC.Types.CostCentre.State
 
 import Control.Monad (ap)
-import qualified Control.Monad.Fail as MonadFail
 import Data.Set      ( Set )
 import qualified Data.Set as S
 
@@ -1653,14 +1652,11 @@ instance Applicative TcPluginM where
   (<*>) = ap
 
 instance Monad TcPluginM where
-#if !MIN_VERSION_base(4,13,0)
-  fail = MonadFail.fail
-#endif
   TcPluginM m >>= k =
     TcPluginM (\ ev -> do a <- m ev
                           runTcPluginM (k a) ev)
 
-instance MonadFail.MonadFail TcPluginM where
+instance MonadFail TcPluginM where
   fail x   = TcPluginM (const $ fail x)
 
 runTcPluginM :: TcPluginM a -> EvBindsVar -> TcM a
diff --git a/compiler/typecheck/TcSMonad.hs b/compiler/typecheck/TcSMonad.hs
index 9e934172fc..bddbcd0451 100644
--- a/compiler/typecheck/TcSMonad.hs
+++ b/compiler/typecheck/TcSMonad.hs
@@ -177,7 +177,6 @@ import Maybes
 
 import GHC.Core.Map
 import Control.Monad
-import qualified Control.Monad.Fail as MonadFail
 import MonadUtils
 import Data.IORef
 import Data.List ( partition, mapAccumL )
@@ -2699,12 +2698,9 @@ instance Applicative TcS where
   (<*>) = ap
 
 instance Monad TcS where
-#if !MIN_VERSION_base(4,13,0)
-  fail = MonadFail.fail
-#endif
   m >>= k   = TcS (\ebs -> unTcS m ebs >>= \r -> unTcS (k r) ebs)
 
-instance MonadFail.MonadFail TcS where
+instance MonadFail TcS where
   fail err  = TcS (\_ -> fail err)
 
 instance MonadUnique TcS where
diff --git a/compiler/utils/Binary.hs b/compiler/utils/Binary.hs
index bedf380d29..98d4e5ad56 100644
--- a/compiler/utils/Binary.hs
+++ b/compiler/utils/Binary.hs
@@ -829,12 +829,10 @@ instance Binary RuntimeRep where
     put_ bh AddrRep         = putByte bh 9
     put_ bh FloatRep        = putByte bh 10
     put_ bh DoubleRep       = putByte bh 11
-#if __GLASGOW_HASKELL__ >= 807
     put_ bh Int8Rep         = putByte bh 12
     put_ bh Word8Rep        = putByte bh 13
     put_ bh Int16Rep        = putByte bh 14
     put_ bh Word16Rep       = putByte bh 15
-#endif
 #if __GLASGOW_HASKELL__ >= 809
     put_ bh Int32Rep        = putByte bh 16
     put_ bh Word32Rep       = putByte bh 17
@@ -855,12 +853,10 @@ instance Binary RuntimeRep where
           9  -> pure AddrRep
           10 -> pure FloatRep
           11 -> pure DoubleRep
-#if __GLASGOW_HASKELL__ >= 807
           12 -> pure Int8Rep
           13 -> pure Word8Rep
           14 -> pure Int16Rep
           15 -> pure Word16Rep
-#endif
 #if __GLASGOW_HASKELL__ >= 809
           16 -> pure Int32Rep
           17 -> pure Word32Rep
diff --git a/compiler/utils/IOEnv.hs b/compiler/utils/IOEnv.hs
index fd6f6722cd..f9da146da5 100644
--- a/compiler/utils/IOEnv.hs
+++ b/compiler/utils/IOEnv.hs
@@ -43,7 +43,6 @@ import Data.IORef       ( IORef, newIORef, readIORef, writeIORef, modifyIORef,
 import System.IO.Unsafe ( unsafeInterleaveIO )
 import System.IO        ( fixIO )
 import Control.Monad
-import qualified Control.Monad.Fail as MonadFail
 import MonadUtils
 import Control.Applicative (Alternative(..))
 
@@ -60,11 +59,8 @@ unIOEnv (IOEnv m) = m
 instance Monad (IOEnv m) where
     (>>=)  = thenM
     (>>)   = (*>)
-#if !MIN_VERSION_base(4,13,0)
-    fail   = MonadFail.fail
-#endif
 
-instance MonadFail.MonadFail (IOEnv m) where
+instance MonadFail (IOEnv m) where
     fail _ = failM -- Ignore the string
 
 instance Applicative (IOEnv m) where
diff --git a/configure.ac b/configure.ac
index 2973edd2ab..3718d272c6 100644
--- a/configure.ac
+++ b/configure.ac
@@ -158,8 +158,8 @@ if test "$WithGhc" = ""
 then
     AC_MSG_ERROR([GHC is required.])
 fi
-FP_COMPARE_VERSIONS([$GhcVersion],[-lt],[8.6],
-    [AC_MSG_ERROR([GHC version 8.6 or later is required to compile GHC.])])
+FP_COMPARE_VERSIONS([$GhcVersion],[-lt],[8.8],
+    [AC_MSG_ERROR([GHC version 8.8 or later is required to compile GHC.])])
 
 if test `expr $GhcMinVersion % 2` = "1"
 then
diff --git a/hadrian/src/Hadrian/Utilities.hs b/hadrian/src/Hadrian/Utilities.hs
index 81b76a8db5..4768780d37 100644
--- a/hadrian/src/Hadrian/Utilities.hs
+++ b/hadrian/src/Hadrian/Utilities.hs
@@ -40,7 +40,6 @@ import Data.Char
 import Data.Dynamic (Dynamic, fromDynamic, toDyn)
 import Data.Functor
 import Data.HashMap.Strict (HashMap)
-import Data.List (isPrefixOf)
 import Data.List.Extra
 import Data.Maybe
 import Data.Typeable (TypeRep, typeOf)
diff --git a/hadrian/src/Settings/Builders/Cabal.hs b/hadrian/src/Settings/Builders/Cabal.hs
index 763f51636b..85350d96e6 100644
--- a/hadrian/src/Settings/Builders/Cabal.hs
+++ b/hadrian/src/Settings/Builders/Cabal.hs
@@ -1,6 +1,5 @@
 module Settings.Builders.Cabal (cabalBuilderArgs) where
 
-import Hadrian.Builder (getBuilderPath, needBuilder)
 import Hadrian.Haskell.Cabal
 
 import Builder
diff --git a/hadrian/src/Settings/Builders/Ghc.hs b/hadrian/src/Settings/Builders/Ghc.hs
index 2549a63e39..17b0a4a359 100644
--- a/hadrian/src/Settings/Builders/Ghc.hs
+++ b/hadrian/src/Settings/Builders/Ghc.hs
@@ -2,8 +2,6 @@
 
 module Settings.Builders.Ghc (ghcBuilderArgs, haddockGhcArgs) where
 
-import Data.List.Extra (splitOn)
-
 import Hadrian.Haskell.Cabal
 import Hadrian.Haskell.Cabal.Type
 
diff --git a/libraries/base/Control/Monad/ST/Lazy/Imp.hs b/libraries/base/Control/Monad/ST/Lazy/Imp.hs
index 5bb1a06caf..625bcb0fe3 100644
--- a/libraries/base/Control/Monad/ST/Lazy/Imp.hs
+++ b/libraries/base/Control/Monad/ST/Lazy/Imp.hs
@@ -8,7 +8,7 @@
 -- Module      :  Control.Monad.ST.Lazy.Imp
 -- Copyright   :  (c) The University of Glasgow 2001
 -- License     :  BSD-style (see the file libraries/base/LICENSE)
--- 
+--
 -- Maintainer  :  libraries@haskell.org
 -- Stability   :  provisional
 -- Portability :  non-portable (requires universal quantification for runST)
@@ -44,7 +44,6 @@ import qualified Control.Monad.ST.Unsafe as ST
 
 import qualified GHC.ST as GHC.ST
 import GHC.Base
-import qualified Control.Monad.Fail as Fail
 
 -- | The lazy @'ST' monad.
 -- The ST monad allows for destructive updates, but is escapable (unlike IO).
@@ -192,7 +191,7 @@ instance Monad (ST s) where
          unST (k r) new_s
 
 -- | @since 4.10
-instance Fail.MonadFail (ST s) where
+instance MonadFail (ST s) where
     fail s = errorWithoutStackTrace s
 
 -- | Return the value computed by an 'ST' computation.
@@ -205,8 +204,8 @@ runST (ST st) = runRW# (\s -> case st (S# s) of (r, _) -> r)
 -- inside the computation.
 -- Note that if @f@ is strict, @'fixST' f = _|_@.
 fixST :: (a -> ST s a) -> ST s a
-fixST m = ST (\ s -> 
-                let 
+fixST m = ST (\ s ->
+                let
                    q@(r,_s') = unST (m r) s
                 in q)
 -- Why don't we need unsafePerformIO in fixST? We create a thunk, q,
@@ -233,7 +232,7 @@ strictToLazyST (GHC.ST.ST m) = ST $ \(S# s) ->
     (# s', a #) -> (a, S# s')
 -- See Note [Lazy ST: not producing lazy pairs]
 
-{-| 
+{-|
 Convert a lazy 'ST' computation into a strict one.
 -}
 lazyToStrictST :: ST s a -> ST.ST s a
diff --git a/libraries/base/GHC/ST.hs b/libraries/base/GHC/ST.hs
index e9061b7433..98ba18b3d8 100644
--- a/libraries/base/GHC/ST.hs
+++ b/libraries/base/GHC/ST.hs
@@ -26,7 +26,7 @@ module GHC.ST (
 
 import GHC.Base
 import GHC.Show
-import qualified Control.Monad.Fail as Fail
+import Control.Monad.Fail
 
 default ()
 
@@ -79,7 +79,7 @@ instance Monad (ST s) where
         (k2 new_s) }})
 
 -- | @since 4.11.0.0
-instance Fail.MonadFail (ST s) where
+instance MonadFail (ST s) where
     fail s = errorWithoutStackTrace s
 
 -- | @since 4.11.0.0
diff --git a/libraries/base/Text/ParserCombinators/ReadPrec.hs b/libraries/base/Text/ParserCombinators/ReadPrec.hs
index df7704549a..bf4dad91f3 100644
--- a/libraries/base/Text/ParserCombinators/ReadPrec.hs
+++ b/libraries/base/Text/ParserCombinators/ReadPrec.hs
@@ -64,7 +64,7 @@ import qualified Text.ParserCombinators.ReadP as ReadP
 import GHC.Num( Num(..) )
 import GHC.Base
 
-import qualified Control.Monad.Fail as MonadFail
+import Control.Monad.Fail
 
 -- ---------------------------------------------------------------------------
 -- The readPrec type
@@ -88,8 +88,8 @@ instance Monad ReadPrec where
   P f >>= k = P (\n -> do a <- f n; let P f' = k a in f' n)
 
 -- | @since 4.9.0.0
-instance MonadFail.MonadFail ReadPrec where
-  fail s    = P (\_ -> MonadFail.fail s)
+instance MonadFail ReadPrec where
+  fail s    = P (\_ -> fail s)
 
 -- | @since 2.01
 instance MonadPlus ReadPrec
diff --git a/libraries/ghci/GHCi/TH.hs b/libraries/ghci/GHCi/TH.hs
index 927376b53d..5351b90ef1 100644
--- a/libraries/ghci/GHCi/TH.hs
+++ b/libraries/ghci/GHCi/TH.hs
@@ -97,7 +97,6 @@ import GHCi.RemoteTypes
 import GHC.Serialized
 
 import Control.Exception
-import qualified Control.Monad.Fail as Fail
 import Control.Monad.IO.Class (MonadIO (..))
 import Data.Binary
 import Data.Binary.Put
@@ -144,11 +143,8 @@ instance Monad GHCiQ where
     do (m', s')  <- runGHCiQ m s
        (a,  s'') <- runGHCiQ (f m') s'
        return (a, s'')
-#if !MIN_VERSION_base(4,13,0)
-  fail = Fail.fail
-#endif
 
-instance Fail.MonadFail GHCiQ where
+instance MonadFail GHCiQ where
   fail err  = GHCiQ $ \s -> throwIO (GHCiQException s err)
 
 getState :: GHCiQ QState
diff --git a/libraries/template-haskell/Language/Haskell/TH/Syntax.hs b/libraries/template-haskell/Language/Haskell/TH/Syntax.hs
index f81c92f38d..8054f599ea 100644
--- a/libraries/template-haskell/Language/Haskell/TH/Syntax.hs
+++ b/libraries/template-haskell/Language/Haskell/TH/Syntax.hs
@@ -52,15 +52,13 @@ import Numeric.Natural
 import Prelude
 import Foreign.ForeignPtr
 
-import qualified Control.Monad.Fail as Fail
-
 -----------------------------------------------------
 --
 --              The Quasi class
 --
 -----------------------------------------------------
 
-class (MonadIO m, Fail.MonadFail m) => Quasi m where
+class (MonadIO m, MonadFail m) => Quasi m where
   qNewName :: String -> m Name
         -- ^ Fresh names
 
@@ -187,12 +185,9 @@ runQ (Q m) = m
 instance Monad Q where
   Q m >>= k  = Q (m >>= \x -> unQ (k x))
   (>>) = (*>)
-#if !MIN_VERSION_base(4,13,0)
-  fail       = Fail.fail
-#endif
 
-instance Fail.MonadFail Q where
-  fail s     = report True s >> Q (Fail.fail "Q monad failure")
+instance MonadFail Q where
+  fail s     = report True s >> Q (fail "Q monad failure")
 
 instance Functor Q where
   fmap f (Q x) = Q (fmap f x)