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.

11 files changed, 9 insertions, 48 deletions

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