diff --git a/Cabal-QuickCheck/src/Test/QuickCheck/Instances/Cabal.hs b/Cabal-QuickCheck/src/Test/QuickCheck/Instances/Cabal.hs
index 0acb8df1786..92eeb8799f0 100644
--- a/Cabal-QuickCheck/src/Test/QuickCheck/Instances/Cabal.hs
+++ b/Cabal-QuickCheck/src/Test/QuickCheck/Instances/Cabal.hs
@@ -41,6 +41,8 @@ import Distribution.Types.VersionRange.Internal
 import Distribution.Utils.NubList
 import Distribution.Verbosity
 import Distribution.Version
+import Distribution.Types.DebugInfoLevel (DebugInfoLevel)
+import Distribution.Types.OptimisationLevel (OptimisationLevel)
 
 import Test.QuickCheck.GenericArbitrary
 
diff --git a/Cabal-syntax/src/Distribution/Parsec.hs b/Cabal-syntax/src/Distribution/Parsec.hs
index 9b43b6e41a2..6d384418370 100644
--- a/Cabal-syntax/src/Distribution/Parsec.hs
+++ b/Cabal-syntax/src/Distribution/Parsec.hs
@@ -252,15 +252,11 @@ instance Parsec Bool where
   parsec = P.munch1 isAlpha >>= postprocess
     where
       postprocess str
-        | str == "True" = pure True
-        | str == "False" = pure False
-        | lstr == "true" = parsecWarning PWTBoolCase caseWarning *> pure True
-        | lstr == "false" = parsecWarning PWTBoolCase caseWarning *> pure False
+        | lstr == "true" = pure True
+        | lstr == "false" = pure False
         | otherwise = fail $ "Not a boolean: " ++ str
         where
           lstr = map toLower str
-          caseWarning =
-            "Boolean values are case sensitive, use 'True' or 'False'."
 
 instance Parsec a => Parsec (Last a) where
   parsec = parsecLast
diff --git a/Cabal-syntax/src/Distribution/Parsec/Warning.hs b/Cabal-syntax/src/Distribution/Parsec/Warning.hs
index 65b1471b054..2472c544b8f 100644
--- a/Cabal-syntax/src/Distribution/Parsec/Warning.hs
+++ b/Cabal-syntax/src/Distribution/Parsec/Warning.hs
@@ -25,8 +25,6 @@ data PWarnType
     PWTOther
   | -- | Invalid UTF encoding
     PWTUTF
-  | -- | @true@ or @false@, not @True@ or @False@
-    PWTBoolCase
   | -- | there are version with tags
     PWTVersionTag
   | -- | New syntax used, but no @cabal-version: >= 1.2@ specified
diff --git a/Cabal-tests/tests/ParserTests.hs b/Cabal-tests/tests/ParserTests.hs
index 42ab00ae9f2..88c40fde27f 100644
--- a/Cabal-tests/tests/ParserTests.hs
+++ b/Cabal-tests/tests/ParserTests.hs
@@ -68,7 +68,6 @@ warningTests = testGroup "warnings triggered"
     , warningTest PWTLexNBSP           "nbsp.cabal"
     , warningTest PWTLexTab            "tab.cabal"
     , warningTest PWTUTF               "utf8.cabal"
-    , warningTest PWTBoolCase          "bool.cabal"
     , warningTest PWTVersionTag        "versiontag.cabal"
     , warningTest PWTNewSyntax         "newsyntax.cabal"
     , warningTest PWTOldSyntax         "oldsyntax.cabal"
diff --git a/Cabal-tests/tests/ParserTests/warnings/bool.cabal b/Cabal-tests/tests/ParserTests/warnings/bool.cabal
deleted file mode 100644
index 34e6e0f8f22..00000000000
--- a/Cabal-tests/tests/ParserTests/warnings/bool.cabal
+++ /dev/null
@@ -1,12 +0,0 @@
-name: bool
-version: 1
-cabal-version: >= 1.8
-
-flag foo
-  manual: true
-
-library
-    build-depends: base >= 4.9 && <4.10
-    if flag(foo)
-      build-depends: containers
-    hs-source-dirs: .
diff --git a/Cabal-tree-diff/src/Data/TreeDiff/Instances/Cabal.hs b/Cabal-tree-diff/src/Data/TreeDiff/Instances/Cabal.hs
index dd3dfaa028f..1251d237bb6 100644
--- a/Cabal-tree-diff/src/Data/TreeDiff/Instances/Cabal.hs
+++ b/Cabal-tree-diff/src/Data/TreeDiff/Instances/Cabal.hs
@@ -18,20 +18,22 @@ import Distribution.Compiler                       (CompilerFlavor, CompilerId,
 import Distribution.InstalledPackageInfo           (AbiDependency, ExposedModule, InstalledPackageInfo)
 import Distribution.ModuleName                     (ModuleName)
 import Distribution.PackageDescription
-import Distribution.Simple.Compiler                (DebugInfoLevel, OptimisationLevel, ProfDetailLevel)
+import Distribution.Simple.Compiler                (ProfDetailLevel)
 import Distribution.Simple.InstallDirs
-import Distribution.Simple.InstallDirs.Internal
+import Distribution.Simple.InstallDirs.Internal    (PathComponent)
 import Distribution.Simple.Setup                   (HaddockTarget, TestShowDetails)
-import Distribution.System
+import Distribution.System                         (OS, Arch)
 import Distribution.Types.AbiHash                  (AbiHash)
 import Distribution.Types.ComponentId              (ComponentId)
 import Distribution.Types.DumpBuildInfo            (DumpBuildInfo)
-import Distribution.Types.PackageVersionConstraint
+import Distribution.Types.PackageVersionConstraint (PackageVersionConstraint)
 import Distribution.Types.UnitId                   (DefUnitId, UnitId)
 import Distribution.Utils.NubList                  (NubList)
 import Distribution.Utils.Path                     (SymbolicPathX)
-import Distribution.Verbosity
-import Distribution.Verbosity.Internal
+import Distribution.Verbosity                      (VerbosityLevel, VerbosityFlags)
+import Distribution.Verbosity.Internal             (VerbosityFlag)
+import Distribution.Types.DebugInfoLevel           (DebugInfoLevel)
+import Distribution.Types.OptimisationLevel        (OptimisationLevel)
 
 import qualified Distribution.Compat.NonEmptySet as NES
 
diff --git a/Cabal/Cabal.cabal b/Cabal/Cabal.cabal
index c91d2c11d89..aba6aff5a82 100644
--- a/Cabal/Cabal.cabal
+++ b/Cabal/Cabal.cabal
@@ -171,11 +171,13 @@ library
     Distribution.TestSuite
     Distribution.Types.AnnotatedId
     Distribution.Types.ComponentInclude
+    Distribution.Types.DebugInfoLevel
     Distribution.Types.DumpBuildInfo
     Distribution.Types.PackageName.Magic
     Distribution.Types.ComponentLocalBuildInfo
     Distribution.Types.LocalBuildConfig
     Distribution.Types.LocalBuildInfo
+    Distribution.Types.OptimisationLevel
     Distribution.Types.TargetInfo
     Distribution.Types.GivenComponent
     Distribution.Types.ParStrat
diff --git a/Cabal/src/Distribution/Simple/Build.hs b/Cabal/src/Distribution/Simple/Build.hs
index e79a1924eb9..a8ce8903df9 100644
--- a/Cabal/src/Distribution/Simple/Build.hs
+++ b/Cabal/src/Distribution/Simple/Build.hs
@@ -309,7 +309,7 @@ dumpBuildInfo verbosity distPref dumpBuildInfoFlag pkg_descr lbi flags = do
     removeFileForcibly buildInfoFile
   where
     buildInfoFile = interpretSymbolicPathLBI lbi $ buildInfoPref distPref
-    shouldDumpBuildInfo = fromFlagOrDefault NoDumpBuildInfo dumpBuildInfoFlag == DumpBuildInfo
+    shouldDumpBuildInfo = fromNoFlag dumpBuildInfoFlag == DumpBuildInfo
 
     -- \| Given the flavor of the compiler, try to find out
     -- which program we need.
diff --git a/Cabal/src/Distribution/Simple/Command.hs b/Cabal/src/Distribution/Simple/Command.hs
index 0fe65f8d751..6f0eced8ad0 100644
--- a/Cabal/src/Distribution/Simple/Command.hs
+++ b/Cabal/src/Distribution/Simple/Command.hs
@@ -79,7 +79,6 @@ module Distribution.Simple.Command
   , reqArg'
   , optArg
   , optArg'
-  , optArgDef'
   , noArg
   , boolOpt
   , boolOpt'
@@ -280,15 +279,6 @@ optArg'
 optArg' ad mkflag showflag =
   optArg ad (succeedReadE (mkflag . Just)) ("", mkflag Nothing) showflag
 
-optArgDef'
-  :: Monoid b
-  => ArgPlaceHolder
-  -> (String, Maybe String -> b)
-  -> (b -> [Maybe String])
-  -> MkOptDescr (a -> b) (b -> a -> a) a
-optArgDef' ad (dv, mkflag) showflag =
-  optArg ad (succeedReadE (mkflag . Just)) (dv, mkflag Nothing) showflag
-
 noArg :: Eq b => b -> MkOptDescr (a -> b) (b -> a -> a) a
 noArg flag sf lf d = choiceOpt [(flag, (sf, lf), d)] sf lf d
 
diff --git a/Cabal/src/Distribution/Simple/Compiler.hs b/Cabal/src/Distribution/Simple/Compiler.hs
index 4f66f90eea8..dd24249f07f 100644
--- a/Cabal/src/Distribution/Simple/Compiler.hs
+++ b/Cabal/src/Distribution/Simple/Compiler.hs
@@ -52,14 +52,6 @@ module Distribution.Simple.Compiler
   , coercePackageDBStack
   , readPackageDb
 
-    -- * Support for optimisation levels
-  , OptimisationLevel (..)
-  , flagToOptimisationLevel
-
-    -- * Support for debug info levels
-  , DebugInfoLevel (..)
-  , flagToDebugInfoLevel
-
     -- * Support for language extensions
   , CompilerFlag
   , languageToFlags
@@ -95,22 +87,30 @@ module Distribution.Simple.Compiler
   , showProfDetailLevel
   ) where
 
-import Distribution.Compat.CharParsing
 import Distribution.Compat.Prelude
-import Distribution.Parsec
-import Distribution.Pretty
+import Distribution.Parsec (CabalParsing, Parsec (..), parsecToken)
+import Distribution.Pretty (prettyShow)
 import Prelude ()
 
 import Distribution.Compiler
 import Distribution.Package (PackageName)
-import Distribution.Simple.Utils
+import Distribution.Simple.Utils (lowercase, safeLast)
 import Distribution.Types.UnitId (UnitId)
 import Distribution.Utils.Path
-import Distribution.Version
-
-import Language.Haskell.Extension
+  ( CWD
+  , FileOrDir (..)
+  , Pkg
+  , PkgDB
+  , SymbolicPath
+  , getSymbolicPath
+  , interpretSymbolicPath
+  , makeSymbolicPath
+  , symbolicPathRelative_maybe
+  )
+import Distribution.Version (Version, mkVersion)
+
+import Language.Haskell.Extension (Extension, Language (..))
 
-import Data.Bool (bool)
 import qualified Data.Map as Map (lookup)
 import System.Directory (canonicalizePath)
 
@@ -303,95 +303,6 @@ coercePackageDBStack = map coercePackageDB
 
 -- ------------------------------------------------------------
 
--- * Optimisation levels
-
--- ------------------------------------------------------------
-
--- | Some compilers support optimising. Some have different levels.
--- For compilers that do not the level is just capped to the level
--- they do support.
-data OptimisationLevel
-  = NoOptimisation
-  | NormalOptimisation
-  | MaximumOptimisation
-  deriving (Bounded, Enum, Eq, Generic, Read, Show)
-
-instance Binary OptimisationLevel
-instance NFData OptimisationLevel
-instance Structured OptimisationLevel
-
-instance Parsec OptimisationLevel where
-  parsec = parsecOptimisationLevel
-
-parsecOptimisationLevel :: CabalParsing m => m OptimisationLevel
-parsecOptimisationLevel = boolParser <|> intParser
-  where
-    boolParser = bool NoOptimisation NormalOptimisation <$> parsec
-    intParser = intToOptimisationLevel <$> integral
-
-flagToOptimisationLevel :: Maybe String -> OptimisationLevel
-flagToOptimisationLevel Nothing = NormalOptimisation
-flagToOptimisationLevel (Just s) = case reads s of
-  [(i, "")] -> intToOptimisationLevel i
-  _ -> error $ "Can't parse optimisation level " ++ s
-
-intToOptimisationLevel :: Int -> OptimisationLevel
-intToOptimisationLevel i
-  | i >= minLevel && i <= maxLevel = toEnum i
-  | otherwise =
-      error $
-        "Bad optimisation level: "
-          ++ show i
-          ++ ". Valid values are "
-          ++ show minLevel
-          ++ ".."
-          ++ show maxLevel
-  where
-    minLevel = fromEnum (minBound :: OptimisationLevel)
-    maxLevel = fromEnum (maxBound :: OptimisationLevel)
-
--- ------------------------------------------------------------
-
--- * Debug info levels
-
--- ------------------------------------------------------------
-
--- | Some compilers support emitting debug info. Some have different
--- levels.  For compilers that do not the level is just capped to the
--- level they do support.
-data DebugInfoLevel
-  = NoDebugInfo
-  | MinimalDebugInfo
-  | NormalDebugInfo
-  | MaximalDebugInfo
-  deriving (Bounded, Enum, Eq, Generic, Read, Show)
-
-instance Binary DebugInfoLevel
-instance NFData DebugInfoLevel
-instance Structured DebugInfoLevel
-
-instance Parsec DebugInfoLevel where
-  parsec = parsecDebugInfoLevel
-
-parsecDebugInfoLevel :: CabalParsing m => m DebugInfoLevel
-parsecDebugInfoLevel = flagToDebugInfoLevel . pure <$> parsecToken
-
-flagToDebugInfoLevel :: Maybe String -> DebugInfoLevel
-flagToDebugInfoLevel Nothing = NormalDebugInfo
-flagToDebugInfoLevel (Just s) = case reads s of
-  [(i, "")]
-    | i >= fromEnum (minBound :: DebugInfoLevel)
-        && i <= fromEnum (maxBound :: DebugInfoLevel) ->
-        toEnum i
-    | otherwise ->
-        error $
-          "Bad debug info level: "
-            ++ show i
-            ++ ". Valid values are 0..3"
-  _ -> error $ "Can't parse debug info level " ++ s
-
--- ------------------------------------------------------------
-
 -- * Languages and Extensions
 
 -- ------------------------------------------------------------
diff --git a/Cabal/src/Distribution/Simple/Configure.hs b/Cabal/src/Distribution/Simple/Configure.hs
index 69b9a80e16f..29b958b8e34 100644
--- a/Cabal/src/Distribution/Simple/Configure.hs
+++ b/Cabal/src/Distribution/Simple/Configure.hs
@@ -163,6 +163,7 @@ import Distribution.Pretty
   )
 import Distribution.Simple.Errors
 import Distribution.Types.AnnotatedId
+import Distribution.Types.DebugInfoLevel (DebugInfoLevel (..))
 import Distribution.Utils.Path
 import Distribution.Utils.Structured (structuredDecodeOrFailIO, structuredEncode)
 import System.Directory
diff --git a/Cabal/src/Distribution/Simple/Flag.hs b/Cabal/src/Distribution/Simple/Flag.hs
index 341fd907e09..b6b92d5600b 100644
--- a/Cabal/src/Distribution/Simple/Flag.hs
+++ b/Cabal/src/Distribution/Simple/Flag.hs
@@ -31,6 +31,8 @@ module Distribution.Simple.Flag
   , maybeToFlag
   , mergeListFlag
   , BooleanFlag (..)
+  , NoFlagValue (..)
+  , fromNoFlag
   ) where
 
 import Data.Monoid (Last (..))
@@ -123,3 +125,15 @@ class BooleanFlag a where
 
 instance BooleanFlag Bool where
   asBool = id
+
+-- | Flag is a Monoid, with 'NoFlag' as the identity element, and 'Flag' as the binary operation.
+--
+-- @since 3.18.0.0
+class NoFlagValue a where
+  noFlagValue :: a
+
+-- | Extracts a value from a 'Flag', and returns the 'noFlagValue' on 'NoFlag'.
+--
+-- @since 3.18.0.0
+fromNoFlag :: NoFlagValue a => Flag a -> a
+fromNoFlag = fromFlagOrDefault noFlagValue
diff --git a/Cabal/src/Distribution/Simple/GHC/Internal.hs b/Cabal/src/Distribution/Simple/GHC/Internal.hs
index ac2116b4b09..b6668b1fea8 100644
--- a/Cabal/src/Distribution/Simple/GHC/Internal.hs
+++ b/Cabal/src/Distribution/Simple/GHC/Internal.hs
@@ -76,11 +76,13 @@ import Distribution.Simple.Utils
 import Distribution.System
 import Distribution.Types.BuildInfo
 import Distribution.Types.ComponentLocalBuildInfo
+import Distribution.Types.DebugInfoLevel (DebugInfoLevel (..))
 import Distribution.Types.GivenComponent
 import qualified Distribution.Types.InstalledPackageInfo as IPI
 import Distribution.Types.Library
 import Distribution.Types.LocalBuildInfo
 import Distribution.Types.ModuleRenaming
+import Distribution.Types.OptimisationLevel (OptimisationLevel (..))
 import Distribution.Types.PackageName
 import Distribution.Types.TargetInfo
 import Distribution.Types.UnitId
diff --git a/Cabal/src/Distribution/Simple/Program/GHC.hs b/Cabal/src/Distribution/Simple/Program/GHC.hs
index e049749ee7e..5dc689573d9 100644
--- a/Cabal/src/Distribution/Simple/Program/GHC.hs
+++ b/Cabal/src/Distribution/Simple/Program/GHC.hs
@@ -48,12 +48,13 @@ import Distribution.Verbosity
 import Distribution.Version
 
 import GHC.IO.Encoding (TextEncoding)
-import Language.Haskell.Extension
+import Language.Haskell.Extension (Extension, Language)
 
 import Data.List (stripPrefix)
 import qualified Data.Map as Map
 import Data.Monoid (All (..), Any (..), Endo (..))
 import qualified Data.Set as Set
+import Distribution.Types.DebugInfoLevel (DebugInfoLevel (..))
 import qualified System.Process as Process
 
 normaliseGhcArgs :: Maybe Version -> PackageDescription -> [String] -> [String]
diff --git a/Cabal/src/Distribution/Simple/Setup/Config.hs b/Cabal/src/Distribution/Simple/Setup/Config.hs
index 92fb879df9a..4da3966acf1 100644
--- a/Cabal/src/Distribution/Simple/Setup/Config.hs
+++ b/Cabal/src/Distribution/Simple/Setup/Config.hs
@@ -63,9 +63,13 @@ import Distribution.Simple.Program
 import Distribution.Simple.Setup.Common
 import Distribution.Simple.Utils
 import Distribution.Types.ComponentId
-import Distribution.Types.DumpBuildInfo
+import Distribution.Types.DebugInfoLevel (DebugInfoLevel (..))
+import qualified Distribution.Types.DebugInfoLevel as DebugInfoLevel
+import Distribution.Types.DumpBuildInfo (DumpBuildInfo (..))
 import Distribution.Types.GivenComponent
 import Distribution.Types.Module
+import Distribution.Types.OptimisationLevel (OptimisationLevel (..))
+import qualified Distribution.Types.OptimisationLevel as OptimisationLevel
 import Distribution.Types.PackageVersionConstraint
 import Distribution.Types.UnitId
 import Distribution.Utils.NubList
@@ -570,18 +574,20 @@ configureOptions showOrParseArgs =
           "optimization"
           configOptimization
           (\v flags -> flags{configOptimization = v})
-          [ optArgDef'
+          [ reqArg'
               "n"
-              (show NoOptimisation, Flag . flagToOptimisationLevel)
+              (Flag . fromString)
               ( \case
-                  Flag NoOptimisation -> []
-                  Flag NormalOptimisation -> [Nothing]
-                  Flag MaximumOptimisation -> [Just "2"]
-                  _ -> []
+                  NoFlag -> []
+                  Flag flag -> [OptimisationLevel.toString flag]
               )
               "O"
               ["enable-optimization", "enable-optimisation"]
-              "Build with optimization (n is 0--2, default is 1)"
+              ( let minLevel = OptimisationLevel.toString minBound
+                    maxLevel = OptimisationLevel.toString maxBound
+                    def = OptimisationLevel.toString noFlagValue
+                 in "Build with optimization (n is " ++ minLevel ++ "--" ++ maxLevel ++ ", default is " ++ def ++ ")"
+              )
           , noArg
               (Flag NoOptimisation)
               []
@@ -592,19 +598,20 @@ configureOptions showOrParseArgs =
           "debug-info"
           configDebugInfo
           (\v flags -> flags{configDebugInfo = v})
-          [ optArg'
+          [ reqArg'
               "n"
-              (Flag . flagToDebugInfoLevel)
+              (Flag . fromString)
               ( \case
-                  Flag NoDebugInfo -> []
-                  Flag MinimalDebugInfo -> [Just "1"]
-                  Flag NormalDebugInfo -> [Nothing]
-                  Flag MaximalDebugInfo -> [Just "3"]
-                  _ -> []
+                  NoFlag -> []
+                  Flag flag -> [DebugInfoLevel.toString flag]
               )
-              ""
+              "g"
               ["enable-debug-info"]
-              "Emit debug info (n is 0--3, default is 0)"
+              ( let minLevel = DebugInfoLevel.toString minBound
+                    maxLevel = DebugInfoLevel.toString maxBound
+                    def = DebugInfoLevel.toString noFlagValue
+                 in "Emit debug info (n is " ++ minLevel ++ "--" ++ maxLevel ++ ", default is " ++ def ++ ")"
+              )
           , noArg
               (Flag NoDebugInfo)
               []
diff --git a/Cabal/src/Distribution/Simple/UHC.hs b/Cabal/src/Distribution/Simple/UHC.hs
index 2a9261a40b0..dae0af37596 100644
--- a/Cabal/src/Distribution/Simple/UHC.hs
+++ b/Cabal/src/Distribution/Simple/UHC.hs
@@ -45,6 +45,7 @@ import Distribution.Simple.Program
 import Distribution.Simple.Utils
 import Distribution.System
 import Distribution.Types.MungedPackageId
+import Distribution.Types.OptimisationLevel (OptimisationLevel (..))
 import Distribution.Utils.Path
 import Distribution.Verbosity
 import Distribution.Version
diff --git a/Cabal/src/Distribution/Types/DebugInfoLevel.hs b/Cabal/src/Distribution/Types/DebugInfoLevel.hs
new file mode 100644
index 00000000000..4087a781e62
--- /dev/null
+++ b/Cabal/src/Distribution/Types/DebugInfoLevel.hs
@@ -0,0 +1,73 @@
+{-# LANGUAGE DataKinds #-}
+{-# LANGUAGE DeriveGeneric #-}
+{-# LANGUAGE DerivingStrategies #-}
+{-# LANGUAGE InstanceSigs #-}
+
+module Distribution.Types.DebugInfoLevel
+  ( DebugInfoLevel (..)
+  , toString
+  )
+where
+
+import Distribution.Compat.CharParsing (integral)
+import Distribution.Compat.Prelude
+import Distribution.Parsec (CabalParsing, Parsec (..))
+import Prelude ()
+
+import Data.Bool (bool)
+import Distribution.Simple.Flag (NoFlagValue (..))
+
+-- ------------------------------------------------------------
+
+-- * Debug info levels
+
+-- ------------------------------------------------------------
+
+-- | Some compilers support emitting debug info. Some have different
+-- levels.  For compilers that do not the level is just capped to the
+-- level they do support.
+data DebugInfoLevel
+  = NoDebugInfo
+  | MinimalDebugInfo
+  | NormalDebugInfo
+  | MaximalDebugInfo
+  deriving stock (Bounded, Enum, Eq, Generic, Read, Show)
+
+instance Binary DebugInfoLevel
+instance NFData DebugInfoLevel
+instance Structured DebugInfoLevel
+
+instance NoFlagValue DebugInfoLevel where
+  noFlagValue :: DebugInfoLevel
+  noFlagValue = NoDebugInfo
+
+instance Parsec DebugInfoLevel where
+  parsec :: CabalParsing m => m DebugInfoLevel
+  parsec = boolParser <|> intParser
+    where
+      boolParser = bool NoDebugInfo NormalDebugInfo <$> parsec
+      intParser = intToDebugInfoLevel <$> integral
+
+instance IsString DebugInfoLevel where
+  fromString :: String -> DebugInfoLevel
+  fromString s = case reads s of
+    [(i, "")] -> intToDebugInfoLevel i
+    _ -> error $ "Can't parse debug info level " ++ s
+
+toString :: DebugInfoLevel -> String
+toString = show . fromEnum
+
+intToDebugInfoLevel :: Int -> DebugInfoLevel
+intToDebugInfoLevel i
+  | i >= minLevel && i <= maxLevel = toEnum i
+  | otherwise =
+      error $
+        "Bad debug info level: "
+          ++ show i
+          ++ ". Valid values are "
+          ++ show minLevel
+          ++ ".."
+          ++ show maxLevel
+  where
+    minLevel = fromEnum (minBound :: DebugInfoLevel)
+    maxLevel = fromEnum (maxBound :: DebugInfoLevel)
diff --git a/Cabal/src/Distribution/Types/DumpBuildInfo.hs b/Cabal/src/Distribution/Types/DumpBuildInfo.hs
index b946093b487..fff8fe8ad60 100644
--- a/Cabal/src/Distribution/Types/DumpBuildInfo.hs
+++ b/Cabal/src/Distribution/Types/DumpBuildInfo.hs
@@ -1,26 +1,38 @@
 {-# LANGUAGE DeriveGeneric #-}
+{-# LANGUAGE DerivingStrategies #-}
+{-# LANGUAGE InstanceSigs #-}
+{-# LANGUAGE LambdaCase #-}
 
 module Distribution.Types.DumpBuildInfo
   ( DumpBuildInfo (..)
+  , toString
   ) where
 
-import Distribution.Compat.Prelude
-import Distribution.Parsec
+import Distribution.Compat.Prelude (Binary, Generic, NFData, Structured)
+import Distribution.Parsec (CabalParsing, Parsec (..))
+import Distribution.Simple.Flag (NoFlagValue (..))
 
 data DumpBuildInfo
   = NoDumpBuildInfo
   | DumpBuildInfo
-  deriving (Read, Show, Eq, Ord, Enum, Bounded, Generic)
+  deriving stock (Read, Show, Eq, Ord, Enum, Bounded, Generic)
 
 instance Binary DumpBuildInfo
 instance NFData DumpBuildInfo
 instance Structured DumpBuildInfo
 
-instance Parsec DumpBuildInfo where
-  parsec = parsecDumpBuildInfo
+instance NoFlagValue DumpBuildInfo where
+  noFlagValue :: DumpBuildInfo
+  noFlagValue = NoDumpBuildInfo
 
-parsecDumpBuildInfo :: CabalParsing m => m DumpBuildInfo
-parsecDumpBuildInfo = boolToDumpBuildInfo <$> parsec
+instance Parsec DumpBuildInfo where
+  parsec :: CabalParsing m => m DumpBuildInfo
+  parsec = boolToDumpBuildInfo <$> parsec
 
 boolToDumpBuildInfo :: Bool -> DumpBuildInfo
 boolToDumpBuildInfo bool = if bool then DumpBuildInfo else NoDumpBuildInfo
+
+toString :: DumpBuildInfo -> String
+toString = \case
+  NoDumpBuildInfo -> "False"
+  DumpBuildInfo -> "True"
diff --git a/Cabal/src/Distribution/Types/LocalBuildConfig.hs b/Cabal/src/Distribution/Types/LocalBuildConfig.hs
index f79d470dcf3..9bd0246b6f6 100644
--- a/Cabal/src/Distribution/Types/LocalBuildConfig.hs
+++ b/Cabal/src/Distribution/Types/LocalBuildConfig.hs
@@ -20,25 +20,23 @@ module Distribution.Types.LocalBuildConfig
 import Distribution.Compat.Prelude
 import Prelude ()
 
-import Distribution.Types.ComponentLocalBuildInfo
-import Distribution.Types.ComponentRequestedSpec
-import Distribution.Types.GivenComponent
-import Distribution.Types.PackageDescription
-import Distribution.Types.UnitId
+import Distribution.Types.ComponentLocalBuildInfo (ComponentLocalBuildInfo)
+import Distribution.Types.ComponentRequestedSpec (ComponentRequestedSpec)
+import Distribution.Types.GivenComponent (PromisedComponent)
+import Distribution.Types.PackageDescription (PackageDescription)
+import Distribution.Types.UnitId (UnitId)
 
-import Distribution.PackageDescription
-import Distribution.Simple.Compiler
-import Distribution.Simple.Flag
-import Distribution.Simple.InstallDirs hiding
-  ( absoluteInstallDirs
-  , prefixRelativeInstallDirs
-  , substPathTemplate
-  )
-import Distribution.Simple.PackageIndex
+import Distribution.PackageDescription (ComponentName, FlagAssignment, PackageName)
+import Distribution.Simple.Compiler (Compiler, PackageDBStack, ProfDetailLevel)
+import Distribution.Simple.Flag (toFlag)
+import Distribution.Simple.InstallDirs (InstallDirTemplates)
+import Distribution.Simple.PackageIndex (InstalledPackageIndex)
 import Distribution.Simple.Program.Db (ProgramDb)
-import Distribution.Simple.Setup.Config
-import Distribution.System
-import Distribution.Utils.Path
+import Distribution.Simple.Setup.Config (ConfigFlags (..))
+import Distribution.System (Platform)
+import Distribution.Types.DebugInfoLevel (DebugInfoLevel)
+import Distribution.Types.OptimisationLevel (OptimisationLevel)
+import Distribution.Utils.Path (FileOrDir (..), Pkg, SymbolicPath)
 
 import Distribution.Compat.Graph (Graph)
 
diff --git a/Cabal/src/Distribution/Types/LocalBuildInfo.hs b/Cabal/src/Distribution/Types/LocalBuildInfo.hs
index f525d397aba..15afb3cc076 100644
--- a/Cabal/src/Distribution/Types/LocalBuildInfo.hs
+++ b/Cabal/src/Distribution/Types/LocalBuildInfo.hs
@@ -131,6 +131,8 @@ import Distribution.System
 import qualified Data.Map as Map
 import Distribution.Compat.Graph (Graph)
 import qualified Distribution.Compat.Graph as Graph
+import Distribution.Types.DebugInfoLevel (DebugInfoLevel)
+import Distribution.Types.OptimisationLevel (OptimisationLevel)
 
 import qualified System.FilePath as FilePath (takeDirectory)
 
diff --git a/Cabal/src/Distribution/Types/OptimisationLevel.hs b/Cabal/src/Distribution/Types/OptimisationLevel.hs
new file mode 100644
index 00000000000..cca447a7bb9
--- /dev/null
+++ b/Cabal/src/Distribution/Types/OptimisationLevel.hs
@@ -0,0 +1,72 @@
+{-# LANGUAGE DataKinds #-}
+{-# LANGUAGE DeriveGeneric #-}
+{-# LANGUAGE DerivingStrategies #-}
+{-# LANGUAGE InstanceSigs #-}
+
+module Distribution.Types.OptimisationLevel
+  ( OptimisationLevel (..)
+  , toString
+  )
+where
+
+import Distribution.Compat.CharParsing (integral)
+import Distribution.Compat.Prelude
+import Distribution.Parsec (CabalParsing, Parsec (..))
+import Prelude ()
+
+import Data.Bool (bool)
+import Distribution.Simple.Flag (NoFlagValue (..))
+
+-- ------------------------------------------------------------
+
+-- * Optimisation levels
+
+-- ------------------------------------------------------------
+
+-- | Some compilers support optimising. Some have different levels.
+-- For compilers that do not the level is just capped to the level
+-- they do support.
+data OptimisationLevel
+  = NoOptimisation
+  | NormalOptimisation
+  | MaximumOptimisation
+  deriving stock (Bounded, Enum, Eq, Generic, Read, Show)
+
+instance Binary OptimisationLevel
+instance NFData OptimisationLevel
+instance Structured OptimisationLevel
+
+instance NoFlagValue OptimisationLevel where
+  noFlagValue :: OptimisationLevel
+  noFlagValue = NormalOptimisation
+
+instance Parsec OptimisationLevel where
+  parsec :: CabalParsing m => m OptimisationLevel
+  parsec = boolParser <|> intParser
+    where
+      boolParser = bool NoOptimisation NormalOptimisation <$> parsec
+      intParser = intToOptimisationLevel <$> integral
+
+instance IsString OptimisationLevel where
+  fromString :: String -> OptimisationLevel
+  fromString s = case reads s of
+    [(i, "")] -> intToOptimisationLevel i
+    _ -> error $ "Can't parse optimisation level " ++ s
+
+toString :: OptimisationLevel -> String
+toString = show . fromEnum
+
+intToOptimisationLevel :: Int -> OptimisationLevel
+intToOptimisationLevel i
+  | i >= minLevel && i <= maxLevel = toEnum i
+  | otherwise =
+      error $
+        "Bad optimisation level: "
+          ++ show i
+          ++ ". Valid values are "
+          ++ show minLevel
+          ++ ".."
+          ++ show maxLevel
+  where
+    minLevel = fromEnum (minBound :: OptimisationLevel)
+    maxLevel = fromEnum (maxBound :: OptimisationLevel)
diff --git a/cabal-install/parser-tests/Tests/ParserTests.hs b/cabal-install/parser-tests/Tests/ParserTests.hs
index 34b65edcb5e..4bb34b824e1 100644
--- a/cabal-install/parser-tests/Tests/ParserTests.hs
+++ b/cabal-install/parser-tests/Tests/ParserTests.hs
@@ -31,7 +31,7 @@ import Distribution.Client.Types.WriteGhcEnvironmentFilesPolicy (WriteGhcEnviron
 import Distribution.Compat.Prelude
 import Distribution.Compiler (CompilerFlavor (..))
 import Distribution.Parsec (simpleParsec)
-import Distribution.Simple.Compiler (DebugInfoLevel (..), OptimisationLevel (..), PackageDBX (..), ProfDetailLevel (..))
+import Distribution.Simple.Compiler (PackageDBX (..), ProfDetailLevel (..))
 import Distribution.Simple.Flag
 import Distribution.Simple.InstallDirs (InstallDirs (..), toPathTemplate)
 import Distribution.Simple.Setup (DumpBuildInfo (..), HaddockTarget (..), TestShowDetails (..))
@@ -50,7 +50,9 @@ import Distribution.Solver.Types.Settings
   )
 import Distribution.System (OS (..), buildOS)
 import Distribution.Types.CondTree (CondTree (..))
+import Distribution.Types.DebugInfoLevel (DebugInfoLevel (..))
 import Distribution.Types.Flag (mkFlagAssignment)
+import Distribution.Types.OptimisationLevel (OptimisationLevel (..))
 import Distribution.Types.PackageId (PackageIdentifier (..))
 import Distribution.Types.PackageName
 import Distribution.Types.PackageVersionConstraint (PackageVersionConstraint (..))
diff --git a/cabal-install/src/Distribution/Client/Config.hs b/cabal-install/src/Distribution/Client/Config.hs
index dd353e422a6..524f4b35c43 100644
--- a/cabal-install/src/Distribution/Client/Config.hs
+++ b/cabal-install/src/Distribution/Client/Config.hs
@@ -1,5 +1,4 @@
 {-# LANGUAGE DeriveGeneric #-}
-{-# LANGUAGE LambdaCase #-}
 {-# LANGUAGE PatternSynonyms #-}
 
 -----------------------------------------------------------------------------
@@ -138,7 +137,6 @@ import Distribution.Compiler
 import Distribution.Deprecated.ParseUtils
   ( FieldDescr (..)
   , PError (..)
-  , PWarning (..)
   , ParseResult (..)
   , liftField
   , lineNo
@@ -166,10 +164,7 @@ import Distribution.Simple.Command
   , ShowOrParseArgs (..)
   , commandDefaultFlags
   )
-import Distribution.Simple.Compiler
-  ( DebugInfoLevel (..)
-  , OptimisationLevel (..)
-  )
+import Distribution.Simple.Flag (Flag, flagElim, flagToMaybe, fromFlagOrDefault, toFlag, pattern Flag, pattern NoFlag)
 import Distribution.Simple.InstallDirs
   ( InstallDirs (..)
   , PathTemplate
@@ -183,7 +178,6 @@ import Distribution.Simple.Setup
   ( BenchmarkFlags (..)
   , CommonSetupFlags (..)
   , ConfigFlags (..)
-  , Flag
   , HaddockFlags (..)
   , TestFlags (..)
   , configureOptions
@@ -191,16 +185,11 @@ import Distribution.Simple.Setup
   , defaultConfigFlags
   , defaultHaddockFlags
   , defaultTestFlags
-  , flagToMaybe
-  , fromFlagOrDefault
   , haddockOptions
   , installDirsOptions
   , optionDistPref
   , programDbOptions
   , programDbPaths'
-  , toFlag
-  , pattern Flag
-  , pattern NoFlag
   )
 import Distribution.Simple.Utils
   ( cabalVersion
@@ -212,6 +201,8 @@ import Distribution.Simple.Utils
   , writeFileAtomic
   )
 import Distribution.Solver.Types.ConstraintSource
+import qualified Distribution.Types.DebugInfoLevel as DebugInfoLevel
+import qualified Distribution.Types.OptimisationLevel as OptimisationLevel
 import Distribution.Utils.Path (getSymbolicPath, unsafeMakeSymbolicPath)
 import Distribution.Verbosity
   ( normal
@@ -1190,75 +1181,23 @@ configFieldDescriptions src =
           (Flag <$> parsec <|> pure NoFlag)
           configHcFlavor
           (\v flags -> flags{configHcFlavor = v})
-      , -- TODO: The following is a temporary fix. The "optimization"
-        -- and "debug-info" fields are OptArg, and viewAsFieldDescr
-        -- fails on that. Instead of a hand-written hackaged parser
-        -- and printer, we should handle this case properly in the
-        -- library.
-        liftField
-          configOptimization
-          ( \v flags ->
-              flags{configOptimization = v}
-          )
-          $ let name = "optimization"
-             in FieldDescr
-                  name
-                  ( \case
-                      Flag NoOptimisation -> Disp.text "False"
-                      Flag NormalOptimisation -> Disp.text "True"
-                      Flag MaximumOptimisation -> Disp.text "2"
-                      _ -> Disp.empty
-                  )
-                  ( \line str _ -> case () of
-                      _
-                        | str == "False" -> ParseOk [] (Flag NoOptimisation)
-                        | str == "True" -> ParseOk [] (Flag NormalOptimisation)
-                        | str == "0" -> ParseOk [] (Flag NoOptimisation)
-                        | str == "1" -> ParseOk [] (Flag NormalOptimisation)
-                        | str == "2" -> ParseOk [] (Flag MaximumOptimisation)
-                        | lstr == "false" -> ParseOk [caseWarning] (Flag NoOptimisation)
-                        | lstr == "true" ->
-                            ParseOk
-                              [caseWarning]
-                              (Flag NormalOptimisation)
-                        | otherwise -> ParseFailed (NoParse name line)
-                        where
-                          lstr = lowercase str
-                          caseWarning =
-                            PWarning $
-                              "The '"
-                                ++ name
-                                ++ "' field is case sensitive, use 'True' or 'False'."
-                  )
+      , liftField configOptimization (\v flags -> flags{configOptimization = v}) $
+          let name = "optimization"
+           in FieldDescr
+                name
+                (flagElim Disp.empty (Disp.text . OptimisationLevel.toString))
+                ( \line str _ -> case maybe NoFlag Flag (simpleParsec str) of
+                    NoFlag -> ParseFailed (NoParse name line)
+                    flag -> ParseOk [] flag
+                )
       , liftField configDebugInfo (\v flags -> flags{configDebugInfo = v}) $
           let name = "debug-info"
            in FieldDescr
                 name
-                ( \case
-                    Flag NoDebugInfo -> Disp.text "False"
-                    Flag MinimalDebugInfo -> Disp.text "1"
-                    Flag NormalDebugInfo -> Disp.text "True"
-                    Flag MaximalDebugInfo -> Disp.text "3"
-                    _ -> Disp.empty
-                )
-                ( \line str _ -> case () of
-                    _
-                      | str == "False" -> ParseOk [] (Flag NoDebugInfo)
-                      | str == "True" -> ParseOk [] (Flag NormalDebugInfo)
-                      | str == "0" -> ParseOk [] (Flag NoDebugInfo)
-                      | str == "1" -> ParseOk [] (Flag MinimalDebugInfo)
-                      | str == "2" -> ParseOk [] (Flag NormalDebugInfo)
-                      | str == "3" -> ParseOk [] (Flag MaximalDebugInfo)
-                      | lstr == "false" -> ParseOk [caseWarning] (Flag NoDebugInfo)
-                      | lstr == "true" -> ParseOk [caseWarning] (Flag NormalDebugInfo)
-                      | otherwise -> ParseFailed (NoParse name line)
-                      where
-                        lstr = lowercase str
-                        caseWarning =
-                          PWarning $
-                            "The '"
-                              ++ name
-                              ++ "' field is case sensitive, use 'True' or 'False'."
+                (flagElim Disp.empty (Disp.text . DebugInfoLevel.toString))
+                ( \line str _ -> case maybe NoFlag Flag (simpleParsec str) of
+                    NoFlag -> ParseFailed (NoParse name line)
+                    flag -> ParseOk [] flag
                 )
       ]
     ++ toSavedConfig
diff --git a/cabal-install/src/Distribution/Client/DistDirLayout.hs b/cabal-install/src/Distribution/Client/DistDirLayout.hs
index 0a6e51b09e7..03ab7974b8c 100644
--- a/cabal-install/src/Distribution/Client/DistDirLayout.hs
+++ b/cabal-install/src/Distribution/Client/DistDirLayout.hs
@@ -43,7 +43,6 @@ import Distribution.Package
   )
 import Distribution.Simple.Compiler
   ( Compiler (..)
-  , OptimisationLevel (..)
   , PackageDBCWD
   , PackageDBStackCWD
   , PackageDBX (..)
@@ -52,6 +51,7 @@ import Distribution.Simple.Configure (interpretPackageDbFlags)
 import Distribution.System
 import Distribution.Types.ComponentName
 import Distribution.Types.LibraryName
+import Distribution.Types.OptimisationLevel (OptimisationLevel (..))
 
 -- | Information which can be used to construct the path to
 -- the build directory of a build.  This is LESS fine-grained
diff --git a/cabal-install/src/Distribution/Client/PackageHash.hs b/cabal-install/src/Distribution/Client/PackageHash.hs
index 3a94c0e028b..5ad31354ab4 100644
--- a/cabal-install/src/Distribution/Client/PackageHash.hs
+++ b/cabal-install/src/Distribution/Client/PackageHash.hs
@@ -40,8 +40,6 @@ import Distribution.Package
 import Distribution.Simple.Compiler
   ( AbiTag (..)
   , CompilerId
-  , DebugInfoLevel (..)
-  , OptimisationLevel (..)
   , PackageDBCWD
   , ProfDetailLevel (..)
   , showProfDetailLevel
@@ -56,10 +54,12 @@ import Distribution.System
   , Platform
   , buildOS
   )
+import Distribution.Types.DebugInfoLevel (DebugInfoLevel (..))
 import Distribution.Types.Flag
   ( FlagAssignment
   , showFlagAssignment
   )
+import Distribution.Types.OptimisationLevel (OptimisationLevel (..))
 import Distribution.Types.PkgconfigVersion (PkgconfigVersion)
 
 import qualified Data.ByteString.Lazy.Char8 as LBS
diff --git a/cabal-install/src/Distribution/Client/ProjectConfig/Legacy.hs b/cabal-install/src/Distribution/Client/ProjectConfig/Legacy.hs
index 0dea2791303..b915449795e 100644
--- a/cabal-install/src/Distribution/Client/ProjectConfig/Legacy.hs
+++ b/cabal-install/src/Distribution/Client/ProjectConfig/Legacy.hs
@@ -88,8 +88,6 @@ import Distribution.PackageDescription.Configuration (simplifyWithSysParams)
 import Distribution.Simple.Compiler
   ( Compiler (..)
   , CompilerInfo (..)
-  , DebugInfoLevel (..)
-  , OptimisationLevel (..)
   , compilerInfo
   , interpretPackageDB
   )
@@ -110,8 +108,6 @@ import Distribution.Simple.Setup
   ( BenchmarkFlags (..)
   , CommonSetupFlags (..)
   , ConfigFlags (..)
-  , DumpBuildInfo (DumpBuildInfo, NoDumpBuildInfo)
-  , Flag
   , HaddockFlags (..)
   , TestFlags (..)
   , benchmarkOptions'
@@ -119,7 +115,6 @@ import Distribution.Simple.Setup
   , defaultBenchmarkFlags
   , defaultHaddockFlags
   , defaultTestFlags
-  , fromFlagOrDefault
   , haddockOptions
   , installDirsOptions
   , programDbPaths'
@@ -127,13 +122,9 @@ import Distribution.Simple.Setup
   , showPackageDb
   , splitArgs
   , testOptions'
-  , toFlag
-  , pattern Flag
-  , pattern NoFlag
   )
 import Distribution.Simple.Utils
   ( debug
-  , lowercase
   , noticeDoc
   )
 import Distribution.Types.CondTree
@@ -144,6 +135,7 @@ import Distribution.Types.CondTree
   , mapTreeData
   , traverseCondTreeV
   )
+import qualified Distribution.Types.DumpBuildInfo as DumpBuildInfo
 import Distribution.Types.SourceRepo (RepoType)
 import Distribution.Utils.NubList
   ( fromNubList
@@ -156,7 +148,6 @@ import Distribution.Client.ParseUtils
 import Distribution.Client.ReplFlags (multiReplOption)
 import Distribution.Deprecated.ParseUtils
   ( PError (..)
-  , PWarning (..)
   , ParseResult (..)
   , commaNewLineListFieldParsec
   , newLineListField
@@ -200,6 +191,9 @@ import qualified Data.ByteString.Char8 as BS
 import Data.Functor ((<&>))
 import qualified Data.Map as Map
 import qualified Data.Set as Set
+import Distribution.Simple.Flag (Flag, flagElim, fromFlagOrDefault, toFlag, pattern Flag, pattern NoFlag)
+import qualified Distribution.Types.DebugInfoLevel as DebugInfoLevel
+import qualified Distribution.Types.OptimisationLevel as OptimisationLevel
 import Network.URI (URI (..), nullURIAuth)
 import System.Directory (makeAbsolute)
 import System.FilePath (splitFileName)
@@ -1693,91 +1687,38 @@ legacyPackageConfigFieldDescrs =
         (\v flags -> flags{configHcFlavor = v})
 
     overrideDumpBuildInfo =
-      liftField
-        configDumpBuildInfo
-        (\v flags -> flags{configDumpBuildInfo = v})
-        $ let name = "build-info"
-           in FieldDescr
-                name
-                ( \case
-                    Flag NoDumpBuildInfo -> Disp.text "False"
-                    Flag DumpBuildInfo -> Disp.text "True"
-                    _ -> Disp.empty
-                )
-                ( \line str _ -> case () of
-                    _
-                      | str == "False" -> ParseOk [] (Flag NoDumpBuildInfo)
-                      | str == "True" -> ParseOk [] (Flag DumpBuildInfo)
-                      | lstr == "false" -> ParseOk [caseWarning name] (Flag NoDumpBuildInfo)
-                      | lstr == "true" -> ParseOk [caseWarning name] (Flag DumpBuildInfo)
-                      | otherwise -> ParseFailed (NoParse name line)
-                      where
-                        lstr = lowercase str
-                )
-
-    -- TODO: [code cleanup] The following is a hack. The "optimization" and
-    -- "debug-info" fields are OptArg, and viewAsFieldDescr fails on that.
-    -- Instead of a hand-written parser and printer, we should handle this case
-    -- properly in the library.
+      liftField configDumpBuildInfo (\v flags -> flags{configDumpBuildInfo = v}) $
+        let name = "build-info"
+         in FieldDescr
+              name
+              (flagElim Disp.empty (Disp.text . DumpBuildInfo.toString))
+              ( \line str _ -> case maybe NoFlag Flag (simpleParsec str) of
+                  NoFlag -> ParseFailed (NoParse name line)
+                  flag -> ParseOk [] flag
+              )
 
     overrideFieldOptimization =
-      liftField
-        configOptimization
-        (\v flags -> flags{configOptimization = v})
-        $ let name = "optimization"
-           in FieldDescr
-                name
-                ( \case
-                    Flag NoOptimisation -> Disp.text "False"
-                    Flag NormalOptimisation -> Disp.text "True"
-                    Flag MaximumOptimisation -> Disp.text "2"
-                    _ -> Disp.empty
-                )
-                ( \line str _ -> case () of
-                    _
-                      | str == "False" -> ParseOk [] (Flag NoOptimisation)
-                      | str == "True" -> ParseOk [] (Flag NormalOptimisation)
-                      | str == "0" -> ParseOk [] (Flag NoOptimisation)
-                      | str == "1" -> ParseOk [] (Flag NormalOptimisation)
-                      | str == "2" -> ParseOk [] (Flag MaximumOptimisation)
-                      | lstr == "false" -> ParseOk [caseWarning name] (Flag NoOptimisation)
-                      | lstr == "true" -> ParseOk [caseWarning name] (Flag NormalOptimisation)
-                      | otherwise -> ParseFailed (NoParse name line)
-                      where
-                        lstr = lowercase str
-                )
+      liftField configOptimization (\v flags -> flags{configOptimization = v}) $
+        let name = "optimization"
+         in FieldDescr
+              name
+              (flagElim Disp.empty (Disp.text . OptimisationLevel.toString))
+              ( \line str _ -> case maybe NoFlag Flag (simpleParsec str) of
+                  NoFlag -> ParseFailed (NoParse name line)
+                  flag -> ParseOk [] flag
+              )
 
     overrideFieldDebugInfo =
       liftField configDebugInfo (\v flags -> flags{configDebugInfo = v}) $
         let name = "debug-info"
          in FieldDescr
               name
-              ( \case
-                  Flag NoDebugInfo -> Disp.text "False"
-                  Flag MinimalDebugInfo -> Disp.text "1"
-                  Flag NormalDebugInfo -> Disp.text "True"
-                  Flag MaximalDebugInfo -> Disp.text "3"
-                  _ -> Disp.empty
-              )
-              ( \line str _ -> case () of
-                  _
-                    | str == "False" -> ParseOk [] (Flag NoDebugInfo)
-                    | str == "True" -> ParseOk [] (Flag NormalDebugInfo)
-                    | str == "0" -> ParseOk [] (Flag NoDebugInfo)
-                    | str == "1" -> ParseOk [] (Flag MinimalDebugInfo)
-                    | str == "2" -> ParseOk [] (Flag NormalDebugInfo)
-                    | str == "3" -> ParseOk [] (Flag MaximalDebugInfo)
-                    | lstr == "false" -> ParseOk [caseWarning name] (Flag NoDebugInfo)
-                    | lstr == "true" -> ParseOk [caseWarning name] (Flag NormalDebugInfo)
-                    | otherwise -> ParseFailed (NoParse name line)
-                    where
-                      lstr = lowercase str
+              (flagElim Disp.empty (Disp.text . DebugInfoLevel.toString))
+              ( \line str _ -> case maybe NoFlag Flag (simpleParsec str) of
+                  NoFlag -> ParseFailed (NoParse name line)
+                  flag -> ParseOk [] flag
               )
 
-    caseWarning name =
-      PWarning $
-        "The '" ++ name ++ "' field is case sensitive, use 'True' or 'False'."
-
     prefixTest name
       | "test-" `isPrefixOf` name = name
       | otherwise = "test-" ++ name
diff --git a/cabal-install/src/Distribution/Client/ProjectConfig/Lens.hs b/cabal-install/src/Distribution/Client/ProjectConfig/Lens.hs
index 03164305a62..2151fcb360d 100644
--- a/cabal-install/src/Distribution/Client/ProjectConfig/Lens.hs
+++ b/cabal-install/src/Distribution/Client/ProjectConfig/Lens.hs
@@ -24,9 +24,7 @@ import Distribution.PackageDescription
   ( FlagAssignment
   )
 import Distribution.Simple.Compiler
-  ( DebugInfoLevel (..)
-  , OptimisationLevel (..)
-  , PackageDBCWD
+  ( PackageDBCWD
   , ProfDetailLevel
   )
 import Distribution.Simple.InstallDirs
@@ -51,6 +49,8 @@ import Distribution.Solver.Types.Settings
   , ReorderGoals (..)
   , StrongFlags (..)
   )
+import Distribution.Types.DebugInfoLevel (DebugInfoLevel (..))
+import Distribution.Types.OptimisationLevel (OptimisationLevel (..))
 import Distribution.Types.PackageVersionConstraint
   ( PackageVersionConstraint
   )
diff --git a/cabal-install/src/Distribution/Client/ProjectConfig/Types.hs b/cabal-install/src/Distribution/Client/ProjectConfig/Types.hs
index 751875be403..eb85995e12a 100644
--- a/cabal-install/src/Distribution/Client/ProjectConfig/Types.hs
+++ b/cabal-install/src/Distribution/Client/ProjectConfig/Types.hs
@@ -69,8 +69,6 @@ import Distribution.PackageDescription
 import Distribution.Simple.Compiler
   ( Compiler
   , CompilerFlavor
-  , DebugInfoLevel (..)
-  , OptimisationLevel (..)
   , PackageDBCWD
   , ProfDetailLevel
   )
@@ -99,6 +97,8 @@ import Distribution.Version
 
 import qualified Data.Map as Map
 import Distribution.Solver.Types.ProjectConfigPath (ProjectConfigPath)
+import Distribution.Types.DebugInfoLevel (DebugInfoLevel)
+import Distribution.Types.OptimisationLevel (OptimisationLevel)
 import Distribution.Types.ParStrat
 import Distribution.Verbosity (VerbosityFlags)
 
diff --git a/cabal-install/src/Distribution/Client/ProjectOrchestration.hs b/cabal-install/src/Distribution/Client/ProjectOrchestration.hs
index 606516065a3..b1d0a717f1d 100644
--- a/cabal-install/src/Distribution/Client/ProjectOrchestration.hs
+++ b/cabal-install/src/Distribution/Client/ProjectOrchestration.hs
@@ -184,17 +184,13 @@ import Distribution.Client.Errors
 import Distribution.Package
 import Distribution.Simple.Command (commandShowOptions)
 import Distribution.Simple.Compiler
-  ( OptimisationLevel (..)
-  , compilerCompatVersion
+  ( compilerCompatVersion
   , compilerId
   , compilerInfo
   , showCompilerId
   )
 import Distribution.Simple.Configure (computeEffectiveProfiling)
-import Distribution.Simple.Flag
-  ( flagToMaybe
-  , fromFlagOrDefault
-  )
+import Distribution.Simple.Flag (flagToMaybe, fromFlagOrDefault, fromNoFlag)
 import Distribution.Simple.LocalBuildInfo
   ( ComponentName (..)
   , pkgComponents
@@ -218,6 +214,7 @@ import Distribution.Types.Flag
   , diffFlagAssignment
   , showFlagAssignment
   )
+import qualified Distribution.Types.OptimisationLevel as OptimisationLevel
 import Distribution.Utils.NubList
   ( fromNubList
   )
@@ -1258,13 +1255,7 @@ printPlan
         "Build profile: "
           ++ unwords
             [ "-w " ++ (showCompilerId . pkgConfigCompiler) elaboratedShared
-            , "-O"
-                ++ ( case globalOptimization <> localOptimization of -- if local is not set, read global
-                      Setup.Flag NoOptimisation -> "0"
-                      Setup.Flag NormalOptimisation -> "1"
-                      Setup.Flag MaximumOptimisation -> "2"
-                      Setup.NoFlag -> "1"
-                   )
+            , "-O" ++ (OptimisationLevel.toString . fromNoFlag) (globalOptimization <> localOptimization)
             ]
           ++ "\n"
 
diff --git a/cabal-install/src/Distribution/Client/ProjectPlanning.hs b/cabal-install/src/Distribution/Client/ProjectPlanning.hs
index f2ace7ff3b5..259b5ae4e37 100644
--- a/cabal-install/src/Distribution/Client/ProjectPlanning.hs
+++ b/cabal-install/src/Distribution/Client/ProjectPlanning.hs
@@ -234,6 +234,8 @@ import qualified Data.Map as Map
 import qualified Data.Set as Set
 import Distribution.Client.Errors
 import Distribution.Solver.Types.ProjectConfigPath
+import Distribution.Types.DebugInfoLevel (DebugInfoLevel (..))
+import Distribution.Types.OptimisationLevel (OptimisationLevel (..))
 import System.Directory (getCurrentDirectory)
 import System.FilePath
 import qualified Text.PrettyPrint as Disp
diff --git a/cabal-install/src/Distribution/Client/ScriptUtils.hs b/cabal-install/src/Distribution/Client/ScriptUtils.hs
index 6f6e03eccf5..8500179ca49 100644
--- a/cabal-install/src/Distribution/Client/ScriptUtils.hs
+++ b/cabal-install/src/Distribution/Client/ScriptUtils.hs
@@ -119,11 +119,10 @@ import Distribution.Parsec
 import qualified Distribution.SPDX.License as SPDX
 import Distribution.Simple.Compiler
   ( Compiler (..)
-  , OptimisationLevel (..)
   )
 import Distribution.Simple.Flag
   ( flagToMaybe
-  , fromFlagOrDefault
+  , fromNoFlag
   )
 import Distribution.Simple.PackageDescription
   ( parseString
@@ -434,7 +433,7 @@ scriptDistDirParams scriptPath ctx compiler platform =
     , distParamComponentName = Just $ CExeName cn
     , distParamCompilerId = compilerId compiler
     , distParamPlatform = platform
-    , distParamOptimization = fromFlagOrDefault NormalOptimisation optimization
+    , distParamOptimization = fromNoFlag optimization
     }
   where
     cn = scriptComponentName scriptPath
diff --git a/cabal-install/tests/UnitTests/Distribution/Client/Configure.hs b/cabal-install/tests/UnitTests/Distribution/Client/Configure.hs
index 21a6bcabb46..07694e82214 100644
--- a/cabal-install/tests/UnitTests/Distribution/Client/Configure.hs
+++ b/cabal-install/tests/UnitTests/Distribution/Client/Configure.hs
@@ -9,9 +9,9 @@ import Distribution.Client.NixStyleOptions
 import Distribution.Client.ProjectConfig.Types
 import Distribution.Client.ProjectFlags
 import Distribution.Client.Setup
-import Distribution.Simple
 import Distribution.Simple.Flag
 import Distribution.Simple.Utils (removeFileForcibly)
+import Distribution.Types.OptimisationLevel (OptimisationLevel (..))
 import Distribution.Verbosity
 import System.Directory
 import System.FilePath
diff --git a/changelog.d/11828.md b/changelog.d/11828.md
new file mode 100644
index 00000000000..be9ddc26e5f
--- /dev/null
+++ b/changelog.d/11828.md
@@ -0,0 +1,12 @@
+---
+synopsis: "Allow case insensitive bools and move to numeric render"
+packages: [Cabal-syntax, Cabal, cabal-install]
+prs: 11828
+---
+The `.cabal` file parser now accepts boolean values in a case‑insensitive manner (`true`, `True`, `TRUE`, `false`, etc.) without emitting a warning.
+Previously only `True` and `False` were treated as “standard”, and other capitalisations triggered a parse warning. That warning was introduced in 2008 as a backwards‑compatibility shim to prevent Hackage uploads that would break very old Cabal versions. Since all supported Cabal releases can now handle case‑insensitive booleans, the warning has been removed as unnecessary.
+
+Additionally, the rendering of certain boolean‑like flags has been changed to use numeric levels:
+- For the `-O` (optimisation) flag, `True` or an unspecified value now renders as `-O1`, while `False` renders as `-O0`.
+- For the `-g` (debug info) flag, `True` or an unspecified value now renders `-g2`, while `False` renders as `-g0`.
+This makes the displayed compiler flags more consistent with their internal semantics (e.g. optimisation level 1 is the default).