Replace internal `class Semigroup` fork by original class

in favour of the real `class Semigroup` provided by the canonical
`semigroups` package for GHC < 8.0

This way we can properly provide `Semigroup` instances as well a
`NonEmpty` lists in Cabal's public API in a uniform way for older GHCs
before GHC 8.0/base-4.9

NB: This incurs a dependency on `semigroups` *only* for GHC versions
    prior to GHC 8.0; similar to the pre-existing conditional
    dependency on the `fail` package

Cabal/Cabal.cabal

@@ -283,9 +283,9 @@ library
     ghc-options: -Wcompat -Wnoncanonical-monad-instances
                  -Wnoncanonical-monadfail-instances
   else
-    -- at least one of lib:Cabal's dependency (`parsec`)
-    -- already depends on `fail` transitively
-    build-depends: fail == 4.9.*
+    -- at least one of lib:Cabal's dependency (i.e. `parsec`)
+    -- already depends on `fail` and `semigroups` transitively
+    build-depends: fail == 4.9.*, semigroups >= 0.18.3 && < 0.20
 
   exposed-modules:
     Distribution.Backpack

Cabal/ChangeLog.md

@@ -25,6 +25,7 @@
   * Change the arguments of `Newtype` class to better suit @DeriveAnyClass@ usage,
     add default implementation in terms of `coerce` / `unsafeCoerce`.
   * Implement support for response file arguments to defaultMain* and cabal-install.
+  * Uniformly provide 'Semigroup' instances for `base < 4.9` via `semigroups` package
 
 ----
 

Cabal/Distribution/Compat/Semigroup.hs

@@ -23,85 +23,12 @@ module Distribution.Compat.Semigroup
 import Distribution.Compat.Binary (Binary)
 
 import GHC.Generics
-#if __GLASGOW_HASKELL__ >= 711
--- Data.Semigroup is available since GHC 8.0/base-4.9
+-- Data.Semigroup is available since GHC 8.0/base-4.9 in `base`
+-- for older GHC/base, it's provided by `semigroups`
 import Data.Semigroup
 import qualified Data.Monoid as Mon
-#else
--- provide internal simplified non-exposed class for older GHCs
-import Data.Monoid as Mon (Monoid(..), All(..), Any(..), Dual(..))
--- containers
-import Data.Set (Set)
-import Data.IntSet (IntSet)
-import Data.Map (Map)
-import Data.IntMap (IntMap)
 
 
-class Semigroup a where
-    (<>) :: a -> a -> a
-
--- several primitive instances
-instance Semigroup () where
-    _ <> _ = ()
-
-instance Semigroup [a] where
-    (<>) = (++)
-
-instance Semigroup a => Semigroup (Dual a) where
-    Dual a <> Dual b = Dual (b <> a)
-
-instance Semigroup a => Semigroup (Maybe a) where
-    Nothing <> b       = b
-    a       <> Nothing = a
-    Just a  <> Just b  = Just (a <> b)
-
-instance Semigroup (Either a b) where
-    Left _ <> b = b
-    a      <> _ = a
-
-instance Semigroup Ordering where
-    LT <> _ = LT
-    EQ <> y = y
-    GT <> _ = GT
-
-instance Semigroup b => Semigroup (a -> b) where
-    f <> g = \a -> f a <> g a
-
-instance Semigroup All where
-    All a <> All b = All (a && b)
-
-instance Semigroup Any where
-    Any a <> Any b = Any (a || b)
-
-instance (Semigroup a, Semigroup b) => Semigroup (a, b) where
-    (a,b) <> (a',b') = (a<>a',b<>b')
-
-instance (Semigroup a, Semigroup b, Semigroup c)
-         => Semigroup (a, b, c) where
-    (a,b,c) <> (a',b',c') = (a<>a',b<>b',c<>c')
-
-instance (Semigroup a, Semigroup b, Semigroup c, Semigroup d)
-         => Semigroup (a, b, c, d) where
-    (a,b,c,d) <> (a',b',c',d') = (a<>a',b<>b',c<>c',d<>d')
-
-instance (Semigroup a, Semigroup b, Semigroup c, Semigroup d, Semigroup e)
-         => Semigroup (a, b, c, d, e) where
-    (a,b,c,d,e) <> (a',b',c',d',e') = (a<>a',b<>b',c<>c',d<>d',e<>e')
-
--- containers instances
-instance Semigroup IntSet where
-  (<>) = mappend
-
-instance Ord a => Semigroup (Set a) where
-  (<>) = mappend
-
-instance Semigroup (IntMap v) where
-  (<>) = mappend
-
-instance Ord k => Semigroup (Map k v) where
-  (<>) = mappend
-#endif
-
 -- | A copy of 'Data.Semigroup.First'.
 newtype First' a = First' { getFirst' :: a }
   deriving (Eq, Ord, Show)