Remove Semigroup instance for UniqDFM (#19654)

The (<>) operator was not associative.

Fortunately, the instance is not used anywhere, except to derive
another unused instance for UniqDSet.

2 files changed, 1 insertions, 11 deletions

diff --git a/compiler/GHC/Types/Unique/DFM.hs b/compiler/GHC/Types/Unique/DFM.hs
index f3009de2a2..f143af2b58 100644
--- a/compiler/GHC/Types/Unique/DFM.hs
+++ b/compiler/GHC/Types/Unique/DFM.hs
@@ -77,7 +77,6 @@ import Data.Data
 import Data.Functor.Classes (Eq1 (..))
 import Data.List (sortBy)
 import Data.Function (on)
-import qualified Data.Semigroup as Semi
 import GHC.Types.Unique.FM (UniqFM, nonDetUFMToList, ufmToIntMap, unsafeIntMapToUFM)
 import Unsafe.Coerce
 
@@ -412,13 +411,6 @@ anyUDFM p (UDFM m _i) = M.foldr ((||) . p . taggedFst) False m
 allUDFM :: (elt -> Bool) -> UniqDFM key elt -> Bool
 allUDFM p (UDFM m _i) = M.foldr ((&&) . p . taggedFst) True m
 
-instance Semi.Semigroup (UniqDFM key a) where
-  (<>) = plusUDFM
-
-instance Monoid (UniqDFM key a) where
-  mempty = emptyUDFM
-  mappend = (Semi.<>)
-
 -- This should not be used in committed code, provided for convenience to
 -- make ad-hoc conversions when developing
 alwaysUnsafeUfmToUdfm :: UniqFM key elt -> UniqDFM key elt
diff --git a/compiler/GHC/Types/Unique/DSet.hs b/compiler/GHC/Types/Unique/DSet.hs
index 0ce46ab083..d4d555f83b 100644
--- a/compiler/GHC/Types/Unique/DSet.hs
+++ b/compiler/GHC/Types/Unique/DSet.hs
@@ -8,7 +8,6 @@
 --
 -- Basically, the things need to be in class 'Uniquable'.
 
-{-# LANGUAGE GeneralizedNewtypeDeriving #-}
 {-# LANGUAGE DeriveDataTypeable #-}
 
 module GHC.Types.Unique.DSet (
@@ -46,14 +45,13 @@ import GHC.Types.Unique
 
 import Data.Coerce
 import Data.Data
-import qualified Data.Semigroup as Semi
 
 -- See Note [UniqSet invariant] in GHC.Types.Unique.Set for why we want a newtype here.
 -- Beyond preserving invariants, we may also want to 'override' typeclass
 -- instances.
 
 newtype UniqDSet a = UniqDSet {getUniqDSet' :: UniqDFM a a}
-                   deriving (Data, Semi.Semigroup, Monoid)
+                   deriving (Data)
 
 emptyUniqDSet :: UniqDSet a
 emptyUniqDSet = UniqDSet emptyUDFM