if the coercion is a Refl or MRefl, lets just keep it.

1 files changed, 9 insertions, 3 deletions

diff --git a/compiler/simplCore/CoreEraseCoercionProofs.hs b/compiler/simplCore/CoreEraseCoercionProofs.hs
index baa0695cb7..d2417416ac 100644
--- a/compiler/simplCore/CoreEraseCoercionProofs.hs
+++ b/compiler/simplCore/CoreEraseCoercionProofs.hs
@@ -8,7 +8,7 @@ import HscTypes         ( ModGuts(..) )
 import Coercion
 import CoreMonad        ( CoreM )
 import DynFlags
-import TyCoRep (Coercion(ErasedCoercion))
+import TyCoRep (Coercion(..))
 {-
 Top-level interface function, @eraseCoercionProgram@.
 
@@ -38,12 +38,18 @@ coreExprEraseProof (Case  scrut v ty alts  )=
     Case (coreExprEraseProof scrut) v ty (map eraseAltPfs alts)
     --- TODO : add mrefl and refl cases,
     --- that should suffice to prevent regresions vs current ghc
-coreExprEraseProof (Cast  e co ) = Cast (coreExprEraseProof e) (ErasedCoercion role lty rty )
+coreExprEraseProof (Cast  e co ) =case co of
+      (Refl _t) -> Cast e  co
+      (GRefl _r _t MRefl) -> Cast e co
+      (_) ->  Coercion (ErasedCoercion role lty rty  Cast (coreExprEraseProof e) (ErasedCoercion role lty rty )
   where
     (Pair lty rty,role) = coercionKindRole co
 coreExprEraseProof (Tick  tick e)= Tick tick (coreExprEraseProof e)
 coreExprEraseProof (Type  t) = Type t
-coreExprEraseProof (Coercion co )=  Coercion (ErasedCoercion role lty rty )
+coreExprEraseProof (Coercion co )= case co of
+      (Refl t) -> Coercion co
+      (GRefl r t MRefl) -> Coercion co
+      (_) ->  Coercion (ErasedCoercion role lty rty )
   where
     (Pair lty rty,role) = coercionKindRole co
 eraseAltPfs :: CoreAlt -> CoreAlt