3 files changed, 6 insertions, 3 deletions
diff --git a/compiler/GHC/Core/Coercion.hs b/compiler/GHC/Core/Coercion.hs
index 06bc235913..1449e2331d 100644
--- a/compiler/GHC/Core/Coercion.hs
+++ b/compiler/GHC/Core/Coercion.hs
@@ -146,6 +146,7 @@ import GHC.Types.Var.Set
 import GHC.Types.Name hiding ( varName )
 import GHC.Types.Basic
 import GHC.Types.Unique
+import GHC.Data.FastString
 import GHC.Data.Pair
 import GHC.Types.SrcLoc
 import GHC.Builtin.Names
@@ -286,7 +287,7 @@ tidyCoAxBndrsForUser init_env tcvs
         (env', bndr') = tidyVarBndr env bndr
         env_wild = (occ_env, extendVarEnv subst bndr wild_bndr)
         wild_bndr = setVarName bndr $
-                    tidyNameOcc (varName bndr) (mkTyVarOcc "_")
+                    tidyNameOcc (varName bndr) (mkTyVarOccFS (fsLit "_"))
                     -- Tidy the binder to "_"
 
     is_wildcard :: Var -> Bool
diff --git a/compiler/GHC/Core/FamInstEnv.hs b/compiler/GHC/Core/FamInstEnv.hs
index 844f753957..e955e5befd 100644
--- a/compiler/GHC/Core/FamInstEnv.hs
+++ b/compiler/GHC/Core/FamInstEnv.hs
@@ -50,6 +50,7 @@ import GHC.Core.RoughMap
 import GHC.Types.Var.Set
 import GHC.Types.Var.Env
 import GHC.Types.Name
+import GHC.Data.FastString
 import GHC.Data.Maybe
 import GHC.Types.Var
 import GHC.Types.SrcLoc
@@ -686,7 +687,7 @@ mkCoAxBranch tvs eta_tvs cvs lhs rhs roles loc
     -- See Note [Tidy axioms when we build them]
     -- See also Note [CoAxBranch type variables] in GHC.Core.Coercion.Axiom
 
-    init_occ_env = initTidyOccEnv [mkTyVarOcc "_"]
+    init_occ_env = initTidyOccEnv [mkTyVarOccFS (fsLit "_")]
     init_tidy_env = mkEmptyTidyEnv init_occ_env
     -- See Note [Always number wildcard types in CoAxBranch]
 
diff --git a/compiler/GHC/Core/TyCo/Tidy.hs b/compiler/GHC/Core/TyCo/Tidy.hs
index 36f1bb015a..2cbebbd411 100644
--- a/compiler/GHC/Core/TyCo/Tidy.hs
+++ b/compiler/GHC/Core/TyCo/Tidy.hs
@@ -17,6 +17,7 @@ module GHC.Core.TyCo.Tidy
   ) where
 
 import GHC.Prelude
+import GHC.Data.FastString
 
 import GHC.Core.TyCo.Rep
 import GHC.Core.TyCo.FVs (tyCoVarsOfTypesWellScoped, tyCoVarsOfTypeList)
@@ -70,7 +71,7 @@ getHelpfulOccName :: TyCoVar -> OccName
 -- this way is a helpful clue for users
 getHelpfulOccName tv
   | isSystemName name, isTcTyVar tv
-  = mkTyVarOcc (occNameString occ ++ "0")
+  = mkTyVarOccFS (occNameFS occ `appendFS` fsLit "0")
   | otherwise
   = occ
   where