diff --git a/compiler/iface/BuildTyCl.hs b/compiler/iface/BuildTyCl.hs
index d13d38e6ffb4197fcd45f429534c19ed5c673657..1b4017abdc1ca4e9bae46d00d30cc5cac05f1fd6 100644
--- a/compiler/iface/BuildTyCl.hs
+++ b/compiler/iface/BuildTyCl.hs
@@ -184,7 +184,7 @@ buildPatSyn src_name declared_infix matcher@(matcher_id,_) builder
-- compatible with the pattern synonym
ASSERT2((and [ univ_tvs `equalLength` univ_tvs1
, ex_tvs `equalLength` ex_tvs1
- , pat_ty `eqType` substTyUnchecked subst pat_ty1
+ , pat_ty `eqType` substTy subst pat_ty1
, prov_theta `eqTypes` substTys subst prov_theta1
, req_theta `eqTypes` substTys subst req_theta1
, arg_tys `eqTypes` substTys subst arg_tys1
diff --git a/compiler/main/InteractiveEval.hs b/compiler/main/InteractiveEval.hs
index b7c2178fc3087b99b0354e9632221b191a052f44..f0df2700441d5341ad0d7adfbb8d0d39f6506ff5 100644
--- a/compiler/main/InteractiveEval.hs
+++ b/compiler/main/InteractiveEval.hs
@@ -543,13 +543,13 @@ bindLocalsAtBreakpoint hsc_env apStack_fhv (Just BreakInfo{..}) = do
let tv_subst = newTyVars us free_tvs
filtered_ids = [ id | (id, Just _hv) <- zip ids mb_hValues ]
(_,tidy_tys) = tidyOpenTypes emptyTidyEnv $
- map (substTyUnchecked tv_subst . idType) filtered_ids
+ map (substTy tv_subst . idType) filtered_ids
new_ids <- zipWith3M mkNewId occs tidy_tys filtered_ids
result_name <- newInteractiveBinder hsc_env (mkVarOccFS result_fs) span
let result_id = Id.mkVanillaGlobal result_name
- (substTyUnchecked tv_subst result_ty)
+ (substTy tv_subst result_ty)
result_ok = isPointer result_id
final_ids | result_ok = result_id : new_ids
diff --git a/compiler/typecheck/TcFlatten.hs b/compiler/typecheck/TcFlatten.hs
index 612f8a691bcb43c58398ad7b53160d3225fbdc3e..f87a302d5c37af8292b95c6cb67a5fbad01cb499 100644
--- a/compiler/typecheck/TcFlatten.hs
+++ b/compiler/typecheck/TcFlatten.hs
@@ -956,7 +956,7 @@ flatten_one (TyConApp tc tys)
-- Expand type synonyms that mention type families
-- on the RHS; see Note [Flattening synonyms]
| Just (tenv, rhs, tys') <- expandSynTyCon_maybe tc tys
- , let expanded_ty = mkAppTys (substTyUnchecked (mkTopTCvSubst tenv) rhs) tys'
+ , let expanded_ty = mkAppTys (substTy (mkTopTCvSubst tenv) rhs) tys'
= do { mode <- getMode
; let used_tcs = tyConsOfType rhs
; case mode of
diff --git a/compiler/typecheck/TcMType.hs b/compiler/typecheck/TcMType.hs
index bb31005c01036d72273d74a71e85ad42830d740b..a160d4ebfe358de1d4bbb25dd62fe483fc8a5a0e 100644
--- a/compiler/typecheck/TcMType.hs
+++ b/compiler/typecheck/TcMType.hs
@@ -307,7 +307,7 @@ tcSuperSkolTyVars :: [TyVar] -> (TCvSubst, [TcTyVar])
-- Moreover, make them "super skolems"; see comments with superSkolemTv
-- see Note [Kind substitution when instantiating]
-- Precondition: tyvars should be ordered by scoping
-tcSuperSkolTyVars = mapAccumL tcSuperSkolTyVar (mkTopTCvSubst [])
+tcSuperSkolTyVars = mapAccumL tcSuperSkolTyVar emptyTCvSubst
tcSuperSkolTyVar :: TCvSubst -> TyVar -> (TCvSubst, TcTyVar)
tcSuperSkolTyVar subst tv
diff --git a/compiler/typecheck/TcSplice.hs b/compiler/typecheck/TcSplice.hs
index fe13226c608f8d5f7aa9a7bad984986ac135638c..ac7e1b707d5917ef5bd91f1f392a00bbeee5a555 100644
--- a/compiler/typecheck/TcSplice.hs
+++ b/compiler/typecheck/TcSplice.hs
@@ -99,7 +99,7 @@ import GHC.Serialized
import ErrUtils
import Util
import Unique
-import VarSet ( isEmptyVarSet, filterVarSet )
+import VarSet ( isEmptyVarSet, filterVarSet, mkVarSet, elemVarSet )
import Data.List ( find )
import Data.Maybe
import FastString
@@ -1395,8 +1395,8 @@ reifyDataCon isGadtDataCon tys dc
name = reifyName dc
-- Universal tvs present in eq_spec need to be filtered out, as
-- they will not appear anywhere in the type.
- subst = mkTopTCvSubst (map eqSpecPair g_eq_spec)
- g_unsbst_univ_tvs = filter (`notElemTCvSubst` subst) g_univ_tvs
+ eq_spec_tvs = mkVarSet (map eqSpecTyVar g_eq_spec)
+ g_unsbst_univ_tvs = filterOut (`elemVarSet` eq_spec_tvs) g_univ_tvs
; r_arg_tys <- reifyTypes (if isGadtDataCon then g_arg_tys else arg_tys)
diff --git a/compiler/types/FamInstEnv.hs b/compiler/types/FamInstEnv.hs
index 1167ac254c548f9c095baa05f441f1fd94c79ee2..4b4cc5d2f6e81e5461309b4fd40bd7d43c9117b8 100644
--- a/compiler/types/FamInstEnv.hs
+++ b/compiler/types/FamInstEnv.hs
@@ -1237,7 +1237,7 @@ normalise_tc_app tc tys
; case expandSynTyCon_maybe tc ntys of
{ Just (tenv, rhs, ntys') ->
do { (co2, ninst_rhs)
- <- normalise_type (substTyUnchecked (mkTopTCvSubst tenv) rhs)
+ <- normalise_type (substTy (mkTopTCvSubst tenv) rhs)
; return $
if isReflCo co2
then (args_co, mkTyConApp tc ntys)
diff --git a/compiler/types/TyCoRep.hs b/compiler/types/TyCoRep.hs
index 2f034d036a2eb66947001e9921ef9752c80bc7b9..a2477250f0bb5177da22a12b89cd8a18b6349bbb 100644
--- a/compiler/types/TyCoRep.hs
+++ b/compiler/types/TyCoRep.hs
@@ -1686,12 +1686,15 @@ zipOpenTCvSubstBinders bndrs tys
is = mkInScopeSet (tyCoVarsOfTypes tys)
tenv = mkVarEnv [ (tv, ty) | (Named tv _, ty) <- zip bndrs tys ]
--- | Called when doing top-level substitutions. Here we expect that the
--- free vars of the range of the substitution will be empty.
-mkTopTCvSubst :: [(TyCoVar, Type)] -> TCvSubst
-mkTopTCvSubst prs = TCvSubst emptyInScopeSet tenv cenv
- where (tenv, cenv) = foldl extend (emptyTvSubstEnv, emptyCvSubstEnv) prs
- extend envs (v, ty) = extendSubstEnvs envs v ty
+-- | Called when doing top-level substitutions. No CoVars, please!
+mkTopTCvSubst :: [(TyVar, Type)] -> TCvSubst
+mkTopTCvSubst prs =
+ ASSERT2( onlyTyVarsAndNoCoercionTy, text "prs" <+> ppr prs )
+ mkOpenTCvSubst tenv emptyCvSubstEnv
+ where tenv = mkVarEnv prs
+ onlyTyVarsAndNoCoercionTy =
+ and [ isTyVar tv && not (isCoercionTy ty)
+ | (tv, ty) <- prs ]
zipTyEnv :: [TyVar] -> [Type] -> TvSubstEnv
zipTyEnv tyvars tys
diff --git a/compiler/types/Type.hs b/compiler/types/Type.hs
index 8b426f131a5492f1c2f9038f30baba1f00ce0e95..c6d51f35f67424b77df38f9133ed88563e06c302 100644
--- a/compiler/types/Type.hs
+++ b/compiler/types/Type.hs
@@ -294,7 +294,11 @@ coreView :: Type -> Maybe Type
-- By being non-recursive and inlined, this case analysis gets efficiently
-- joined onto the case analysis that the caller is already doing
coreView (TyConApp tc tys) | Just (tenv, rhs, tys') <- expandSynTyCon_maybe tc tys
- = Just (mkAppTys (substTyUnchecked (mkTopTCvSubst tenv) rhs) tys')
+ = Just (mkAppTys (substTy (mkTopTCvSubst tenv) rhs) tys')
+ -- The free vars of 'rhs' should all be bound by 'tenv', so it's
+ -- ok to use 'substTy' here.
+ -- See also Note [Generating the in-scope set for a substitution]
+ -- in TyCoRep.
-- Its important to use mkAppTys, rather than (foldl AppTy),
-- because the function part might well return a
-- partially-applied type constructor; indeed, usually will!