Zonk the free tvs of a RULE lhs to TyVars

Previously we were making them into skolem TcTyVars,
which is wrong for the output of the type checker, which
no TcTyVars should surive.

See Note [Zonking the LHS of a RULE] in TcHsSyn

This was flushed out by the new IfaceTcTyVar thing;
I found some more TcTyVars that were being serialised into
an interface file, which is wrong wrong wrong.

3 files changed, 16 insertions, 8 deletions

diff --git a/compiler/deSugar/DsBinds.hs b/compiler/deSugar/DsBinds.hs
index 143d209b42..4253255bae 100644
--- a/compiler/deSugar/DsBinds.hs
+++ b/compiler/deSugar/DsBinds.hs
@@ -895,7 +895,7 @@ Consider
 After type checking the LHS becomes (foo alpha (C alpha)), where alpha
 is an unbound meta-tyvar.  The zonker in TcHsSyn is careful not to
 turn the free alpha into Any (as it usually does).  Instead it turns it
-into a skolem 'a'.  See TcHsSyn Note [Zonking the LHS of a RULE].
+into a TyVar 'a'.  See TcHsSyn Note [Zonking the LHS of a RULE].
 
 Now we must quantify over that 'a'.  It's /really/ inconvenient to do that
 in the zonker, because the HsExpr data type is very large.  But it's /easy/
diff --git a/compiler/typecheck/TcHsSyn.hs b/compiler/typecheck/TcHsSyn.hs
index 5a455ead32..ebdc6176b2 100644
--- a/compiler/typecheck/TcHsSyn.hs
+++ b/compiler/typecheck/TcHsSyn.hs
@@ -187,7 +187,9 @@ the environment manipulation is tiresome.
 -- Confused by zonking? See Note [What is zonking?] in TcMType.
 type UnboundTyVarZonker = TcTyVar -> TcM Type
         -- How to zonk an unbound type variable
-        -- The TcTyVar is (a) a MetaTv (b) Flexi and
+        -- The TcTyVar is
+        --     (a) a MetaTv
+        --     (b) Flexi and
         --     (c) its kind is alrady zonked
         -- Note [Zonking the LHS of a RULE]
 
@@ -1617,8 +1619,13 @@ zonkTvSkolemising :: UnboundTyVarZonker
 -- This variant is used for the LHS of rules
 -- See Note [Zonking the LHS of a RULE].
 zonkTvSkolemising tv
-  = do { tv' <- skolemiseUnboundMetaTyVar tv
-       ; return (mkTyVarTy tv') }
+  = do { let tv' = mkTyVar (tyVarName tv) (tyVarKind tv)
+                  -- NB: the kind of tv is already zonked
+             ty = mkTyVarTy tv'
+                  -- Make a proper TyVar (remember we
+                  -- are now done with type checking)
+       ; writeMetaTyVar tv ty
+       ; return ty }
 
 zonkTypeZapping :: UnboundTyVarZonker
 -- This variant is used for everything except the LHS of rules
@@ -1652,9 +1659,10 @@ over it!
 
 We do this in two stages.
 
-* During zonking, we skolemise 'alpha' to 'a'.  We do this by using
-  zonkTvSkolemising as the UnboundTyVarZonker in the ZonkEnv.
-  (This is the whole reason that the ZonkEnv has a UnboundTyVarZonker.)
+* During zonking, we skolemise the TcTyVar 'alpha' to TyVar 'a'.  We
+  do this by using zonkTvSkolemising as the UnboundTyVarZonker in the
+  ZonkEnv.  (This is in fact the whole reason that the ZonkEnv has a
+  UnboundTyVarZonker.)
 
 * In DsBinds, we quantify over it.  See DsBinds
   Note [Free tyvars on rule LHS]
diff --git a/compiler/typecheck/TcMType.hs b/compiler/typecheck/TcMType.hs
index eae7305b58..b5104a1a32 100644
--- a/compiler/typecheck/TcMType.hs
+++ b/compiler/typecheck/TcMType.hs
@@ -68,7 +68,7 @@ module TcMType (
   zonkTidyTcType, zonkTidyOrigin,
   mkTypeErrorThing, mkTypeErrorThingArgs,
   tidyEvVar, tidyCt, tidySkolemInfo,
-  skolemiseUnboundMetaTyVar, skolemiseRuntimeUnk,
+  skolemiseRuntimeUnk,
   zonkTcTyVar, zonkTcTyVars, zonkTcTyVarToTyVar,
   zonkTyCoVarsAndFV, zonkTcTypeAndFV,
   zonkTyCoVarsAndFVList,