Fix Trac #4401: meta-tyvars allocated by the constraint solver are always touchable

[ghc-hetmet.git] / compiler / typecheck / TcType.lhs

diff --git a/compiler/typecheck/TcType.lhs b/compiler/typecheck/TcType.lhs
index b49dbff..b20d32e 100644 (file)
--- a/compiler/typecheck/TcType.lhs
+++ b/compiler/typecheck/TcType.lhs
@@ -302,6 +302,11 @@ data MetaInfo
                   -- The Name is the name of the function from whose
                   -- type signature we got this skolem
 
+   | TcsTv        -- A MetaTv allocated by the constraint solver
+                  -- Its particular property is that it is always "touchable"
+                  -- Nevertheless, the constraint solver has to try to guess
+                  -- what type to instantiate it to
+
 ----------------------------------
 -- SkolemInfo describes a site where 
 --   a) type variables are skolemised
@@ -408,6 +413,7 @@ pprTcTyVarDetails :: TcTyVarDetails -> SDoc
 pprTcTyVarDetails (SkolemTv _)         = ptext (sLit "sk")
 pprTcTyVarDetails (FlatSkol {})        = ptext (sLit "fsk")
 pprTcTyVarDetails (MetaTv TauTv _)     = ptext (sLit "tau")
+pprTcTyVarDetails (MetaTv TcsTv _)     = ptext (sLit "tcs")
 pprTcTyVarDetails (MetaTv (SigTv _) _) = ptext (sLit "sig")
 
 pprUserTypeCtxt :: UserTypeCtxt -> SDoc
@@ -433,8 +439,9 @@ pprSkolTvBinding tv
   where
     ppr_details (SkolemTv info)      = ppr_skol info
     ppr_details (FlatSkol {})       = ptext (sLit "is a flattening type variable")
-    ppr_details (MetaTv TauTv _)     = ptext (sLit "is a meta type variable")
-    ppr_details (MetaTv (SigTv n) _) = ptext (sLit "is bound by the type signature for") <+> quotes (ppr n)
+    ppr_details (MetaTv (SigTv n) _) = ptext (sLit "is bound by the type signature for")
+                                       <+> quotes (ppr n)
+    ppr_details (MetaTv _ _)         = ptext (sLit "is a meta type variable")
 
     ppr_skol UnkSkol       = ptext (sLit "is an unknown type variable")        -- Unhelpful
     ppr_skol RuntimeUnkSkol = ptext (sLit "is an unknown runtime type")
@@ -615,8 +622,8 @@ isTyConableTyVar tv
        -- not a SigTv
   = ASSERT( isTcTyVar tv) 
     case tcTyVarDetails tv of
-       MetaTv TauTv _ -> True
-       _              -> False
+       MetaTv (SigTv _) _ -> False
+       _                  -> True
        
 isSkolemTyVar tv 
   = ASSERT2( isTcTyVar tv, ppr tv )