Fix Trac #2573; and explanatory comment

diff --git a/compiler/typecheck/TcMType.lhs b/compiler/typecheck/TcMType.lhs
index 1addfe4..02de940 100644 (file)
--- a/compiler/typecheck/TcMType.lhs
+++ b/compiler/typecheck/TcMType.lhs
@@ -524,6 +524,7 @@ writeMetaTyVar tyvar ty
     -- TOM: It should also work for coercions
     -- ASSERT2( k2 `isSubKind` k1, (ppr tyvar <+> ppr k1) $$ (ppr ty <+> ppr k2) )
     do { ASSERTM2( do { details <- readMetaTyVar tyvar; return (isFlexi details) }, ppr tyvar )
+       ; traceTc (text "writeMetaTyVar" <+> ppr tyvar <+> text ":=" <+> ppr ty)
        ; writeMutVar (metaTvRef tyvar) (Indirect ty) }
   where
     _k1 = tyVarKind tyvar
@@ -997,18 +998,22 @@ checkValidType ctxt ty = do
                   | otherwise = Rank n
        rank
          = case ctxt of
-                GenPatCtxt     -> MustBeMonoType
                 DefaultDeclCtxt-> MustBeMonoType
                 ResSigCtxt     -> MustBeMonoType
                 LamPatSigCtxt  -> gen_rank 0
                 BindPatSigCtxt -> gen_rank 0
                 TySynCtxt _    -> gen_rank 0
+                GenPatCtxt     -> gen_rank 1
+                       -- This one is a bit of a hack
+                       -- See the forall-wrapping in TcClassDcl.mkGenericInstance              
+
                 ExprSigCtxt    -> gen_rank 1
                 FunSigCtxt _   -> gen_rank 1
                 ConArgCtxt _   | polycomp -> gen_rank 2
                                 -- We are given the type of the entire
                                 -- constructor, hence rank 1
                                | otherwise -> gen_rank 1
+
                 ForSigCtxt _   -> gen_rank 1
                 SpecInstCtxt   -> gen_rank 1