Fix rank-validity testing

GHC does not now do "hoisting" as it used to.  Instead, it allows
foralls to the right of fuction arrows, as well as to the left.

But the type-validity tester hadn't caught up.  This commit fixes
it. The test is tc203.

Incidentally, GHC still doesn't let you write
	forall a. Eq a => forall b. b -> b
because we get a zillion reduce/reduce errors if we allow that.  I'm
sure it's fixable.  But meanwhile you have to use an auxiliary type
synonym, which is a bit stupid.

diff --git a/compiler/typecheck/TcMType.lhs b/compiler/typecheck/TcMType.lhs
index fa129d3..6a2041e 100644 (file)
--- a/compiler/typecheck/TcMType.lhs
+++ b/compiler/typecheck/TcMType.lhs
@@ -731,14 +731,16 @@ check_poly_type (Rank 0) ubx_tup ty
   = check_tau_type (Rank 0) ubx_tup ty
 
 check_poly_type rank ubx_tup ty 
-  = let
-       (tvs, theta, tau) = tcSplitSigmaTy ty
-    in
-    check_valid_theta SigmaCtxt theta          `thenM_`
-    check_tau_type (decRank rank) ubx_tup tau  `thenM_`
-    checkFreeness tvs theta                    `thenM_`
-    checkAmbiguity tvs theta (tyVarsOfType tau)
-
+  | null tvs && null theta
+  = check_tau_type rank ubx_tup ty
+  | otherwise
+  = do { check_valid_theta SigmaCtxt theta
+       ; check_poly_type rank ubx_tup tau      -- Allow foralls to right of arrow
+       ; checkFreeness tvs theta
+       ; checkAmbiguity tvs theta (tyVarsOfType tau) }
+  where
+    (tvs, theta, tau) = tcSplitSigmaTy ty
+   
 ----------------------------------------
 check_arg_type :: Type -> TcM ()
 -- The sort of type that can instantiate a type variable,
@@ -781,8 +783,8 @@ check_tau_type rank ubx_tup (PredTy sty) = getDOpts         `thenM` \ dflags ->
 
 check_tau_type rank ubx_tup (TyVarTy _)       = returnM ()
 check_tau_type rank ubx_tup ty@(FunTy arg_ty res_ty)
-  = check_poly_type rank UT_NotOk arg_ty       `thenM_`
-    check_poly_type rank UT_Ok    res_ty
+  = check_poly_type (decRank rank) UT_NotOk arg_ty     `thenM_`
+    check_poly_type rank          UT_Ok    res_ty
 
 check_tau_type rank ubx_tup (AppTy ty1 ty2)
   = check_arg_type ty1 `thenM_` check_arg_type ty2