1 files changed, 35 insertions, 0 deletions
diff --git a/testsuite/tests/typecheck/should_compile/tc229.hs b/testsuite/tests/typecheck/should_compile/tc229.hs
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+++ b/testsuite/tests/typecheck/should_compile/tc229.hs
@@ -0,0 +1,35 @@
+
+-- trac #1406: Constraint doesn't reduce in the presence of quantified
+--             type variables
+
+{-# LANGUAGE FlexibleInstances, UndecidableInstances, Rank2Types,
+             MultiParamTypeClasses, FunctionalDependencies #-}
+
+module Problem where
+
+data Z
+data S a
+
+class HPrefix l
+instance (NSub (S Z) ndiff, HDrop ndiff l l) => HPrefix l
+
+class NSub n1 n3 | n1 -> n3
+instance NSub Z Z
+instance NSub n1 n3 => NSub (S n1) n3
+
+class HDrop n l1 l2 | n l1 -> l2
+instance HDrop Z l l
+
+t_hPrefix :: HPrefix l => l -> ()
+t_hPrefix = undefined
+
+-- In ghc 6.6.1 this works...
+thr' :: (forall r. l -> a) -> a
+thr' f = f undefined
+thP4' = thr' t_hPrefix
+
+-- ... but this doesn't work...?
+thr :: (forall r. r -> a) -> a
+thr f = f undefined
+thP4 = thr t_hPrefix
+