1 files changed, 27 insertions, 0 deletions
diff --git a/testsuite/tests/typecheck/should_compile/twins.hs b/testsuite/tests/typecheck/should_compile/twins.hs
new file mode 100644
index 0000000000..6e46f860db
--- /dev/null
+++ b/testsuite/tests/typecheck/should_compile/twins.hs
@@ -0,0 +1,27 @@
+{-# LANGUAGE RankNTypes, LiberalTypeSynonyms #-}
+
+-- This test checks that deep skolemisation and deep 
+-- instanatiation work right.  A buggy prototype
+-- of GHC 7.0, where the type checker generated wrong
+-- code, sent applyTypeToArgs into a loop.
+
+module Twins where
+
+import Data.Data
+
+type GenericQ r = forall a. Data a => a -> r
+type GenericM m = forall a. Data a => a -> m a
+
+gzip :: GenericQ (GenericM Maybe) -> GenericQ (GenericM Maybe)
+gzip f x y
+  = f x y
+     `orElse`
+    if toConstr x == toConstr y
+    then gzipWithM (gzip f) x y
+    else Nothing
+
+gzipWithM :: Monad m => GenericQ (GenericM m) -> GenericQ (GenericM m)
+gzipWithM = error "urk"
+
+orElse :: Maybe a -> Maybe a -> Maybe a
+orElse = error "urk"
+\ No newline at end of file