Test Trac #9569

1 files changed, 25 insertions, 0 deletions

diff --git a/testsuite/tests/polykinds/T9569.hs b/testsuite/tests/polykinds/T9569.hs
new file mode 100644
index 0000000000..012d61fc31
--- /dev/null
+++ b/testsuite/tests/polykinds/T9569.hs
@@ -0,0 +1,25 @@
+{-# LANGUAGE RankNTypes, ConstraintKinds, KindSignatures, DataKinds, TypeFamilies #-}
+module T9569 where
+
+import GHC.Prim
+
+data Proxy (c :: Constraint)
+
+class Deferrable (c :: Constraint) where
+  defer :: Proxy c -> (c => a) -> a
+
+deferPair :: (Deferrable c1, Deferrable c2) =>
+                  Proxy (c1,c2) -> ((c1,c2) => a) -> a
+deferPair = undefined
+
+instance (Deferrable c1, Deferrable c2) => Deferrable (c1,c2) where
+    -- defer p f = deferPair p f     -- Succeeds
+    defer = deferPair                -- Fails
+
+{-
+  [G] Deferrable c1, Deferrable c2
+
+  [W] Proxy (c1,c2) -> ((c1,c2) => a) -> a ~ Proxy (c1x,c2x) -> ((c1x,c2x) => ax) -> ax
+  [w] Deferrable c1x
+  [w] Deferrable c2x
+-}
\ No newline at end of file