Fix Trac #10670

In dataConCannotMatch we were using a GADT data con without
properly instantiating the existential type variables.
The fix is easy, and the code is tighter.

1 files changed, 24 insertions, 0 deletions

diff --git a/testsuite/tests/polykinds/T10670.hs b/testsuite/tests/polykinds/T10670.hs
new file mode 100644
index 0000000000..5b9cc72e21
--- /dev/null
+++ b/testsuite/tests/polykinds/T10670.hs
@@ -0,0 +1,24 @@
+{-# LANGUAGE ScopedTypeVariables, RankNTypes, GADTs, PolyKinds #-}
+
+module T10670 where
+
+import Unsafe.Coerce
+
+data TypeRepT (a::k) where
+  TRCon :: TypeRepT a
+
+data G2 c a where
+  G2 :: TypeRepT a -> TypeRepT b -> G2 c (c a b)
+
+getT2 :: TypeRepT (c :: k2 -> k1 -> k) -> TypeRepT (a :: k) -> Maybe (G2 c a)
+{-# NOINLINE getT2 #-}
+getT2 c t = Nothing
+
+tyRepTArr :: TypeRepT (->)
+{-# NOINLINE tyRepTArr #-}
+tyRepTArr = TRCon
+
+s :: forall a x. TypeRepT (a :: *) -> Maybe x
+s tf = case getT2 tyRepTArr tf :: Maybe (G2 (->) a) of
+          Just (G2 _ _) -> Nothing
+          _ -> Nothing