Test Trac #7973

testsuite/tests/polykinds/T7973.hs

+{-# LANGUAGE DataKinds, PolyKinds, KindSignatures #-}
+{-# LANGUAGE ExistentialQuantification, UndecidableInstances, TypeFamilies #-}
+
+module Test where
+
+-- Kind-level proxies.
+data {-kind-} K (a :: *) = KP
+
+-- A type with 1 kind-polymorphic type argument.
+data T (n :: k)
+
+-- A type with 1 kind argument.
+data F (kp :: K k)
+
+-- A class with 1 kind argument.
+class (kp ~ KP) => C (kp :: K k) where
+  f :: T (a :: k) -> F kp
+
+-- A type with 1 kind argument.
+-- Contains an existentially quantified type-variable of this kind.
+data SomeT (kp :: K k) = forall (n :: k). Mk (T n)
+
+-- Show `SomeT` by converting it to `F`, using `C`.
+instance (C kp, Show (F kp)) => Show (SomeT kp) where
+  show (Mk x) = show (f x)

testsuite/tests/polykinds/all.T

@@ -87,3 +87,4 @@ test('T7524', normal, compile_fail,[''])
 test('T7601', normal, compile,[''])
 test('T7805', normal, compile_fail,[''])
 test('T7916', normal, compile,[''])
+test('T7973', normal, compile,['-O'])