Test Trac #12055

2 files changed, 46 insertions, 0 deletions

diff --git a/testsuite/tests/polykinds/T12055.hs b/testsuite/tests/polykinds/T12055.hs
new file mode 100644
index 0000000000..3ffc221b7b
--- /dev/null
+++ b/testsuite/tests/polykinds/T12055.hs
@@ -0,0 +1,45 @@
+{-# LANGUAGE ConstraintKinds #-}
+{-# LANGUAGE MultiParamTypeClasses #-}
+{-# LANGUAGE NoImplicitPrelude #-}
+{-# LANGUAGE RankNTypes #-}
+{-# LANGUAGE TypeFamilies #-}
+{-# LANGUAGE TypeInType #-}
+{-# LANGUAGE TypeOperators #-}
+{-# LANGUAGE TypeInType #-}
+
+-- The code from the ticket lacked these extensions,
+-- but crashed the compiler with "GHC internal error"
+-- It doesn't crash now; and in this test case I've added
+-- the extensions, which makes it compile cleanly
+{-# LANGUAGE FlexibleContexts, FlexibleInstances, UndecidableInstances, FunctionalDependencies #-}
+
+
+module T12055 where
+
+import GHC.Base ( Constraint, Type )
+import GHC.Exts ( type (~~) )
+
+type Cat k = k -> k -> Type
+
+class Category (p :: Cat k) where
+    type Ob p :: k -> Constraint
+
+class (Category (Dom f), Category (Cod f)) => Functor (f :: j -> k) where
+    type Dom f :: Cat j
+    type Cod f :: Cat k
+    functor :: forall a b.
+               Iso Constraint (:-) (:-)
+               (Ob (Dom f) a)     (Ob (Dom f) b)
+               (Ob (Cod f) (f a)) (Ob (Cod f) (f b))
+
+class (Functor f , Dom f ~ p, Cod f ~ q) =>
+    Fun (p :: Cat j) (q :: Cat k) (f :: j -> k) | f -> p q
+instance (Functor f , Dom f ~ p, Cod f ~ q) =>
+    Fun (p :: Cat j) (q :: Cat k) (f :: j -> k)
+
+data Nat (p :: Cat j) (q :: Cat k) (f :: j -> k) (g :: j -> k)
+
+type Iso k (c :: Cat k) (d :: Cat k) s t a b =
+    forall p. (Cod p ~~ Nat d (->)) => p a b -> p s t
+
+data (p :: Constraint) :- (q :: Constraint)
diff --git a/testsuite/tests/polykinds/all.T b/testsuite/tests/polykinds/all.T
index 2c3d1df866..c731441679 100644
--- a/testsuite/tests/polykinds/all.T
+++ b/testsuite/tests/polykinds/all.T
@@ -148,3 +148,4 @@ test('T11648b', normal, compile_fail, [''])
 test('KindVType', normal, compile_fail, [''])
 test('T11821', normal, compile, [''])
 test('T11640', normal, compile, [''])
+test('T12055', normal, compile, [''])