Add regression test for #14246

testsuite/tests/indexed-types/should_fail/T14246.hs

+{-# LANGUAGE RankNTypes, GADTs, TypeOperators, PolyKinds, DataKinds, TypeFamilies, AllowAmbiguousTypes, UndecidableInstances, TypeInType #-}
+
+module T14246 where
+
+import Data.Kind -- necessary for *
+
+data Nat = Z | S Nat
+
+data Vect :: Nat -> Type -> Type where
+  Nil  :: Vect Z a
+  Cons :: a -> Vect n a -> Vect (S n) a
+
+data Label a = Label a
+
+data L
+
+type family KLN (n :: k) :: Nat where
+    KLN (f :: v -> k) = S (KLN (forall t. f t))
+    KLN (f :: *) = Z
+
+type family Reveal (n :: k) (l :: Vect (KLN n) L) :: * where
+    Reveal (f :: v -> k) (Cons (Label (t :: v)) l) = Reveal (f t) l
+    Reveal (a :: *) Nil = a

testsuite/tests/indexed-types/should_fail/T14246.stderr

+
+T14246.hs:18:5: error:
+    • Illegal polymorphic type: forall (t :: v). f t
+    • In the equations for closed type family ‘KLN’
+      In the type family declaration for ‘KLN’
+
+T14246.hs:22:27: error:
+    • Expected kind ‘Vect (KLN f) L’,
+        but ‘Cons (Label (t :: v)) l’ has kind ‘Vect ('S (KLN (f t))) *’
+    • In the second argument of ‘Reveal’, namely
+        ‘(Cons (Label (t :: v)) l)’
+      In the type family declaration for ‘Reveal’
+
+T14246.hs:22:67: error:
+    • Expected kind ‘Vect (KLN (f t)) L’,
+        but ‘l’ has kind ‘Vect (KLN (f t)) *’
+    • In the second argument of ‘Reveal’, namely ‘l’
+      In the type ‘Reveal (f t) l’
+      In the type family declaration for ‘Reveal’
+
+T14246.hs:23:21: error:
+    • Expected kind ‘Vect (KLN a) L’, but ‘Nil’ has kind ‘Vect 'Z L’
+    • In the second argument of ‘Reveal’, namely ‘Nil’
+      In the type family declaration for ‘Reveal’

testsuite/tests/indexed-types/should_fail/all.T

@@ -141,5 +141,6 @@ test('T14033', normal, compile_fail, [''])
 test('T14045a', normal, compile_fail, [''])
 test('T14175', normal, compile_fail, [''])
 test('T14179', normal, compile_fail, [''])
+test('T14246', normal, compile_fail, [''])
 test('T14369', normal, compile_fail, [''])
 test('T15172', normal, compile_fail, [''])