1 files changed, 58 insertions, 0 deletions
diff --git a/testsuite/tests/typecheck/should_compile/T16204a.hs b/testsuite/tests/typecheck/should_compile/T16204a.hs
new file mode 100644
index 0000000000..63cae41d8c
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+++ b/testsuite/tests/typecheck/should_compile/T16204a.hs
@@ -0,0 +1,58 @@
+{-# LANGUAGE DataKinds #-}
+{-# LANGUAGE FlexibleContexts #-}
+{-# LANGUAGE GADTs #-}
+{-# LANGUAGE PolyKinds #-}
+{-# LANGUAGE RankNTypes #-}
+{-# LANGUAGE ScopedTypeVariables #-}
+{-# LANGUAGE TypeApplications #-}
+{-# LANGUAGE TypeFamilies #-}
+module T16204 where
+
+import Data.Kind
+
+-----
+-- singletons machinery
+-----
+
+data family Sing :: forall k. k -> Type
+data SomeSing :: Type -> Type where
+  SomeSing :: Sing (a :: k) -> SomeSing k
+
+-----
+-- (Simplified) GHC.Generics
+-----
+
+class Generic (a :: Type) where
+    type Rep a :: Type
+    from :: a -> Rep a
+    to   :: Rep a -> a
+
+class PGeneric (a :: Type) where
+  -- type PFrom ...
+  type PTo (x :: Rep a) :: a
+
+class SGeneric k where
+  -- sFrom :: ...
+  sTo :: forall (a :: Rep k). Sing a -> Sing (PTo a :: k)
+
+-----
+
+class SingKind k where
+  type Demote k :: Type
+  -- fromSing :: ...
+  toSing :: Demote k -> SomeSing k
+
+genericToSing :: forall k.
+                 ( SingKind k, SGeneric k, SingKind (Rep k)
+                 , Generic (Demote k), Rep (Demote k) ~ Demote (Rep k) )
+              => Demote k -> SomeSing k
+genericToSing d = withSomeSing @(Rep k) (from d) $ SomeSing . sTo
+
+withSomeSing :: forall k r
+              . SingKind k
+             => Demote k
+             -> (forall (a :: k). Sing a -> r)
+             -> r
+withSomeSing x f =
+  case toSing x of
+    SomeSing x' -> f x'