1 files changed, 22 insertions, 0 deletions
diff --git a/testsuite/tests/simplCore/should_compile/T20040.hs b/testsuite/tests/simplCore/should_compile/T20040.hs
new file mode 100644
index 0000000000..a50323175f
--- /dev/null
+++ b/testsuite/tests/simplCore/should_compile/T20040.hs
@@ -0,0 +1,22 @@
+{-# LANGUAGE DataKinds           #-}
+{-# LANGUAGE GADTs               #-}
+{-# LANGUAGE RankNTypes          #-}
+{-# LANGUAGE ScopedTypeVariables #-}
+{-# LANGUAGE TypeApplications    #-}
+{-# LANGUAGE BangPatterns        #-}
+
+module T20040 where
+
+import Data.Coerce
+
+data Nat = Z | S Nat
+
+data Vec n a where
+  Nil  :: Vec 'Z a
+  Cons :: a -> Vec n a -> Vec ('S n) a
+
+newtype Succ b n = Succ { unSucc :: b (S n) }
+
+ifoldl' :: forall b n a. (forall m. b m -> a -> b ('S m)) -> b 'Z -> Vec n a -> b n
+ifoldl' f  z Nil = z
+ifoldl' f !z (Cons x xs) = unSucc $ ifoldl' (\(Succ m) a -> Succ (f m a)) (Succ $ f z x) xs