1 files changed, 20 insertions, 0 deletions
diff --git a/testsuite/tests/simplCore/should_compile/T22347a.hs b/testsuite/tests/simplCore/should_compile/T22347a.hs
new file mode 100644
index 0000000000..727eedf8db
--- /dev/null
+++ b/testsuite/tests/simplCore/should_compile/T22347a.hs
@@ -0,0 +1,20 @@
+{-# LANGUAGE ScopedTypeVariables #-}
+module M where
+
+import Control.Monad.ST
+import Data.Array.ST
+import Data.Array.Unboxed
+
+bfs :: Array Int [Int] -> ST s (STArray s Int ())
+bfs g = do
+    vis :: STArray s Int () <- newArray (bounds g) ()
+    ch :: STArray s Int () <- newArray (bounds g) ()
+    let go [] = pure () :: ST s ()
+        go q = do
+            flip mapM_ q $ \u -> do
+                readArray vis (head (g!u))
+                readArray ch u
+                writeArray ch u ()
+            go []
+    go []
+    pure ch