1 files changed, 48 insertions, 0 deletions
diff --git a/testsuite/tests/deriving/should_compile/T8138.hs b/testsuite/tests/deriving/should_compile/T8138.hs
new file mode 100644
index 0000000000..2e7e47bcaa
--- /dev/null
+++ b/testsuite/tests/deriving/should_compile/T8138.hs
@@ -0,0 +1,48 @@
+{-# LANGUAGE ScopedTypeVariables #-}
+
+module Main where
+
+import Control.Monad.ST
+import Data.Primitive
+
+main :: IO ()
+main = do
+    let xs :: [Float] = runST $ do
+        barr <- mutableByteArrayFromList [1..fromIntegral n::Float]
+        peekByteArray n barr
+    print xs
+  where
+    n = 13
+
+mutableByteArrayFromList :: forall s a . (Prim a)
+                         => [a]
+                         -> ST s (MutableByteArray s)
+mutableByteArrayFromList xs = do
+    arr <- newByteArray (length xs*sizeOf (undefined :: a))
+    loop arr 0 xs
+    return arr
+  where
+    loop :: (Prim a) => MutableByteArray s -> Int -> [a] -> ST s ()
+    loop _ _ [] = return ()
+
+    loop arr i (x : xs) = do
+        writeByteArray arr i x
+        loop arr (i+1) xs
+
+peekByteArray :: (Prim a)
+              => Int
+              -> MutableByteArray s
+              -> ST s [a]
+peekByteArray n arr =
+    loop 0 arr
+  where
+    loop :: (Prim a)
+         => Int
+         -> MutableByteArray s
+         -> ST s [a]
+    loop i _ | i >= n = return []
+
+    loop i arr = do
+        x  <- readByteArray arr i
+        xs <- loop (i+1) arr
+        return (x : xs)