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diff --git a/testsuite/tests/stranal/sigs/T13331.hs b/testsuite/tests/stranal/sigs/T13331.hs
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index 0000000000..5f4a4a1631
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+++ b/testsuite/tests/stranal/sigs/T13331.hs
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+{-# LANGUAGE MagicHash, BangPatterns #-}
+
+module T13331 (naiveInsertInt) where
+
+data Map k a = Bin !Int !k a (Map k a) (Map k a)
+             | Tip
+
+singleton :: k -> a -> Map k a
+singleton k a = Bin 1 k a Tip Tip
+
+balanceL :: k -> a -> Map k a -> Map k a -> Map k a
+balanceL !_ _ !_ !_ = undefined
+{-# NOINLINE balanceL #-}
+
+balanceR :: k -> a -> Map k a -> Map k a -> Map k a
+balanceR !_ _ !_ !_ = undefined
+{-# NOINLINE balanceR #-}
+
+-- | Should not unbox `kx`.
+naiveInsertInt :: Int -> a -> Map Int a -> Map Int a
+naiveInsertInt !kx x Tip = singleton kx x
+naiveInsertInt !kx x t@(Bin sz ky y l r) =
+    case compare kx ky of
+        LT -> balanceL ky y l' r
+           where !l' = naiveInsertInt kx x l
+        GT -> balanceR ky y l r'
+           where !r' = naiveInsertInt kx x r
+        EQ -> Bin sz kx x l r
+