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diff --git a/testsuite/tests/simplCore/should_compile/T20820.hs b/testsuite/tests/simplCore/should_compile/T20820.hs
new file mode 100644
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+++ b/testsuite/tests/simplCore/should_compile/T20820.hs
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+module T20820 ( ) where
+
+import Prelude hiding (concat)
+import Data.Semigroup           (Semigroup (sconcat, stimes))
+import Data.List.NonEmpty       (NonEmpty ((:|)))
+
+data ByteString = BS
+
+instance Semigroup ByteString where
+    (<>)    = undefined
+    sconcat (b:|bs) = concat (b:bs)
+    stimes  = stimesPolymorphic
+instance Monoid ByteString where
+    mempty  = undefined
+
+concat :: [ByteString] -> ByteString
+concat = undefined
+{-# NOINLINE concat #-}
+
+{-# RULES
+"ByteString concat [] -> mempty"
+   concat [] = mempty
+ #-}
+
+stimesPolymorphic :: Integral a => a -> ByteString -> ByteString
+stimesPolymorphic nRaw bs = stimesInt (fromIntegral nRaw) bs
+
+stimesInt :: Int -> ByteString -> ByteString
+stimesInt _ BS = mempty