1 files changed, 29 insertions, 0 deletions
diff --git a/testsuite/tests/simplCore/should_compile/T18603.hs b/testsuite/tests/simplCore/should_compile/T18603.hs
new file mode 100644
index 0000000000..d85f77c66a
--- /dev/null
+++ b/testsuite/tests/simplCore/should_compile/T18603.hs
@@ -0,0 +1,29 @@
+{-# LANGUAGE NoImplicitPrelude #-}
+
+module Test where
+
+import GHC.Base (build, foldr, id, Maybe(..))
+
+catMaybes :: [Maybe a] -> [a]
+catMaybes = mapMaybe id
+
+mapMaybe          :: (a -> Maybe b) -> [a] -> [b]
+mapMaybe _ []     = []
+mapMaybe f (x:xs) =
+ let rs = mapMaybe f xs in
+ case f x of
+  Nothing -> rs
+  Just r  -> r:rs
+{-# NOINLINE [1] mapMaybe #-}
+
+{-# RULES
+"mapMaybe"     [~1] forall f xs. mapMaybe f xs
+                    = build (\c n -> foldr (mapMaybeFB c f) n xs)
+"mapMaybeList" [1]  forall f. foldr (mapMaybeFB (:) f) [] = mapMaybe f
+  #-}
+
+{-# INLINE [0] mapMaybeFB #-} -- See Note [Inline FB functions] in GHC.List
+mapMaybeFB :: (b -> r -> r) -> (a -> Maybe b) -> a -> r -> r
+mapMaybeFB cons f x next = case f x of
+  Nothing -> next
+  Just r -> cons r next