Add regression test for T17904

Closes #17904

testsuite/tests/codeGen/should_compile/T17904.hs

+{-# LANGUAGE MagicHash #-}
+{-# LANGUAGE ScopedTypeVariables #-}
+{-# LANGUAGE UnboxedTuples #-}
+{-# LANGUAGE TypeInType #-}
+{-# OPTIONS_GHC -fno-full-laziness  #-}
+
+module T17904
+    ( difference
+    , differenceWith
+    ) where
+
+import GHC.Exts ( TYPE, Int (..)  )
+import Prelude hiding (lookup)
+
+{-# ANN module ("HLint: ignore Eta reduce" :: String) #-}
+
+class Hashable a where
+    hashWithSalt :: Int -> a -> Int
+
+data Leaf k v = L k v
+
+data HashMap k v
+    = Empty
+    | Leaf Word (Leaf k v)
+
+lookup :: (Eq k, Hashable k) => k -> HashMap k v -> Maybe v
+lookup k m = case lookupCont (\_ -> (# (# #) | #)) (\v _i -> (# | v #)) (fromIntegral $ (hashWithSalt undefined) k) k m of
+  _ -> undefined
+{-# INLINE lookup #-}
+
+lookupCont ::
+  forall rep (r :: TYPE rep) k v.
+     Eq k
+  => ((# #) -> r)
+  -> (v -> Int -> r)
+  -> Word
+  -> k -> HashMap k v -> r
+lookupCont _absent _present _h0 _k0 _m0 = go undefined undefined undefined undefined
+  where
+    go :: Word -> k -> Int -> HashMap k v -> r
+    go h k _ _
+        | h == undefined && k == undefined = undefined
+        | otherwise          = undefined
+
+difference :: (Eq k, Hashable k) => HashMap k v -> HashMap k w -> HashMap k v
+difference a b = foldlWithKey' go undefined a
+  where
+    go _m k _v = case lookup k b of
+                 Nothing -> undefined
+                 _       -> undefined
+
+differenceWith :: (Eq k, Hashable k) => a -> HashMap k v -> HashMap k w -> HashMap k v
+differenceWith _f a b = foldlWithKey' go undefined a
+  where
+    go _m k _v = case lookup k b of
+                 Nothing -> undefined
+                 _  -> undefined
+
+foldlWithKey' :: (a -> k -> v -> a) -> a -> HashMap k v -> a
+foldlWithKey' _f = go
+  where
+    go _z Empty          = undefined
+    go _z (Leaf _ _)     = undefined

testsuite/tests/codeGen/should_compile/all.T

@@ -91,3 +91,4 @@ test('T14373d', [],
      switch_skeleton_and_entries_only])
 
 test('T17648', normal, makefile_test, [])
+test('T17904', normal, compile, ['-O'])