testsuite: Add test for #16846

testsuite/tests/codeGen/should_run/T16846.hs

+{-# LANGUAGE CPP  #-}
+{-# LANGUAGE ExistentialQuantification  #-}
+module Main (main) where
+
+import Control.Concurrent.STM
+
+data Free f a = Pure a | Free (f (Free f a))
+
+data SuspendF a
+  = forall r. StepSTM (STM r)
+  | forall r. StepIO (IO r)
+
+effect :: STM a -> Free SuspendF a
+effect a = Free $ StepSTM a
+
+io :: IO a -> Free SuspendF a
+io a = Free $ StepIO a
+
+comb :: [Free SuspendF a] -> Free SuspendF a
+comb vs = io $ do
+  _ <- mapM go vs
+  undefined
+
+go :: Free SuspendF a -> IO (STM ())
+go (Free (StepIO a))  = a >>= \_ -> go $ Pure undefined
+go (Free (StepSTM a)) = pure $ a >>= \_ -> pure ()
+go (Pure _)           = pure $ pure ()
+
+runWidget :: Free SuspendF a -> IO a
+runWidget w = case w of
+  Free (StepIO io) -> do
+    _ <- io
+    undefined
+
+-- Uncommenting this hid the original bug.
+--main :: IO ()
+main = runWidget $ comb $ replicate 10000000 (effect retry)

testsuite/tests/codeGen/should_run/T16846.stderr

+T16846: Prelude.undefined
+CallStack (from HasCallStack):
+  error, called at libraries/base/GHC/Err.hs:80:14 in base:GHC.Err
+  undefined, called at T16846.hs:22:3 in main:Main

testsuite/tests/codeGen/should_run/all.T

@@ -197,3 +197,4 @@ test('T15892',
      compile_and_run, ['-O'])
 test('T16617', normal, compile_and_run, [''])
 test('T16449_2', exit_code(0), compile_and_run, [''])
+test('T16846', [only_ways(['optasm']), exit_code(1)], compile_and_run, [''])