Regression in 9.4.1 (alpha2): unused-top-binds reported for used functions (RecursiveDo)

added needs triage label

Thanks for reporting this bug. Indeed, that seems wrong: mergeNullViews is used in combineConcat which is a local function in the body of patternToQ, and patternToQ is exported by the module. seqTake has an identical usage pattern.

It would be nice to get a smaller reproducer so we can get a better idea of what's going wrong.

added Pnormal Tbug error messages renamer labels

removed needs triage label

The problem seems to be connected to mdo. If I replace it with do, the spurious warnings disappear.

Shrunk file:

{-# LANGUAGE  RecursiveDo #-}

module Main
  ( patternToQ
  , main )
  where

import Control.Monad           (liftM2)
import Control.Monad.RWS       (RWS, runRWS)

import Data.IntMap.EnumMap2    (EnumMap)
import Data.IntSet.EnumSet2    (EnumSet)

import Text.Regex.TDFA.Common
import Text.Regex.TDFA.Pattern (Pattern(..), starTrans)

data Q = Q
  { nullQ :: NullView
  , takes :: (Position, Maybe Position)
  }

newtype SetTestInfo = SetTestInfo (EnumMap WhichTest (EnumSet DoPa))

instance Semigroup SetTestInfo where
  SetTestInfo x <> SetTestInfo y = SetTestInfo (x <> y)

instance Monoid SetTestInfo where
  mempty = SetTestInfo mempty
  mappend = (<>)

type NullView = [(SetTestInfo,TagList)]

data HandleTag = NoTag
               | Advice Tag

mergeNullViews :: NullView -> NullView -> NullView
mergeNullViews s1 s2 = do
  (test1,tag1) <- s1
  (test2,tag2) <- s2
  return (mappend test1 test2, mappend tag1 tag2)

seqTake :: (Int, Maybe Int) -> (Int, Maybe Int) -> (Int, Maybe Int)
seqTake (x1,y1) (x2,y2) = (x1+x2,liftM2 (+) y1 y2)

type PM = RWS (Maybe GroupIndex) [Either Tag GroupInfo] ([OP]->[OP],Tag)

type HHQ = HandleTag
        -> HandleTag
        -> PM Q

patternToQ :: CompOption -> (Pattern,(GroupIndex,DoPa)) -> Q
patternToQ _ (pOrig, _) = tnfa
  where
  (tnfa, _, _) = runRWS monad startReader startState

  monad = go (starTrans pOrig) (Advice 0) (Advice 1)

  go :: Pattern -> HHQ
  go pIn m1 m2 =
    case pIn of
         PConcat ps -> combineConcat ps m1 m2
         _ -> error ""

  startReader :: Maybe GroupIndex
  startReader = Just 0

  startState :: ([OP]->[OP],Tag)
  startState = (id , 2)

  combineConcat :: [Pattern] -> HHQ
  combineConcat = foldr1 combineSeq . map go
    where combineSeq :: HHQ -> HHQ -> HHQ
          combineSeq pFront pEnd m1 m2 = mdo  -- changing this to 'do' fixes the problem
            (qFront,qEnd) <- liftM2 (,) (pFront m1 NoTag) (pEnd NoTag m2)

            return $ Q { nullQ = mergeNullViews (nullQ qFront) (nullQ qEnd)
                       , takes = seqTake (takes qFront) (takes qEnd)
                       }

main :: IO ()
main = putStrLn "Hello, Haskell!"

{- With GHC 9.4 alpha2 and mdo

app/Main.hs:17:10: error: [-Wunused-top-binds, -Werror=unused-top-binds]
    Defined but not used: data constructor ‘Q’
   |
17 | data Q = Q
   |          ^...

app/Main.hs:18:5: error: [-Wunused-top-binds, -Werror=unused-top-binds]
    Defined but not used: ‘nullQ’
   |
18 |   { nullQ :: NullView
   |     ^^^^^

app/Main.hs:19:5: error: [-Wunused-top-binds, -Werror=unused-top-binds]
    Defined but not used: ‘takes’
   |
19 |   , takes :: (Position, Maybe Position)
   |     ^^^^^

app/Main.hs:37:1: error: [-Wunused-top-binds, -Werror=unused-top-binds]
    Defined but not used: ‘mergeNullViews’
   |
37 | mergeNullViews s1 s2 = do
   | ^^^^^^^^^^^^^^

app/Main.hs:43:1: error: [-Wunused-top-binds, -Werror=unused-top-binds]
    Defined but not used: ‘seqTake’
   |
43 | seqTake (x1,y1) (x2,y2) = (x1+x2,liftM2 (+) y1 y2)
   | ^^^^^^^

-}

Cabal file:

cabal-version:      1.24
name:               unused-warning
version:            0.1.0.0
license:            MIT
license-file:       LICENSE
build-type:         Simple

executable unused-warning
  hs-source-dirs:   app
  main-is:          Main.hs

  build-depends:    base >=4.17 && <4.18
                  , mtl
                  , regex-tdfa

  default-language: Haskell2010

  ghc-options:
      -Wall
      -Werror

Thanks for the smaller reproducer. I was able to minimise it a bit more:

{-# LANGUAGE RecursiveDo #-}
{-# OPTIONS_GHC -Wunused-top-binds #-}

module T21654
  ( patternToQ )
  where

import Data.Functor.Identity

mergeNullViews :: () -> ()
mergeNullViews _ = ()

patternToQ :: ()
patternToQ = runIdentity $ combineSeq
  where
  combineSeq :: Identity ()
  combineSeq = mdo  -- changing this to 'do' fixes the problem
    q <- Identity ()
    return $ mergeNullViews q

Great, @sheaf! After 1 hour of shrinking, I stopped after I found out the culprit (mdo). The problem seems to be systematic.
Wild guess: Maybe it is a fixed-point computation with the wrong initial value "nothing is used" when it should be "all is used".

I believe commit acb188e0 by @mpickering introduced this regression. Fix in !8350 (closed). Thanks for taking the time to boil the reproducer down Andreas, that helped a lot.

added backport needed:9.4 label

changed title from Regression in 9.4.1 (alpha2): unused-top-binds reported for used functions to Regression in 9.4.1 (alpha2): unused-top-binds reported for used functions (RecursiveDo)

mentioned in commit 7e60909b

closed with commit b54f6c4f

mentioned in commit b54f6c4f

Re-opening as the patch still needs to be backported to the 9.4 branch.

reopened

mentioned in commit c1b46818

Thanks, this is fixed in 9.4.0.20220623 (9.4.1 alpha3)!

closed

removed backport needed:9.4 label

Regression in 9.4.1 (alpha2): unused-top-binds reported for used functions (RecursiveDo)

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