GitLab

This program will crash when compiled with GHC 9.4 or HEAD:

{-# LANGUAGE TemplateHaskell #-}
module Main where

main :: IO ()
main = print
  $([| let f :: Int -> Int
           f x = x + 1
           {-# OPAQUE f #-}
       in f 41
     |])

$ ~/Software/ghc-9.3.20220413/bin/ghc Bug.hs -fforce-recomp
[1 of 2] Compiling Main             ( Bug.hs, Bug.o, Bug.dyn_o )
GHC error in desugarer lookup in Main:
  Can't find interface-file declaration for data constructor Opaque
    Probable cause: bug in .hi-boot file, or inconsistent .hi file
    Use -ddump-if-trace to get an idea of which file caused the error

This program works if the intervening Template Haskell splice is removed, however:

module Main where

main :: IO ()
main = print $
  let f :: Int -> Int
      f x = x + 1
      {-# OPAQUE f #-}
  in f 41

$ ~/Software/ghc-9.3.20220413/bin/ghc Bug.hs -fforce-recomp && ./Bug
[1 of 2] Compiling Main             ( Bug.hs, Bug.o )
[2 of 2] Linking Bug
42