[GHC] #15970: Recompilation bug with default class methods

29 Nov 2018

      #15970: Recompilation bug with default class methods
-------------------------------------+-------------------------------------
           Reporter:  simonmar       |             Owner:  (none)
               Type:  bug            |            Status:  new
           Priority:  highest        |         Milestone:  8.6.3
          Component:  Compiler       |           Version:  8.6.2
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  Compile-time
  Unknown/Multiple                   |  crash or panic
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
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 Repro as follows.

 A.hs:
 {{{
 {-# OPTIONS_GHC -fno-full-laziness #-}
 module A (toTypedData, toTypedDataNoDef) where

 toTypedData :: String -> IO Int
 toTypedData s = wrapPrint "yoyo" $ toTypedDataNoDef s

 wrapPrint :: String -> IO Int -> IO Int
 wrapPrint s act = do
     putStrLn s
     act

 toTypedDataNoDef  :: String -> IO Int
 toTypedDataNoDef s = return $ length s
 }}}

 B.hs:
 {{{
 module B ( TypeClass(..) ) where

 import A

 class Show a => TypeClass a where
     getSize :: a -> IO Int
     getSize a = toTypedData (show a)

 printA :: a -> IO ()
 }}}

 C.hs:
 {{{
 module Main where

 import B

 data MyDataType = MyDataType String Int deriving Show

 instance TypeClass MyDataType where
     printA = putStrLn . show

 main :: IO ()
 main = do
     let myValue = MyDataType "haha" 99
     sz <- getSize myValue
     putStrLn $ show sz
 printA myValue
 }}}

 1. Comment out the `-fno-full-laziness` option in A.hs
 2. `rm *.o *.hi; ghc -O2 C.hs`
 3. Re-enable the `-fno-full-laziness` option in A.hs
 4. `ghc -O2 C.hs`

 Produces a linker error:

 {{{
 C.o:Main_main1_info: error: undefined reference to
 'A_toTypedData2_closure'
 C.o(.data.rel.ro+0x48): error: undefined reference to
 'A_toTypedData2_closure'
 collect2: error: ld returned 1 exit status
 }}}

 Reproduced in 8.0, 8.4 and master. Probably happens in all released
 versions of GHC.

-- 
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/15970
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