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

7 Dec 2018

      #15970: Recompilation bug with default class methods
-------------------------------------+-------------------------------------
        Reporter:  simonmar          |                Owner:  simonmar
            Type:  bug               |               Status:  patch
        Priority:  highest           |            Milestone:  8.8.1
       Component:  Compiler          |              Version:  8.6.2
      Resolution:                    |             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):  Phab:D5394
       Wiki Page:                    |
-------------------------------------+-------------------------------------
Description changed by bgamari:

Old description:
...
Repro as follows.
A.hs:
{{{#!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:
{{{#!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.
New description:

 Repro as follows.

 A.hs:
 {{{#!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:
 {{{#!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:
 {{{#!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#comment:4
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