[GHC] #12027: ghc 8.0rc4 treats INLINE pragmas for methods with default (implicit) definitions as errors

6 May 2016

      #12027: ghc 8.0rc4 treats INLINE pragmas for methods with default (implicit)
definitions as errors
-------------------------------------+-------------------------------------
           Reporter:  carter         |             Owner:
               Type:  bug            |            Status:  new
           Priority:  high           |         Milestone:
          Component:  Compiler       |           Version:  8.0.1
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  GHC rejects
  Unknown/Multiple                   |  valid program
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
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 I was getting around to testing some of my maths code with ghc 8.0 and i
 noticed that whereas in ghc 7.10 and earlier i could have an INLINE pragma
 for a method of a type class method that has a default implementation
 (whether this did anything or not is another question), in ghc 8.0 this
 now gives an ERROR (with not even a warning in ghc 7.10)

 is this a deliberate design change or an accident? I dont see it in the
 release notes!

 heres a small program that illustrates the differences (builds fine with
 no mention of the INLINE pragma matter under -Wall for 7.10, errors during
 build in 8.0)

 {{{

 {-# LANGUAGE NoImplicitPrelude #-}

 module Main where
 import Prelude (putStrLn)
 import qualified Data.Functor as Fun
 import qualified  Data.Foldable as F
 import Prelude hiding
 (map,foldl,foldr,init,scanl,scanr,scanl1,scanr1,foldl1,foldr1)

 newtype ListWrap a = ListWrap  { unListWrap  :: [a] } deriving (Eq, Show)

 instance Foldable ListWrap where
   {-# INLINE foldMap #-}
   {-# INLINE foldr #-}
   foldMap f (ListWrap ls)=   (F.foldMap f ls )

 main = putStrLn "hello"
 }}}

 this may be a *valid* design change, but i've not seen it documented
 anywhere, such as the release notes in the RC or in
 https://ghc.haskell.org/trac/ghc/wiki/Migration/8.0 ... so i'm assuming
 its a regression pending dicussion ;)

--
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/12027
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