[GHC] #14918: GHC 8.4.1 regression: derived Read instances with field names containing # no longer parse

13 Mar 2018


      #14918: GHC 8.4.1 regression: derived Read instances with field names containing #
no longer parse
-------------------------------------+-------------------------------------
           Reporter:  RyanGlScott    |             Owner:  (none)
               Type:  bug            |            Status:  new
           Priority:  high           |         Milestone:  8.4.2
          Component:  Compiler       |           Version:  8.4.1
           Keywords:  deriving       |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  Incorrect result
  Unknown/Multiple                   |  at runtime
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
-------------------------------------+-------------------------------------
 (Originally noticed [here](https://github.com/ekmett/transformers-
 compat/issues/32).)

 Consider the following program:

 {{{#!hs
 {-# LANGUAGE MagicHash #-}
 module Bug where

 data T a = MkT { runT# :: a }
   deriving (Read, Show)

 t1, t2 :: T Int
 t1 = MkT 1
 t2 = read $ show t1

 main :: IO ()
 main = print t2
 }}}

 In GHC 8.2.1, this runs without issue:

 {{{
 $ /opt/ghc/8.2.2/bin/runghc Bug.hs
 MkT {runT# = 1}
 }}}

 In GHC 8.4.1, however, this produces a runtime error:

 {{{
 $ ~/Software/ghc-8.4.1/bin/runghc Bug.hs
 Bug.hs: Prelude.read: no parse
 }}}

-- 
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/14918
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