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

[GHC]

#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
      Resolution:                    |             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:                    |
-------------------------------------+-------------------------------------
Description changed by RyanGlScott:

Old description:

> (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
> }}}

New description:

 (Originally noticed [https://github.com/ekmett/transformers-
 compat/issues/32 here].)

 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#comment:1>
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