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Created by Ryan Scott@RyanGlScottMaintainer

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

(Originally noticed here.)

Consider the following program:

{-# 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
Edited by Ryan Scott