[GHC] #14692: Deriving Show with -XEmptyDataDeriving cases on the wrong argument

20 Jan 2018

      #14692: Deriving Show with -XEmptyDataDeriving cases on the wrong argument
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           Reporter:  RyanGlScott    |             Owner:  (none)
               Type:  bug            |            Status:  new
           Priority:  high           |         Milestone:  8.4.1
          Component:  Compiler       |           Version:  8.4.1-alpha1
           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:
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 Running this program in GHC 8.4.1-alpha:

 {{{#!hs
 {-# LANGUAGE EmptyDataDeriving #-}
 {-# OPTIONS_GHC -ddump-deriv #-}
 module Main (main) where

 data Empty deriving Show

 loop :: Empty
 loop = let x = x in x

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

 One would expect this to loop infinitely at runtime, but in practice, that
 is not the case:

 {{{
 $ /opt/ghc/8.4.1/bin/runghc Bug.hs

 ==================== Derived instances ====================
 Derived class instances:
   instance GHC.Show.Show Main.Empty where
     GHC.Show.showsPrec z_a1Iu = case z_a1Iu of

 Derived type family instances:

 ==================== Filling in method body ====================
 GHC.Show.Show [Main.Empty]
   GHC.Show.show = GHC.Show.$dmshow @(Main.Empty)

 ==================== Filling in method body ====================
 GHC.Show.Show [Main.Empty]
   GHC.Show.showList = GHC.Show.$dmshowList @(Main.Empty)

 Bug.hs: Bug.hs:5:21-24: Non-exhaustive patterns in case
 }}}

 The `-ddump-deriv` output reveals why: the `showsPrec` implementation for
 `Empty` is casing on the //precedence// argument, not the actual value of
 type `Empty`! This results in the non-exhaustive patterns error.

 This is my fault, so I'll prepare a fix :)

-- 
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/14692
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