Deriving Show with -XEmptyDataDeriving cases on the wrong argument

Running this program in GHC 8.4.1-alpha:

{-# 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 :)

Trac metadata

Trac field	Value
Version	8.4.1-alpha1
Type	Bug
TypeOfFailure	OtherFailure
Priority	high
Resolution	Unresolved
Component	Compiler
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