Panic with orphan instance on 9.0
This is a bug in 9.0 that is apparently fixed in 9.2+. However, it hasn't been reported so far, so I'm ticketing it just in case.
Given one-line Main.hs
instance Num Bool
load ghci -this-unit-id my-package -Wno-missing-methods Main
and use :l Main
:
$ ghci -this-unit-id my-package -Wno-missing-methods Main
GHCi, version 9.0.2: https://www.haskell.org/ghc/ :? for help
[1 of 1] Compiling Main ( Main.hs, interpreted )
Failed to load interface for ‘Main’
no unit id matching ‘main’ was found
ghci> :l Main
[1 of 1] Compiling Main ( Main.hs, interpreted )
ghc: panic! (the 'impossible' happened)
(GHC version 9.0.2:
lookupVers1
Main $fNumBool
Call stack:
CallStack (from HasCallStack):
callStackDoc, called at compiler/GHC/Utils/Outputable.hs:1230:37 in ghc:GHC.Utils.Outputable
pprPanic, called at compiler/GHC/Iface/Recomp.hs:1455:19 in ghc:GHC.Iface.Recomp
Credits to Dominic Steinitz for the initial report.