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Opened by Jakob Brünker@JakobBruenker

panic lookup_final_id with -fdefer-type-errors and kind error in instance definition

Summary

This file

{-# OPTIONS_GHC -fdefer-type-errors #-}
{-# LANGUAGE ExplicitForAll
           , TypeApplications
           , KindSignatures
           , FlexibleInstances
#-}

foo :: forall (f :: * -> *) . String
foo = ""

instance Show a where
  show _ = foo @Int

produces the following error:

Bug.hs:12:17: warning: [-Wdeferred-type-errors]
    • Expected kind ‘* -> *’, but ‘Int’ has kind ‘*’
    • In the type ‘Int’
      In the expression: foo @Int
      In an equation for ‘show’: show _ = foo @Int
   |
12 |   show _ = foo @Int
   |                 ^^^
ghc: panic! (the 'impossible' happened)
  (GHC version 9.0.0.20201227:
        lookup_final_id
  $fShowa
  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/Tidy.hs:200:12 in ghc:GHC.Iface.Tidy

Without -fdefer-type-errors, only the kind error would be shown, not the panic.

Expected behavior

GHC shouldn't panic

Environment

  • GHC version used: 9.0.0.20201227 as well as 8.10.2

Optional:

  • Operating System: Ubuntu inside WSL2 inside Windows 10
  • System Architecture: x86_64
Edited by Jakob Brünker