Panic with GHC.TypeLits

[vladfi1]

The following short program caused a panic:

{-# LANGUAGE TypeFamilies #-}
{-# LANGUAGE PolyKinds #-}
{-# LANGUAGE TypeOperators #-}
{-# LANGUAGE FlexibleContexts #-}

module Panic where

import GHC.TypeLits

type family TypeTable (s :: Symbol) :: k

class (KnownNat (Index t), t ~ TypeTable (Index t)) => Typeable t where
  type Index t :: Symbol

Here's the result:

Panic.hs:12:18: error:ghc: panic! (the 'impossible' happened)
  (GHC version 7.11.20151105 for x86_64-unknown-linux):
    AThing evaluated unexpectedly tcTyVar Index

The issue is that I accidentally used KnownNat instead of KnownSymbol.

[goldfirere]

Thanks for the report! My understanding is that this panic should happen only with type-incorrect code. It will be fixed, but if you ever see it, you know your code has a bug somewhere. (Not all that helpful, I know!)