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I like to mix features to make code fail, code inspired by T10318:

-- /tmp/panic.hs
{-# LANGUAGE FlexibleContexts        #-}
{-# LANGUAGE FlexibleInstances       #-}
{-# LANGUAGE InstanceSigs            #-}
{-# LANGUAGE TypeFamilies            #-}
{-# LANGUAGE TypeSynonymInstances    #-}
{-# LANGUAGE UndecidableSuperClasses #-}

class (Frac (Frac a) ~ Frac a, Fractional (Frac a), ID (Frac a)) => ID a where
  type Frac a
  embed :: a -> Frac a

instance ID Rational where
  type Frac Rational = Int
  embed :: Rational -> Rational
  embed = undefined

When running it with defer-type-errors it causes GHC to panic:

% ghci -fdefer-type-errors -ignore-dot-ghci panic.hs &> panic.log

actual error included in panic.log. GHC asks me to report this as a bug and I do as I'm told.

Trac metadata

Trac field	Value
Version	7.11
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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