id,summary,reporter,owner,description,type,status,priority,milestone,component,version,resolution,keywords,cc,os,architecture,failure,difficulty,testcase,blockedby,blocking,related
3067,GHCi panics with 'initTc:LIE' while :stepping on code with funny types,mnislaih,igloo,"This is due to runtime type reconstruction not dealing correctly with type functions.

I managed to come up a minimal example.
{{{
{-# LANGUAGE TypeFamilies #-}

type family Id x
type instance Id Int = Int
type instance Id Bool = Bool

class Convert x y where convert :: x -> y
instance Convert x x where convert = id

f :: Convert a (Id a) => a -> Id a
f x = convert x

g :: Convert a (Id a) => a -> Id a
g x = let x' = f x in x'
}
{{{
[1 of 1] Compiling Main             ( ../code/debuggerExamples/lie.hs, interpreted )
Ok, modules loaded: Main.
*Main GOA> :step g False
:step g False
Stopped at ../code/debuggerExamples/lie.hs:15:0-23
_result :: Id a = _
14  g :: Convert a (Id a) => a -> Id a
15  g x = let x' = f x in x'
    ^^^^^^^^^^^^^^^^^^^^^^^^
[../code/debuggerExamples/lie.hs:15:0-23] *Main GOA> :step
:step
ghc: panic! (the 'impossible' happened)
  (GHC version 6.10.1 for i386-apple-darwin):
	initTc:LIE

Please report this as a GHC bug:  http://www.haskell.org/ghc/reportabug
}}}",merge,closed,normal,,GHCi,6.10.1,fixed,,,Unknown/Multiple,Unknown/Multiple,,Unknown,break028,,,