id	summary	reporter	owner	description	type	status	priority	milestone	component	version	resolution	keywords	cc	os	architecture	failure	difficulty	testcase	blockedby	blocking	related
7438	Tc panic when type declaration is missing	heisenbug	simonpj	"Compile the attached snippet with HEAD GHC.

> ghc: panic! (the 'impossible' happened)
>   (GHC version 7.7.20121114 for x86_64-unknown-linux):
>         tcTyVarDetails
> <<details unavailable>>

You need to install the ""thrist-0.3"" package from Hackage.

When I just paste the 'Thrist' definition into the code the panic does not happen. (It resulted in this message for me:
>     Occurs check: cannot construct the infinite type: t2 ~ 'St t2
>     Expected type: Thrist Nat (At t4) t2 t3
>       Actual type: Thrist Nat (At t4) b t3
>     Relevant bindings include
>       go :: Thrist Nat (At t4) t2 t3 -> t -> t1
>         (bound at GHC-7438.hs:20:11)
>     In the first argument of `go', namely `rest'
>     In the second argument of `(:)', namely `go rest acc'
>     In the expression: c : go rest acc
> Failed, modules loaded: none.
)

Looks like interaction of non-generalized (local) let in conjunction with imported polykinded GADT.
"	bug	closed	normal		Compiler	7.7	fixed			Unknown/Multiple	Unknown/Multiple	GHCi crash	Unknown