To be clear, I don't think (although could be wrong) this program should compile; afterall, the type of `gunbox` requires that we represent a levity polymorphic result, which we cannot do. Hence, the real bug here is
#13929: GHC panic with levity polymorphism -------------------------------------+------------------------------------- Reporter: vagarenko | Owner: (none) Type: bug | Status: new Priority: high | Milestone: 8.2.1 Component: Compiler | Version: 8.2.1-rc2 Resolution: | Keywords: TypeInType, | LevityPolymorphism Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: None/Unknown | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Comment (by vagarenko): Replying to [comment:7 bgamari]: the panic instead of a proper type error. I think levity polymorphism paper says you can't have functions with levity-polymorphic parameters, levity-polymorphic results are OK. This compiles fine: {{{#!hs instance (GUnbox f r) => GUnbox (M1 i t f) r where type GUnboxed (M1 i t f) r = GUnboxed f r gunbox (M1 x) = gunbox x }}} But unboxed sum also fails to compile: {{{#!hs instance (GUnbox f rf, GUnbox g rg) => GUnbox (f :+: g) ('SumRep '[rf, rg]) where type GUnboxed (f :+: g) ('SumRep '[rf, rg]) = (# GUnboxed f rf | GUnboxed g rg #) gunbox (L1 l) = (# gunbox l | #) gunbox (R1 r) = (# | gunbox r #) }}} with the same message. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/13929#comment:8 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler