#13264: GHC panic with (->) generalization branch while compiling lens -------------------------------------+------------------------------------- Reporter: bgamari | Owner: Type: bug | Status: new Priority: normal | Milestone: 8.2.1 Component: Compiler | Version: 8.1 Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: None/Unknown | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Changes (by bgamari): * cc: simonpj, goldfire (added) Comment: It appears that replacing the `tcRepSplitTyApp_maybe` callsite in `can_eq_nc'` with a variant which returns `Nothing` instead of panicking allows compilation to proceed, {{{#!hs -- Try to decompose type constructor applications -- Including FunTy (s -> t) can_eq_nc' _flat _rdr_env _envs ev eq_rel ty1 _ ty2 _ | Just (tc1, tys1) <- tcRepSplitTyConApp_maybe' ty1 , Just (tc2, tys2) <- tcRepSplitTyConApp_maybe' ty2 , not (isTypeFamilyTyCon tc1) , not (isTypeFamilyTyCon tc2) = canTyConApp ev eq_rel tc1 tys1 tc2 tys2 }}} Where `tcRepSplitTyConApp_maybe'` is the variant described above. While I'm far from convinced that this is the right, it does seem plausible. Afterall, the failing invocation involves a `FunTy` and an `AppTy`, so the guards really should just fail. It would be nice to hear Simon or Richard's opinion on this. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/13264#comment:4 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler