#13658: Assertion failure on HEAD: "optCoercion changed types!" -------------------------------------+------------------------------------- Reporter: bgamari | Owner: (none) Type: bug | Status: new Priority: high | Milestone: 8.2.1 Component: Compiler | Version: 8.3 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: | -------------------------------------+------------------------------------- Comment (by bgamari): The problem here appears to be that the `TyConAppCo` case of `OptCoercion.opt_univ` fires, {{{#!hs opt_univ env sym prov role oty1 oty2 | Just (tc1, tys1) <- splitTyConApp_maybe oty1 , Just (tc2, tys2) <- splitTyConApp_maybe oty2 , tc1 == tc2 = pprTrace "opt_univ(tyconapp)" (ppr tc1 <+> ppr tys1 $$ ppr tc2 <+> ppr tys2) $ let roles = tyConRolesX role tc1 arg_cos = zipWith3 (mkUnivCo prov) roles tys1 tys2 arg_cos' = zipWith (opt_co4 env sym False) roles arg_cos in mkTyConAppCo role tc1 arg_cos' }}} This emits, {{{ opt_univ(tyconapp) Any [Any -> Any, Any] Any [Any -> Any] }}} for the problematic coercion. The issue here is that we end up dropping the second argument from `tys1` due to the `zipWith`. I suspect we want to add a guard to this equation asserting that `tys1` and `tys2` are of equal lengths. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/13658#comment:4 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler