#15499: Panic in occurence analysis phase (?), getRuntimeRep -------------------------------------+------------------------------------- Reporter: _deepfire | Owner: RyanGlScott Type: bug | Status: new Priority: high | Milestone: 8.4.4 Component: Compiler | Version: 8.4.3 Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: Type of failure: Compile-time | Unknown/Multiple crash or panic | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Changes (by RyanGlScott): * owner: (none) => RyanGlScott * os: Linux => Unknown/Multiple Comment: Ugh, this is my fault. I introduced this regression in ef26182e2014b0a2a029ae466a4b121bf235e4e4 (`Track the order of user-written tyvars in DataCon`). The `-v4` logs are a bit misleading, as the real problem lies in desugaring, not occurrence analysis. Essentially, GHC tries to desugar this: {{{#!hs foo :: forall p. ADT p foo = undefined {b=undefined} }}} Into this: {{{#!hs foo :: forall p. ADT p foo @p = case undefined of ADT @a x y -> ADT @p @a x undefined }}} If the fact that I wrote `ADT @p @a` looks strange to you, that's because it is! The //real// type of the `ADT` constructor is: {{{#!hs ADT :: forall a p. a -> Integer -> ADT p }}} Due to the fact that type variables in GADT type signatures are now quantified in toposorted order after the aforementioned commit. This means that the correct order of arguments //should// be `ADT @a @p`. However, the code in `dsExpr` assumes that the universally quantified type variables always come before the existentially quantified type variables, so `@p` ends up being applied before `@a`, leading to utter disaster and chaos down the line. Thankfully, this isn't too difficult to fix. Patch incoming. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/15499#comment:2 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler