#10195: GHC forgets constraints when matching on GADTs -------------------------------------+------------------------------------- Reporter: crockeea | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 7.8.4 Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: Type of failure: GHC rejects | Unknown/Multiple valid program | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Revisions: -------------------------------------+------------------------------------- Comment (by rwbarton): I'm no expert on this part of the compiler, but it looks like GHC reduces the wanted constraint `Bar m m'` (from the use of `magic`) to `BarFamily m m' ~ 'True` (because of the `Bar` instance), and then it can no longer deduce that from `Bar m m'`. A peculiar scenario. It certainly looks wrong though, and FWIW GHC 7.6.3 accepts the program. The robust workaround is to add the superclass constraint `BarFamily m m' ~ 'True` to `Bar m m'`, since then (as you describe as "even more bizarrely") GHC can deduce either of `BarFamily m m' ~ 'True` and `Bar m m'` from the other, and then can't get stuck in this kind of situation. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/10195#comment:1 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler