#13758: Deriving can't find an instance that holds, standalone deriving works -------------------------------------+------------------------------------- Reporter: Iceland_jack | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.0.1 Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: None/Unknown | Test Case: Blocked By: | Blocking: Related Tickets: #2721, #8165 | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Changes (by RyanGlScott): * related: => #2721, #8165 Comment: Hah! You tickled a bug that I originally ran into when I was working on the fix for #2721/#8165 (making `GeneralizedNewtypeDeriving` work for classes with associated type families). The issue was that we weren't updating the type family instance environment early enough in type checking, which resulted in the strange staging error you experienced. I had thought that the only way to trigger that bug was with classes with associated type families, but you have just proven me wrong! The fix made it into GHC 8.2.1, and this program does indeed typecheck with that version. However, I'll keep this ticket open for now, as this program makes for a good GHC regression test. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/13758#comment:3 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler