#14831: QuantifiedConstraints: Odd superclass constraint -------------------------------------+------------------------------------- Reporter: Iceland_jack | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.5 Resolution: | Keywords: | QuantifiedConstraints wipT2893 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 simonpj):
but my hunch is that you're experiencing the same bug as in...
Indeed: see comment:4 above, which gives exactly the same diagnosis as I gave in https://ghc.haskell.org/trac/ghc/ticket/5927#comment:33. In both comments I point out that a local instance declaration that claims to produce evidence for `Semigroup xx` '''for any xx''' is very suspicious (comment:4 above). Now you have two ways of proving `SemiGroup (Free ...)`: from the top- level instance or from the local instance (QC). And currently the local one "wins". For now I'm saying "not a bug" and "you probably didn't really want that program anyway". But I could obviously be wrong about the latter. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/14831#comment:7 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler