It's possible that this is just #16059 manifesting in a different way,
#16140: Cannot create type synonym for quantified constraint without ImpredicativeTypes -------------------------------------+------------------------------------- Reporter: Ashley Yakeley | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.6.3 Resolution: | Keywords: | QuantifiedConstraints, | ImpredicativeTypes Operating System: Unknown/Multiple | Architecture: Type of failure: GHC rejects | Unknown/Multiple valid program | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Comment (by simonpj): though. Sounds likely to me. And we have a plan for #16059, in comment:4. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/16140#comment:7 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler