#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 RyanGlScott): This has nothing to do with this ticket, but rather #16059. Moreover, this is expected behavior—the fix for #16059 revealed an illegal use of impredicativity in `Cabal-2.4.1.0` that previously went unnoticed (see the discussion starting at https://ghc.haskell.org/trac/ghc/ticket/16059#comment:14). The current version of `Cabal` that is bundled with GHC HEAD, `Cabal-2.5.0.0`, fixes this issue. **tl;dr** You want to use `Cabal-2.5.0.0`, not `Cabal-2.4.1.0`. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/16140#comment:21 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler