#15507: Deriving with QuantifiedConstraints is unable to penetrate type families -------------------------------------+------------------------------------- Reporter: isovector | Owner: (none) Type: bug | Status: closed Priority: normal | Milestone: 8.6.1 Component: Compiler | Version: Resolution: duplicate | Keywords: | QuantifiedConstraints Operating System: Unknown/Multiple | Architecture: Type of failure: GHC rejects | Unknown/Multiple valid program | Test Case: Blocked By: | Blocking: Related Tickets: #14860 | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Changes (by RyanGlScott): * status: new => closed * resolution: => duplicate * related: => #14860 Comment: Alas, this is expected behavior. See #14860. On GHC HEAD, you'll get a more informative error message, at least: {{{ GHCi, version 8.7.20180806: http://www.haskell.org/ghc/ :? for help Loaded GHCi configuration from /home/rgscott/.ghci [1 of 1] Compiling QuantifiedConstraints ( Bug.hs, interpreted ) Bug.hs:20:19: error: • Illegal type synonym family application in instance: HKD f a • In the quantified constraint ‘forall a. Eq a => Eq (HKD f a)’ In the stand-alone deriving instance for ‘(forall a. Eq a => Eq (HKD f a)) => Eq (Foo f)’ | 20 | deriving instance (forall a. Eq a => Eq (HKD f a)) => Eq (Foo f) | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ }}} -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/15507#comment:2 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler