[GHC] #13813: DeriveFunctor spuriously rejects existential context with type synonym mentioning the last type variable

[GHC]

#13813: DeriveFunctor spuriously rejects existential context with type synonym
mentioning the last type variable
-------------------------------------+-------------------------------------
           Reporter:  RyanGlScott    |             Owner:  (none)
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.0.1
  (Type checker)                     |
           Keywords:                 |  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:
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 This code does not compile:

 {{{#!hs
 {-# LANGUAGE ConstraintKinds #-}
 {-# LANGUAGE DeriveFunctor #-}
 {-# LANGUAGE ExistentialQuantification #-}
 {-# LANGUAGE KindSignatures #-}
 {-# LANGUAGE StandaloneDeriving #-}
 module Bug where

 import GHC.Exts (Constraint)

 type C (a :: Constraint) b = a

 data T a b = C (Show a) b => MkT b
 deriving instance Functor (T a)
 }}}
 {{{
 GHCi, version 8.2.0.20170523: http://www.haskell.org/ghc/  :? for help
 Loaded GHCi configuration from /home/rgscott/.ghci
 [1 of 1] Compiling Bug              ( Bug.hs, interpreted )

 Bug.hs:13:1: error:
     • Can't make a derived instance of ‘Functor (T a)’:
         Constructor ‘MkT’ must be truly polymorphic in the last argument
 of the data type
     • In the stand-alone deriving instance for ‘Functor (T a)’
    |
 13 | deriving instance Functor (T a)
    | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 }}}

 But it should, since if you expand `C (Show a) b`, you're left with `Show
 a`, which doesn't mention the last type variable `b` at all.

 Fix incoming.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13813>
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