[GHC] #14763: GHC 8.4.1-alpha regression with FunctionalDependencies

5 Feb 2018

      #14763: GHC 8.4.1-alpha regression with FunctionalDependencies
-------------------------------------+-------------------------------------
           Reporter:  RyanGlScott    |             Owner:  (none)
               Type:  bug            |            Status:  new
           Priority:  highest        |         Milestone:  8.4.1
          Component:  Compiler       |           Version:  8.4.1-alpha3
  (Type checker)                     |
           Keywords:  FunDeps        |  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:
-------------------------------------+-------------------------------------
 This regression prevents `esqeueleto-2.5.3` from building with GHC 8.4.1.
 Here is a minimized example of the problem:

 {{{#!hs
 {-# LANGUAGE FunctionalDependencies #-}
 {-# LANGUAGE GADTs #-}
 {-# LANGUAGE KindSignatures #-}
 {-# LANGUAGE MultiParamTypeClasses #-}
 module Bug where

 data Value a = Value a

 data SomeValue expr where
   SomeValue :: Esqueleto query expr backend => expr (Value a) -> SomeValue
 expr

 class Esqueleto (query :: * -> *) (expr :: * -> *) backend
         | query -> expr backend, expr -> query backend

 data SqlQuery a

 data SqlBackend

 data SqlExpr a where
   ECompositeKey :: SqlExpr (Value a)

 instance Esqueleto SqlQuery SqlExpr SqlBackend

 match' :: SomeValue SqlExpr -> a
 match' (SomeValue ECompositeKey) = undefined
 }}}

 On GHC 8.2.2, this typechecks without issue. On GHC 8.4.1-alpha (version
 8.4.0.20180204), this fails with:

 {{{
 $ /opt/ghc/8.4.1/bin/ghci Bug.hs
 GHCi, version 8.4.0.20180204: http://www.haskell.org/ghc/  :? for help
 Loaded GHCi configuration from /home/rgscott/.ghci
 [1 of 1] Compiling Bug              ( Bug.hs, interpreted )

 Bug.hs:25:19: error:
     • Could not deduce: query ~ SqlQuery
         arising from a functional dependency between:
           constraint ‘Esqueleto query SqlExpr backend’
             arising from a pattern with constructor:
                            SomeValue :: forall (query :: * -> *) (expr ::
 * -> *) backend a.
                                         Esqueleto query expr backend =>
                                         expr (Value a) -> SomeValue expr,
                          in an equation for ‘match'’
           instance ‘Esqueleto SqlQuery SqlExpr SqlBackend’ at
 Bug.hs:22:10-46
       from the context: Value a1 ~ Value a2
         bound by a pattern with constructor:
                    ECompositeKey :: forall a. SqlExpr (Value a),
                  in an equation for ‘match'’
         at Bug.hs:25:19-31
       ‘query’ is a rigid type variable bound by
         a pattern with constructor:
           SomeValue :: forall (query :: * -> *) (expr :: * -> *) backend
 a.
                        Esqueleto query expr backend =>
                        expr (Value a) -> SomeValue expr,
         in an equation for ‘match'’
         at Bug.hs:25:9-31
       Inaccessible code in
         a pattern with constructor:
           ECompositeKey :: forall a. SqlExpr (Value a),
         in an equation for ‘match'’
     • In the pattern: ECompositeKey
       In the pattern: SomeValue ECompositeKey
       In an equation for ‘match'’:
           match' (SomeValue ECompositeKey) = undefined
    |
 25 | match' (SomeValue ECompositeKey) = undefined
    |                   ^^^^^^^^^^^^^
 }}}

-- 
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/14763
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