[GHC] #9563: Support for deriving Generic1 for data families

[GHC]

#9563: Support for deriving Generic1 for data families
-------------------------------------+-------------------------------------
       Reporter:  mnislaih           |                   Owner:
           Type:  bug                |                  Status:  new
       Priority:  normal             |               Milestone:
      Component:  Compiler           |                 Version:  7.8.2
       Keywords:                     |        Operating System:  MacOS X
   Architecture:  Unknown/Multiple   |         Type of failure:  GHC
     Difficulty:  Unknown            |  rejects valid program
     Blocked By:                     |               Test Case:
Related Tickets:                     |                Blocking:
                                     |  Differential Revisions:
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 The following code:
 {{{#!hs

 import GHC.Generics

 data family F typ :: * -> *
 data A
 data instance F A a = AData a deriving Generic1
 }}}
 produces an error when run with ghc 7.8.2 or 7.8.3:
 {{{
 Couldn't match type ‘Rep1 (F A)’
               with ‘M1 t0 t1 (M1 t2 t3 (M1 t4 t5 Par1))’
 The type variables ‘t0’, ‘t1’, ‘t2’, ‘t3’, ‘t4’, ‘t5’ are ambiguous
 Expected type: Rep1 (F A) a
   Actual type: M1 t0 t1 (M1 t2 t3 (M1 t4 t5 Par1)) a
 In the pattern: M1 (M1 (M1 g1))
 In an equation for ‘to1’: to1 (M1 (M1 (M1 g1))) = AData (unPar1 g1)
 In the instance declaration for ‘Generic1 (F A)’
 }}}

 whereas ghc 7.6.3 simply refuses to go ahead with the message:
 {{{

 generic1.hs:14:40:
     Derived instance `Generic1 (F A)'
       requires illegal partial application of data type family F
     In the data instance declaration for `F'
 }}}

 Either a check has gone missing in 7.8.x, or a bug has crept up in the
 Generic1 support. I have gone through past tickets and
 https://ghc.haskell.org/trac/ghc/ticket/5936 suggests that it is the
 latter.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9563>
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