[GHC] #8128: Standalone deriving fails for GADTs due to inaccessible code

[GHC]

#8128: Standalone deriving fails for GADTs due to inaccessible code
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        Reporter:  adamgundry                 |            Owner:
            Type:  bug                        |           Status:  new
        Priority:  normal                     |        Milestone:
       Component:  Compiler (Type checker)    |          Version:  7.7
      Resolution:                             |         Keywords:
Operating System:  Unknown/Multiple           |     Architecture:
 Type of failure:  GHC rejects valid program  |  Unknown/Multiple
       Test Case:                             |       Difficulty:  Unknown
        Blocking:                             |       Blocked By:
                                              |  Related Tickets:
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Comment (by goldfire):

 This seems impossible to do correctly, in the general case. For example:

 {{{
 type family F a

 data Bar a where
   B1 :: Bar Int
   B2 :: Bar (F Bool)

 deriving instance Eq (Bar Int)
 }}}

 The instance generated by the `deriving instance` will depend on the
 instances for `F` that are in scope, which seems quite fragile. In
 particular, if the instance for `Eq` were derived out of scope of any
 instance for `F`, then the instance would pattern-match only on `B1`. If,
 later (that is, in another module), we know that `F Bool` is `Int`, then
 it seems we could get a pattern-match failure in the `Eq`  instance.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8128#comment:2>
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