[GHC] #14728: Is (GeneralizedNewtypeDeriving + associated type classes) completely bogus?

[GHC]

#14728: Is (GeneralizedNewtypeDeriving + associated type classes) completely bogus?
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        Reporter:  RyanGlScott       |                Owner:  (none)
            Type:  bug               |               Status:  merge
        Priority:  normal            |            Milestone:  8.4.1
       Component:  Compiler (Type    |              Version:  8.2.2
  checker)                           |             Keywords:  deriving,
      Resolution:                    |  TypeFamilies
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):  Phab:D4402
       Wiki Page:                    |
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Comment (by Ben Gamari <ben@…>):

 In [changeset:"1ede46d415757f53af33bc6672bd9d3fba7f205d/ghc"
 1ede46d4/ghc]:
 {{{
 #!CommitTicketReference repository="ghc"
 revision="1ede46d415757f53af33bc6672bd9d3fba7f205d"
 Implement stopgap solution for #14728

 It turns out that one can produce ill-formed Core by
 combining `GeneralizedNewtypeDeriving`, `TypeInType`, and
 `TypeFamilies`, as demonstrated in #14728. The root of the problem
 is allowing the last parameter of a class to appear in a //kind// of
 an associated type family, as our current approach to deriving
 associated type family instances simply doesn't work well for that
 situation.

 Although it might be possible to properly implement this feature
 today (see https://ghc.haskell.org/trac/ghc/ticket/14728#comment:3
 for a sketch of how this might work), there does not currently exist
 a performant implementation of the algorithm needed to accomplish
 this. Until such an implementation surfaces, we will make this corner
 case of `GeneralizedNewtypeDeriving` an error.

 Test Plan: make test TEST="T14728a T14728b"

 Reviewers: bgamari

 Reviewed By: bgamari

 Subscribers: rwbarton, thomie, carter

 GHC Trac Issues: #14728

 Differential Revision: https://phabricator.haskell.org/D4402
 }}}

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14728#comment:13>
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