[GHC] #9821: DeriveAnyClass support for higher-kinded classes + some more comments

[GHC]

#9821: DeriveAnyClass support for higher-kinded classes + some more comments
-------------------------------------+-------------------------------------
        Reporter:  dreixel           |                Owner:  dreixel
            Type:  bug               |               Status:  patch
        Priority:  normal            |            Milestone:  8.2.1
       Component:  Compiler          |              Version:  7.9
      Resolution:                    |             Keywords:  Generics
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:  #5462, #9968,     |  Differential Rev(s):  Phab:D2961
  #12144                             |
       Wiki Page:                    |
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Comment (by Ryan Scott <ryan.gl.scott@…>):

 In [changeset:"639e702b6129f501c539b158b982ed8489e3d09c/ghc" 639e702/ghc]:
 {{{
 #!CommitTicketReference repository="ghc"
 revision="639e702b6129f501c539b158b982ed8489e3d09c"
 Refactor DeriveAnyClass's instance context inference

 Summary:
 Currently, `DeriveAnyClass` has two glaring flaws:

 * It only works on classes whose argument is of kind `*` or `* -> *`
 (#9821).
 * The way it infers constraints makes no sense. It basically co-opts the
   algorithms used to infer contexts for `Eq` (for `*`-kinded arguments) or
   `Functor` (for `(* -> *)`-kinded arguments). This tends to produce
 overly
   constrained instances, which in extreme cases can lead to legitimate
 things
   failing to typecheck (#12594). Or even worse, it can trigger GHC panics
   (#12144 and #12423).

 This completely reworks the way `DeriveAnyClass` infers constraints to fix
 these two issues. It now uses the type signatures of the derived class's
 methods to infer constraints (and to simplify them). A high-level
 description
 of how this works is included in the GHC users' guide, and more technical
 notes
 on what is going on can be found as comments (and a Note) in
 `TcDerivInfer`.

 Fixes #9821, #12144, #12423, #12594.

 Test Plan: ./validate

 Reviewers: dfeuer, goldfire, simonpj, austin, bgamari

 Subscribers: dfeuer, thomie

 Differential Revision: https://phabricator.haskell.org/D2961
 }}}

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9821#comment:18>
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