[GHC] #12801: Misleading error message when deriving Functor

[GHC]

#12801: Misleading error message when deriving Functor
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        Reporter:  erikd             |                Owner:  (none)
            Type:  bug               |               Status:  closed
        Priority:  normal            |            Milestone:  8.2.1
       Component:  Compiler          |              Version:  8.0.1
      Resolution:  duplicate         |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  Poor/confusing    |  Unknown/Multiple
  error message                      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by Ryan Scott <ryan.gl.scott@…>):

 In [changeset:"6bb32ba78580271921e3d5c3c98afac2c1b68de4/ghc" 6bb32ba/ghc]:
 {{{
 #!CommitTicketReference repository="ghc"
 revision="6bb32ba78580271921e3d5c3c98afac2c1b68de4"
 Fix #10684 by processing deriving clauses with finer grain

 Summary:
 Previously, one could experience error cascades with deriving clauses
 when one class in a set of many failed to derive, causing the other
 derived
 classes to be skipped entirely and resulting in other errors down the
 line.
 The solution is to process each class in a data type's set of deriving
 clauses
 individually, and engineer it so that failure to derive an individual
 class
 within that set doesn't cancel out the others.

 Test Plan: make test TEST="T10684 T12801"

 Reviewers: austin, bgamari, simonpj

 Reviewed By: simonpj

 Subscribers: simonpj, rwbarton, thomie

 GHC Trac Issues: #10684, #12801

 Differential Revision: https://phabricator.haskell.org/D3771
 }}}

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12801#comment:3>
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