[GHC] #12583: Deriving standalone Ix instance for GADT leads to GHC panic

[GHC]

#12583: Deriving standalone Ix instance for GADT leads to GHC panic
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        Reporter:  RyanGlScott       |                Owner:
            Type:  bug               |               Status:  patch
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.0.1
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  Compile-time      |  Unknown/Multiple
  crash                              |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):  Phab:D2521
       Wiki Page:                    |
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Comment (by Ryan Scott <ryan.gl.scott@…>):

 In [changeset:"7b7ea8f40e7400b8c183595a85bb2c65c9f9bb29/ghc" 7b7ea8f/ghc]:
 {{{
 #!CommitTicketReference repository="ghc"
 revision="7b7ea8f40e7400b8c183595a85bb2c65c9f9bb29"
 Fix derived Ix instances for one-constructor GADTs

 Summary:
 Standalone-derived `Ix` instances would panic on GADTs with exactly
 one constructor, since the list of fields was being passed to a function
 that
 uses `foldl1` in order to generate an implementation for `inRange`. This
 adds a
 simple check that makes `inRange` be `True` whenever a product type has no
 fields.

 Fixes #12583.

 Test Plan: make test TEST=12583

 Reviewers: simonpj, austin, bgamari

 Reviewed By: bgamari

 Subscribers: thomie

 Differential Revision: https://phabricator.haskell.org/D2521

 GHC Trac Issues: #12583
 }}}

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12583#comment:2>
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