[GHC] #13474: GHC HEAD regression: Prelude.!!: index too large

[GHC]

#13474: GHC HEAD regression: Prelude.!!: index too large
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        Reporter:  RyanGlScott       |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  highest           |            Milestone:  8.2.1
       Component:  Compiler          |              Version:  8.1
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  GHC rejects       |  Unknown/Multiple
  valid program                      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Changes (by RyanGlScott):

 * cc: bgamari (added)


Comment:

 The commit that caused this regression is somewhat surprising. It's
 d2f48495ebe79b5ef5808a4891b3d03dfd297d35:

 {{{
 From d2f48495ebe79b5ef5808a4891b3d03dfd297d35 Mon Sep 17 00:00:00 2001
 From: Ben Gamari <bgamari.foss at gmail.com>
 Date: Sun, 26 Feb 2017 15:33:54 -0500
 Subject: [PATCH] Coercion: Try dropping constraintIsLifted axiom

 While working through the FunCo patch I encountered some lint issues
 which suggested that `Constraint` wasn't being considered equal to `TYPE
 'LiftedRep`.  Consequently I introduced this axiom and associated messy
 ball of logic to explicitly coerce `Constraint`.

 However, as @goldfire points out on D3208 this really shouldn't be
 necessary.  Indeed, I tried ripping out the axiom and things appear to
 just work. I suspect the issue motivating the axiom was a bug elsewhere
 in the FunCo branch that I fixed during development.
 }}}

 Thoughts, Ben?

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13474#comment:3>
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