[GHC] #13972: GHC 8.2 error message around indexes for associated type instances is baffling

[GHC]

#13972: GHC 8.2 error message around indexes for associated type instances is
baffling
-------------------------------------+-------------------------------------
        Reporter:  RyanGlScott       |                Owner:  (none)
            Type:  bug               |               Status:  patch
        Priority:  normal            |            Milestone:
       Component:  Compiler (Type    |              Version:  8.2.1-rc2
  checker)                           |             Keywords:  TypeFamilies,
      Resolution:                    |  TypeInType
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  Poor/confusing    |  Unknown/Multiple
  error message                      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):  Phab:D3820
       Wiki Page:                    |
-------------------------------------+-------------------------------------
Changes (by RyanGlScott):

 * status:  new => patch
 * differential:   => Phab:D3820


Comment:

 I decided to go with approach (2) in Phab:D3820.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13972#comment:1>
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