[GHC] #13758: Deriving can't find an instance that holds, standalone deriving works

[GHC]

#13758: Deriving can't find an instance that holds, standalone deriving works
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        Reporter:  Iceland_jack      |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.0.1
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:  #2721, #8165      |  Differential Rev(s):
       Wiki Page:                    |
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Changes (by RyanGlScott):

 * related:   => #2721, #8165


Comment:

 Hah! You tickled a bug that I originally ran into when I was working on
 the fix for #2721/#8165 (making `GeneralizedNewtypeDeriving` work for
 classes with associated type families). The issue was that we weren't
 updating the type family instance environment early enough in type
 checking, which resulted in the strange staging error you experienced. I
 had thought that the only way to trigger that bug was with classes with
 associated type families, but you have just proven me wrong!

 The fix made it into GHC 8.2.1, and this program does indeed typecheck
 with that version. However, I'll keep this ticket open for now, as this
 program makes for a good GHC regression test.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13758#comment:3>
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