[GHC] #12768: 8.0.2 derives invalid code when class method is constrained by itself

[GHC]

#12768: 8.0.2 derives invalid code when class method is constrained by itself
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        Reporter:  jophish           |                Owner:
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:  8.0.2
       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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Comment (by Ryan Scott <ryan.gl.scott@…>):

 In [changeset:"ead83db8a7db772a9f248af9767a4283218a5c9f/ghc" ead83db/ghc]:
 {{{
 #!CommitTicketReference repository="ghc"
 revision="ead83db8a7db772a9f248af9767a4283218a5c9f"
 Describe symptoms of (and the cure for) #12768 in 8.0.2 release notes

 GHC 8.0.2 introduced a bugfix involving GeneralizedNewtypeDeriving in
 96d451450923a80b043b5314c5eaaa9d0eab7c56. This made typechecking of
 GND-produced code a bit stricter, and an unfortunate side effect of this
 was
 that there were a couple of corner-case programs that stopped compiling
 when transitioning from GHC 8.0.1 to 8.0.2.

 Since the number of affected programs seems quite small, and since the fix
 is so straightforward, we opt to simply note this discrepancy in the 8.0.2
 release notes.

 Resolves #12768.
 }}}

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12768#comment:9>
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