[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
-------------------------------------+-------------------------------------
        Reporter:  jophish           |                Owner:
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       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:                    |
-------------------------------------+-------------------------------------

Comment (by rwbarton):

 Can't you just delete the `C m =>` constraint on `foo`? Is this reduced
 from a real program?

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12768#comment:1>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler