[GHC] #11427: superclasses aren't considered because context is no smaller than the instance head

[GHC]

#11427: superclasses aren't considered because context is no smaller than the
instance head
-------------------------------------+-------------------------------------
        Reporter:  phadej            |                Owner:
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.0.1-rc1
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  Incorrect         |  Unknown/Multiple
  warning at compile-time            |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
-------------------------------------+-------------------------------------

Comment (by simonpj):

 It doesn't "reduce the normal language".  It does reduce the scope of a
 tricky and ill-documented extension of Haskell, implemented by GHC, namely
 the ability to solve recursive instances.  And it's tied up with an
 implementation trick called "silent superclasses" which I used to help,
 but which ultimately turned out to have several horrid and unforseen
 consequences.

 In short, I do not know now to do better.  But maybe someone else will!

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11427#comment:14>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler