[GHC] #12791: Superclass methods could be more aggressively specialised.

[GHC]

#12791: Superclass methods could be more aggressively specialised.
-------------------------------------+-------------------------------------
        Reporter:  mpickering        |                Owner:  danharaj
            Type:  bug               |               Status:  closed
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.0.1
      Resolution:  fixed             |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:  #5835, #13943,    |  Differential Rev(s):  Phab:D2714
  #14434                             |
       Wiki Page:                    |
-------------------------------------+-------------------------------------

Comment (by Simon Peyton Jones <simonpj@…>):

 In [changeset:"30058b0e45e920319916be999de9c4d77da136e7/ghc"
 30058b0e/ghc]:
 {{{
 #!CommitTicketReference repository="ghc"
 revision="30058b0e45e920319916be999de9c4d77da136e7"
 Fix another dark corner in the shortcut solver

 The shortcut solver for type classes (Trac #12791) was eagerly
 solving a constaint from an OVERLAPPABLE instance. It happened
 to be the only one in scope, so it was unique, but since it's
 specfically flagged as overlappable it's really a bad idea to
 solve using it, rather than using the Given dictionary.

 This led to Trac #14434, a nasty and hard to identify bug.
 }}}

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12791#comment:34>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler