[GHC] #11480: UndecidableSuperClasses causes the compiler to spin with UndecidableInstances

[GHC]

#11480: UndecidableSuperClasses causes the compiler to spin with
UndecidableInstances
-------------------------------------+-------------------------------------
        Reporter:  ekmett            |                Owner:
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler (Type    |              Version:  8.0.1-rc1
  checker)                           |             Keywords:  PolyKinds,
      Resolution:                    |  UndecidableSuperClasses
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  GHC rejects       |  Unknown/Multiple
  valid program                      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:  #10318            |  Differential Rev(s):
       Wiki Page:                    |
-------------------------------------+-------------------------------------

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

 In [changeset:"ff21795a0b9253e811a45626d5686e981ed07f82/ghc"
 ff21795a/ghc]:
 {{{
 #!CommitTicketReference repository="ghc"
 revision="ff21795a0b9253e811a45626d5686e981ed07f82"
 Special-case implicit params in superclass expansion

 This issue came up in Trac #11480, and is documented in
 Note [When superclasses help] in TcRnTypes.

 We were getting a spurious warning
   T11480.hs:1:1: warning:
      solveWanteds: too many iterations (limit = 4)

 The fix is easy.  A bit of refactoring along the way.

 The original bug report in Trac #11480 appears to work
 fine in HEAD and the 8.0 branch but I added a regression
 test in this commit as well.
 }}}

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11480#comment:8>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler