[GHC] #11523: Infinite Loop when mixing UndecidableSuperClasses and the class/instance constraint synonym trick.

[GHC]

#11523: Infinite Loop when mixing UndecidableSuperClasses and the class/instance
constraint synonym trick.
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        Reporter:  ekmett            |                Owner:
            Type:  bug               |               Status:  new
        Priority:  highest           |            Milestone:  8.0.1
       Component:  Compiler (Type    |              Version:  8.0.1-rc1
  checker)                           |             Keywords:
      Resolution:                    |  UndecidableSuperClasses
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  GHC rejects       |            Test Case:
  valid program                      |  polykinds/T11523
      Blocked By:                    |             Blocking:
 Related Tickets:  #11480            |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by Iceland_jack):

 Replying to [comment:17 simonpj]:
 > Replying to [comment:16 Iceland_jack]:
 > > Brought up on #ghc, are there technical reasons why `(,)` ''cannot''
 be partially applied with the same meaning as `(&)`?
 >
 > I'm lost. What is `(&)`?  How is this connected with this ticket?

 `(&) :: Constraint -> Constraint -> Constraint` is defined by

 {{{#!hs
 class    (p, q) => p & q
 instance (p, q) => p & q
 }}}

 at the beginning of this ticket. This would actually be solved by #11715
 (per your comment ticket:11715#comment:14)

 > * Allow partial applications like `(,) (Eq a) :: Constraint ->
 Constraint`

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11523#comment:18>
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