[GHC] #10592: Allow cycles in class declarations

[GHC]

#10592: Allow cycles in class declarations
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        Reporter:  MikeIzbicki       |                Owner:
            Type:  feature request   |               Status:  infoneeded
        Priority:  normal            |            Milestone:
       Component:  Compiler (Type    |              Version:  7.10.1
  checker)                           |
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by Simon Peyton Jones <simonpj@…>):

 In [changeset:"ff752a1a5eb69955c0e4fda8647f495409a2c384/ghc"
 ff752a1a/ghc]:
 {{{
 #!CommitTicketReference repository="ghc"
 revision="ff752a1a5eb69955c0e4fda8647f495409a2c384"
 tcCheckSatisfiability: less aggressive superclass expansion

 In the pattern-match check we are looking for a proof of
 *unsatisfiablity* among a bunch of givens.  The unsat-ness might be
 hidden in the superclasses, so we must expand them.  But in the common
 case where the constraints are satisfiable, we don't want to expand
 a recursive superclass forever.

 This is all a bit arbitrary, but then the whole question is
 undecidable anyway.

 The bug in Trac #10592 comment:12 was that I expanded superclasses
 forever.  This patch fixes it.
 }}}

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10592#comment:15>
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