[GHC] #12734: Missed use of solved dictionaries leads to context stack overflow

[GHC]

#12734: Missed use of solved dictionaries leads to context stack overflow
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        Reporter:  danilo2           |                Owner:
            Type:  bug               |               Status:  new
        Priority:  high              |            Milestone:
       Component:  Compiler          |              Version:  8.0.1
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  GHC rejects       |  Unknown/Multiple
  valid program                      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by Simon Peyton Jones <simonpj@…>):

 In [changeset:"1c4a39d3a8d36803382792ff78b4709794358883/ghc"
 1c4a39d3/ghc]:
 {{{
 #!CommitTicketReference repository="ghc"
 revision="1c4a39d3a8d36803382792ff78b4709794358883"
 Prioritise class-level equality costraints

 This patch fixes Trac #12734 by prioritising the class-level
 variants of equality constraints, namely (a~b) and (a~~b).

 See comment:10 of Trac #12734 for a description of what
 went wrong, and Note [Prioritise class equalities] in TcSMonad.

 The fix is still not great, but it's a definite step forward, and
 cures the particular problem.

 Worth merging to 8.0.
 }}}

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12734#comment:11>
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