[GHC] #14774: Recusive superclass epansion is over-keen

[GHC]

#14774: Recusive superclass epansion is over-keen
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        Reporter:  simonpj           |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.2.2
      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:"65069806ea3637882d584e785dcb9e650271e4b6/ghc"
 65069806/ghc]:
 {{{
 #!CommitTicketReference repository="ghc"
 revision="65069806ea3637882d584e785dcb9e650271e4b6"
 Fix solveOneFromTheOther for RecursiveSuperclasses

 This patch fixes the redundant superclass expansion
 in Trac #14774.

 The main change is to fix TcInterac.solveOneFromTheOther, so
 that it does not prefer a work-item with a binding if that binding
 transitively depends on the inert item we are comparing it with.

 Explained in Note [Replacement vs keeping] in TcInert, esp
 item (c) of the "Constraints coming from the same level" part.

 To make this work I refactored out a new function
 TcEvidence.findNeededEvVars, which was previously buried
 inside TcSimplify.neededEvVars.

 I added quite a few more comments and signposts about superclass
 expansion.
 }}}

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14774#comment:1>
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