[GHC] #12175: Instance resolution regression

[GHC]

#12175: Instance resolution regression
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        Reporter:  crockeea          |                Owner:
            Type:  bug               |               Status:  new
        Priority:  normal            |            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:"ce97b7298d54bdfccd9dcf366a69c5617b4eb43f/ghc" ce97b72/ghc]:
 {{{
 #!CommitTicketReference repository="ghc"
 revision="ce97b7298d54bdfccd9dcf366a69c5617b4eb43f"
 Expand given superclasses more eagerly

 This patch fixes Trac #12175, another delicate corner case of
 Note [Instance and Given overlap] in TcInteract.

 In #12175 we were not expanding given superclasses eagerly
 enough. It was easy to fix, and is actually rather neater than
 before.

 See Note [Eagerly expand given superclasses] in TcCanonical.
 The main change is to move the eager expansion of given superclasses
 to canClassNC.
 }}}

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12175#comment:1>
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