[GHC] #7437: peculiar behaviour with default instances and type variables

[GHC]

#7437: peculiar behaviour with default instances and type variables
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        Reporter:  bos               |                Owner:  simonpj
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Test Suite        |              Version:  7.6.1
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  GHC accepts       |  Unknown/Multiple
  invalid program                    |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:  #12151            |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by simonpj):

 In [changeset:"85aa6ef09346e841abf4e089b24c7f783286cd74/ghc"
 85aa6ef0/ghc]:
 {{{
 #!CommitTicketReference repository="ghc"
 revision="85aa6ef09346e841abf4e089b24c7f783286cd74"
 Check generic-default method for ambiguity

 Fixes Trac #7497 and #12151.   In some earlier upheaval I introduced
 a bug in the ambiguity check for genreric-default method.

 This patch fixes it.  But in fixing it I realised that the
 sourc-location of any such error message was bogus, so I fixed
 that too, which involved a slightly wider change; see the
 comments with TcMethInfo.
 }}}
 The commit message has a typo about the ticket.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/7437#comment:10>
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