[GHC] #12151: GHC 8 allows ambiguous type without AllowAmbiguousTypes

[GHC]

#12151: GHC 8 allows ambiguous type without AllowAmbiguousTypes
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        Reporter:  thomie            |                Owner:
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler (Type    |              Version:  8.0.1
  checker)                           |
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  Incorrect         |  Unknown/Multiple
  warning at compile-time            |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:  #7437             |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by Simon Peyton Jones <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.
 }}}

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12151#comment:1>
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