[GHC] #13490: Ambiguous types with constraints on new variables

[GHC]

#13490: Ambiguous types with constraints on new variables
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        Reporter:  crockeea          |                Owner:  (none)
            Type:  bug               |               Status:  closed
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.0.2
      Resolution:  worksforme        |             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 crockeea):

 Simon, your explanation seems similar to #11948. Are they related?

 I'm still wondering about this ticket. Why am I getting an ambiguous type
 error, and since the definition is `undefined`, why does GHC have to do
 any work at all?

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13490#comment:4>
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