[GHC] #8477: Allow inferring ambiguous types

[GHC]

#8477: Allow inferring ambiguous types
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        Reporter:  aavogt            |            Owner:
            Type:  feature request   |           Status:  new
        Priority:  normal            |        Milestone:
       Component:  Compiler          |          Version:  7.6.3
      Resolution:                    |         Keywords:
Operating System:  Unknown/Multiple  |     Architecture:  Unknown/Multiple
 Type of failure:  None/Unknown      |       Difficulty:  Unknown
       Test Case:                    |       Blocked By:
        Blocking:                    |  Related Tickets:  8390
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Comment (by simonpj):

 PS: my second bullet was wrong. The ambiguity check for instance
 declarations doesn't subsume the coverage check.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8477#comment:4>
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