[GHC] #10532: Unnecessary constraints when using overlapping instances

[GHC]

#10532: Unnecessary constraints when using overlapping instances
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        Reporter:  MikeIzbicki       |                   Owner:
            Type:  bug               |                  Status:  new
        Priority:  normal            |               Milestone:
       Component:  Compiler (Type    |                 Version:  7.10.1
  checker)                           |                Keywords:
      Resolution:                    |            Architecture:
Operating System:  Unknown/Multiple  |  Unknown/Multiple
 Type of failure:  None/Unknown      |               Test Case:
      Blocked By:                    |                Blocking:
 Related Tickets:                    |  Differential Revisions:
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Comment (by goldfire):

 This looks like expected behavior to me. In the overlapping instances
 case, GHC can't know which instance to choose when calling `foo` from `f`.
 If you want GHC to decide based on whether `a` is `Int` at a particular
 call site, then that's exactly what adding the constraint to `f`'s type
 signature means.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10532#comment:2>
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