[GHC] #14273: Typed holes' "valid substitutions" suggestions are oblivious to type class constraints

[GHC]

#14273: Typed holes' "valid substitutions" suggestions are oblivious to type class
constraints
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        Reporter:  RyanGlScott       |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler (Type    |              Version:  8.3
  checker)                           |
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  Poor/confusing    |  Unknown/Multiple
  error message                      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:  #9091             |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by Tritlo):

 Ah, I see where the problem lies. It wraps the hole with the constraints
 from the implications from the context, but in this case, it is the `show`
 function which imposes new constraints on the hole, but as you say, this
 is not reflected in the type of the hole (i.e. the subtype checker is
 checking whether `forall a. Num a => a -> a -> a` fits the hole of type
 `Show a => a0_a1m3[tau:2]`, where (as we can see), the mentioned `a` is
 not `a0_a1m3[tau:2]`, which is the ambiguous type variable in question.
 I'll get to work on fixing that.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14273#comment:7>
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