[GHC] #12677: Type equality in constraint not used?

[GHC]

#12677: Type equality in constraint not used?
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        Reporter:  Iceland_jack      |                Owner:  (none)
            Type:  feature request   |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.0.1
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:  #13337, #15710    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by simonpj):

 > The problem is that this would require using the equality proof for Type
 ~ k as the body of η

 Doesn't coercion quantification help with this?  (Though as so often, I'm
 worried about the difference between `(Type ~ k)` and `(Type ~# k)`.)

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12677#comment:5>
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