[GHC] #15489: TestCoercion should be a superclass of TestEquality

[GHC]

#15489: TestCoercion should be a superclass of TestEquality
-------------------------------------+-------------------------------------
           Reporter:  Zemyla         |             Owner:  (none)
               Type:  feature        |            Status:  new
  request                            |
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.4.3
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  None/Unknown
  Unknown/Multiple                   |
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
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 Given that you can implement

 {{{#!hs
 testCoercionFromEquality :: TestEquality f => f a -> f b -> Maybe
 (Coercion a b)
 testCoercionFromEquality fa fb = fmap repr $ testEquality fa fb
 }}}

 that shows the superclass relationship.

 Are there enough libraries that implement new instances of `TestEquality`
 that adding this superclass would break?

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15489>
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