[GHC] #12421: TestEquality and TestCoercion documentation is confusing

[GHC]

#12421: TestEquality and TestCoercion documentation is confusing
-------------------------------------+-------------------------------------
        Reporter:  dfeuer            |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Core Libraries    |              Version:  8.0.1
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  Documentation     |  Unknown/Multiple
  bug                                |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
-------------------------------------+-------------------------------------

Comment (by Zemyla):

 An instance of `TestCoercion` doesn't need to be a GADT, though. `MutVar`
 and `MVar`, for instance, have fairly sensible semantics for a
 hypothetical `TestCoercion` instance: If two mutable variables point at
 the same spot, then their runtime representation must be the same. The
 same applies to `StableName`: If two `StableName`s match, then they were
 created using the same object.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12421#comment:3>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler