[GHC] #12669: Add some weird Kmettian tests to the test suite

[GHC]

#12669: Add some weird Kmettian tests to the test suite
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           Reporter:  dfeuer         |             Owner:
               Type:  feature        |            Status:  new
  request                            |
           Priority:  normal         |         Milestone:  8.2.1
          Component:  Test Suite     |           Version:  8.0.1
           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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 `reflection` does some funny business essentially coercing between `->`
 and `=>`. `constraints` has some code that's very sensitive to superclass
 cycle detection. And there's some other package (`structures`?) that
 coerces between `*` and `#` to stick raw `MutArr#` pointers into arrays. I
 believe Kmett also uses `unsafeCoerce# reallyUnsafePtrEquality#` to
 compare pointers of kind `#` to others (of possibly different types).

 None of these things are anything like officially supported. However,
 breaking them could be expensive for the ecosystem. Dealing with breakage
 when someone discovers it months later can be very hard, so I think some
 of these things should probably be added to the GHC test suite. Failures
 shouldn't categorically prevent changes from being merged, but they would
 provide an early warning and get conversations started earlier.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12669>
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