[GHC] #15928: Reduction stack overflow when using "coerce"

[GHC]

#15928: Reduction stack overflow when using "coerce"
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        Reporter:  harendra          |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:  8.6.3
       Component:  Compiler          |              Version:  8.6.2
      Resolution:                    |             Keywords:
Operating System:  MacOS X           |         Architecture:
 Type of failure:  Compile-time      |  Unknown/Multiple
  crash or panic                     |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by harendra):

 Thanks for patiently and promptly explaining the problem. I appreciate
 that. Phantom role explains the inconsistency and everything falls in
 place. I did not realize it, to create an absolutely minimal example I
 removed the use of the parameter and it became phantom, it was not so in
 the original code. I like the `coerce` feature and I am perfectly ok with
 the theory as long as the behavior is documented, errors are easy to
 understand and the documentation is discoverable.

 So perhaps we can make the error message better, if you think it can be
 improved. Does it make sense to spit out the role information in the error
 message? We can also add one or more examples to explain the
 pitfalls/incompleteness in the user guide.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15928#comment:7>
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