[GHC] #15683: coerce fails for Coercible type families

[GHC]

#15683: coerce fails for Coercible type families
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        Reporter:  Iceland_jack      |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:  8.6.1
       Component:  Compiler          |              Version:  8.6.1
      Resolution:                    |             Keywords:  TypeFamilies
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:  #8177             |  Differential Rev(s):
       Wiki Page:                    |
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Changes (by RyanGlScott):

 * related:   => #8177


Comment:

 I think what you likely want is the ability to declare type families that
 aren't nominal in their arguments. Something like:

 {{{#!hs
 type role X representational
 type family X a
 }}}

 To be clear, GHC doesn't currently grant the ability to do this, but if it
 hypothetically did, then this would solve your problem. This is the scope
 of #8177. I'd go as far to say that this ticket is a duplicate of that
 one—do you agree?

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15683#comment:4>
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