[GHC] #15245: Data family promotion is possible

[GHC]

#15245: Data family promotion is possible
-------------------------------------+-------------------------------------
        Reporter:  int-index         |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:  8.6.1
       Component:  Documentation     |              Version:  8.4.3
      Resolution:                    |             Keywords:  TypeInType
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
-------------------------------------+-------------------------------------

Comment (by goldfire):

 According to
 [https://ghc.haskell.org/trac/ghc/wiki/DependentHaskell/Internal#User-
 facingdesignissues these mutterings], we need to abstract over a coercion
 to promote data family instances properly. I haven't paged it all back in,
 but this is an implementation error, not a documentation one.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15245#comment:1>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler