[GHC] #14333: GHC doesn't use the fact that Coercible is symmetric

[GHC]

#14333: GHC doesn't use the fact that Coercible is symmetric
-------------------------------------+-------------------------------------
        Reporter:  Iceland_jack      |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.2.1
      Resolution:                    |             Keywords:  TypeFamilies,
                                     |  Roles
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 Iceland_jack):

 Replying to [comment:2 goldfire]:
 > Do you have a concrete use case?

 My use case is [https://github.com/Icelandjack/deriving-
 via/blob/master/examples/Generics.hs this] where `swap` is required to
 express symmetry. I use this to witness a coercion between `Rep a x` and
 `Rep b x`

 {{{#!hs
 sameRep :: SameRep a b :- (Rep a x `Coercible` Rep b x)
 }}}

 but I also need

 {{{#!hs
 SameRep a b :- (Rep b x `Coercible` Rep a x)
 }}}

 for which I use `swap`

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14333#comment:4>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler