[GHC] #8555: Simplify given `Coercible` constraints

[GHC]

#8555: Simplify given `Coercible` constraints
-------------------------------------+-------------------------------------
       Reporter:  nomeata            |             Owner:
           Type:  feature request    |            Status:  new
       Priority:  normal             |         Milestone:
      Component:  Compiler (Type     |           Version:  7.7
  checker)                           |  Operating System:  Unknown/Multiple
       Keywords:                     |   Type of failure:  GHC rejects
   Architecture:  Unknown/Multiple   |  valid program
     Difficulty:  Unknown            |         Test Case:
     Blocked By:                     |          Blocking:
Related Tickets:  #8503              |
-------------------------------------+-------------------------------------
 It would be feasible and possibly useful if

 {{{
 foo :: Coercible [a] [b] => a -> b
 foo = coerce
 }}}
 would work. This involve simplifying `CtGiven`s similar to how given
 (nominal) equalities are simplified.

 I’ll defer working on this, as it is not strictly required, it seems.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8555>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler