[GHC] #8177: Roles for type families

[GHC]

#8177: Roles for type families
-------------------------------------+-------------------------------------
              Reporter:  simonpj     |            Owner:  goldfire
                  Type:  bug         |           Status:  new
              Priority:  normal      |        Milestone:
             Component:  Compiler    |          Version:  7.6.3
            Resolution:              |         Keywords:
      Operating System:              |     Architecture:  Unknown/Multiple
  Unknown/Multiple                   |       Difficulty:  Unknown
       Type of failure:              |       Blocked By:
  None/Unknown                       |  Related Tickets:
             Test Case:              |
              Blocking:              |
Differential Revisions:              |
-------------------------------------+-------------------------------------

Comment (by dmcclean):

 Yes. I was badly confused when I wrote that comment, sorry.

 See 9580 for where I broke this out as requested by goldfire. It turns out
 that the rule is indeed supposed to be that you can only do this coercion
 when `Quantity'` is in scope, which makes sense because that is when you
 could do it longhand, but that 7.8.3 currently implements a more lenient
 unintentional rule that also lets you do it (at least sometimes?) when you
 only have the data family in scope.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8177#comment:25>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler