[GHC] #12551: Make type indices take local constraints into account in type instance declaration

[GHC]

#12551: Make type indices take local constraints into account in type instance
declaration
-------------------------------------+-------------------------------------
        Reporter:  Iceland_jack      |                Owner:
            Type:  feature request   |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.0.1
      Resolution:                    |             Keywords:  TypeFamilies
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):

 I vaguely remember that discussion. That's fine, I will add the
 [https://gist.github.com/Icelandjack/5afdaa32f41adf3204ef9025d9da2a70#workaround
 workaround].

 Could this be mentioned in the error message?

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12551#comment:4>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler