[GHC] #11062: Type families + hs-boot files = panic

[GHC]

#11062: Type families + hs-boot files = panic
-------------------------------------+-------------------------------------
        Reporter:  goldfire          |                Owner:
            Type:  bug               |               Status:  patch
        Priority:  high              |            Milestone:  8.2.1
       Component:  Compiler          |              Version:  7.11
      Resolution:                    |             Keywords:  TypeFamilies
                                     |  hs-boot
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):  Phab:D2215
       Wiki Page:                    |
-------------------------------------+-------------------------------------

Comment (by ezyang):

 I attempted to do (2) but actually it is not that easy. The difficulty
 stems from the fact that some of the functions that query FamInstEnv,
 e.g., `normaliseType :: FamInstEnvs -> Role -> Type -> (Coercion, Type)`
 never fail; that is, they assume that there are no conflicting instances.
 So it would seem all of these functions would need to have their
 signatures changed that they might fail due to a conflicting instance.
 Might be worth it, but certainly not a non-invasive change.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11062#comment:11>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler