[GHC] #8958: Allow role inference on datatype contexts

[GHC]

#8958: Allow role inference on datatype contexts
-----------------------------------------------+---------------------------
        Reporter:  goldfire                    |            Owner:
            Type:  feature request             |  goldfire
        Priority:  normal                      |           Status:  merge
       Component:  Compiler                    |        Milestone:  7.8.1
      Resolution:                              |          Version:
Operating System:  Unknown/Multiple            |  7.8.1-rc2
 Type of failure:  None/Unknown                |         Keywords:
       Test Case:  roles/should_compile/T8958  |     Architecture:
        Blocking:                              |  Unknown/Multiple
                                               |       Difficulty:  Unknown
                                               |       Blocked By:
                                               |  Related Tickets:
-----------------------------------------------+---------------------------
Changes (by goldfire):

 * status:  new => merge
 * testcase:   => roles/should_compile/T8958
 * milestone:   => 7.8.1


Comment:

 This commit also fixed a latent bug in that the datatype context was not
 fully zonked in tcDataDefn.

 Please merge if possible into 7.8.1 -- this tiny patch allows lightweight
 role annotations, all done in user space, as suggested by Mark Lentczner.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8958#comment:2>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler