[GHC] #14101: Type synonyms can make roles too conservative

[GHC]

#14101: Type synonyms can make roles too conservative
-------------------------------------+-------------------------------------
        Reporter:  dfeuer            |                Owner:  (none)
            Type:  bug               |               Status:  patch
        Priority:  normal            |            Milestone:  8.4.1
       Component:  Compiler (Type    |              Version:  8.2.1
  checker)                           |
      Resolution:                    |             Keywords:  roles
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  GHC rejects       |  Unknown/Multiple
  valid program                      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:  #8234             |  Differential Rev(s):  Phab:D3838
       Wiki Page:                    |
-------------------------------------+-------------------------------------

Comment (by goldfire):

 comment:7 and comment:8 seem out of scope for this ticket. This ticket is
 about type synonyms, not type families.

 comment:7 seems utterly out of reach. As explained in the
 [http://repository.brynmawr.edu/cgi/viewcontent.cgi?article=1010&context=compsci_pubs
 paper], any type application that's not a declared parameter of a type is
 considered nominal. I think we'd need to overhaul the roles design to get
 that.

 comment:8, though, seems approachable, in two steps. Step 1: use data
 families, not type families. Step 2: #8177, which will allow non-nominal
 roles on data families.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14101#comment:10>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler