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

[GHC]

#14101: Type synonyms can make roles too conservative
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        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:                    |
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Comment (by dfeuer):

 My concern is that this might not always do the best job with closed type
 families. In particular, we can produce a bound on the role of an
 application of a closed type family even if we can't reduce it.

 {{{#!hs
 type family Foo a where
   Foo 'True = Maybe
   Foo 'False = Either Int

 newtype Bar a b = Bar (Foo a b)
 }}}

 We know that `Foo a` must be `Maybe`, `Either Int`, or stuck. I believe it
 should therefore be safe to give `b` a representational role. My guess is
 that the right thing is to give type signatures and closed type families
 role annotations with the number of components determined by the number of
 arrows in their kinds.
 }}}

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14101#comment:7>
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