[GHC] #9918: GHC chooses an instance between two overlapping, but cannot resolve a clause within the similar closed type family

[GHC]

#9918: GHC chooses an instance between two overlapping, but cannot resolve a
clause within the similar closed type family
-------------------------------------+-------------------------------------
        Reporter:  qnikst            |                   Owner:
            Type:  bug               |                  Status:  new
        Priority:  normal            |               Milestone:
       Component:  Compiler          |                 Version:  7.8.3
      Resolution:                    |                Keywords:
Operating System:  Unknown/Multiple  |            Architecture:
 Type of failure:  None/Unknown      |  Unknown/Multiple
      Blocked By:                    |               Test Case:
 Related Tickets:                    |                Blocking:
                                     |  Differential Revisions:
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Comment (by simonpj):

 GHC has always permitted types that are uninhabited (except by bottom);
 consider
 {{{
 data Void  -- No constructors
 }}}
 So I don't see why `(undefined :: TT a)` should be a problem.  I certainly
 don't see a problem with `test4` or `test5`.  Your instance of `C [a]`
 does not require `C a`, and so of course we can compute the length of a
 list `[TT a]` without knowing anything about the type `TT a`.  That's just
 parametricity.

 Simon

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9918#comment:20>
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