[GHC] #16241: Avoid orphan instances with OVERLAPPABLE (sometimes)

[GHC]

#16241: Avoid orphan instances with OVERLAPPABLE (sometimes)
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        Reporter:  AntC              |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.6.1
      Resolution:                    |             Keywords:
Operating System:  Windows           |         Architecture:
 Type of failure:  Poor/confusing    |  Unknown/Multiple
  error message                      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:  #15135            |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by simonpj):

 > I think only if the instance is not marked OVERLAPPABLE

 I'm not sure that's right. Currently
 [http://downloads.haskell.org/~ghc/master/users-guide/glasgow_exts.html
 #overlapping-instances the user manual for instance resolution] says this
 {{{
 Eliminate any candidate IX for which there is another candidate IY such
 that both of the following hold:

 * IY is strictly more specific than IX. That is, IY is a substitution
   instance of IX but not vice versa.

 * Either IX is overlappable, or IY is overlapping. (This “either/or”
   design, rather than a “both/and” design, allow a client to
   deliberately override an instance from a library, without requiring
   a change to the library.)
 }}}
 The second bullet means that you don't need to say "OVERLAPPABLE" for an
 instance to be overlappable.  It's enough to say "OVERLAPPING" in the
 overlapping instance.  So you can't deduce anything much from the absence
 of "OVERLAPPABLE", sadly.

 I don't know if this is the right design.  If we you could only overlap an
 explicitly-overlappable instance, we could commit to them more
 aggressively.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/16241#comment:2>
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