[GHC] #15927: Weird interaction between fundeps and overlappable instances

[GHC]

#15927: Weird interaction between fundeps and overlappable instances
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        Reporter:  Darwin226         |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:  8.6.3
       Component:  Compiler          |              Version:  8.6.2
      Resolution:                    |             Keywords:
                                     |  FunctionalDependencies
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  GHC accepts       |  Unknown/Multiple
  invalid program                    |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:  10675, 15632      |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by Darwin226):

 I'm sorry if you addressed this in your comment and I'm just not getting
 it, but note that I'm not just declaring `f` with and unsatisfyable
 constraint. I'm also able to satisfy it! So writing

 {{{#!hs
 g :: StateT Int (StateT Char IO) ()
 g = f
 }}}

 works.

 So are you saying that this behavior can be relied on? Is my code likely
 to get broken in newer GHC versions?

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15927#comment:2>
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