[GHC] #15039: Bizarre pretty-printing of inferred Coercible constraint in partial type signature

[GHC]

#15039: Bizarre pretty-printing of inferred Coercible constraint in partial type
signature
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        Reporter:  RyanGlScott       |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:  8.6.1
       Component:  Compiler (Type    |              Version:  8.4.1
  checker)                           |             Keywords:
      Resolution:                    |  PartialTypeSignatures, TypeInType
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  Poor/confusing    |  Unknown/Multiple
  error message                      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by goldfire):

 I think comment:6 is a red herring. Why is GHC attempting to report about
 `~R#` at all here? The type of the hole is really `Coercible a b`. I don't
 think this is a pretty-printing issue.

 Instead, the solver needs to be taught somewhere to transmute `a ~R# b`
 constraints to `Coercible a b` constraints. But where? Do we do this for
 nominal equality anywhere?

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15039#comment:7>
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