[GHC] #15244: Ambiguity checks in QuantifiedConstraints

[GHC]

#15244: Ambiguity checks in QuantifiedConstraints
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        Reporter:  bitwiseshiftleft  |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:  8.6.1
       Component:  Compiler          |              Version:  8.5
      Resolution:                    |             Keywords:
                                     |  QuantifiedConstraints
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by bitwiseshiftleft):

 Thanks for the quick response Simon.

 Right, {{{bar}}} is only to help track down the cause.  It's the Blah case
 that's more likely, in particular when the Blah instance comes into scope
 from a pattern match.  Something like (and I didn't test this example):

 {{{
 data GenericBlah t where
     GB :: (Typeable a, Blah a) => a t -> GenericBlah t

 instance Eq (GenericBlah t) where
     (GB (a::ca t)) == (GB (b::cb t)) = case eqT @ca @cb of
         Nothing -> False
         Just Refl -> a==b
 }}}

 The constraint inside the {{{Eq}}} instance is equivalent to {{{baz}}}.

 I'm exploring QuantifiedConstraints, and might eventually use them at my
 job.  However, QuantifiedConstraints would only be used in a few places.
 This pattern would be used rare and the bug isn't difficult to work
 around, so it's not urgent.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15244#comment:3>
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