[GHC] #15316: Regarding coherence and implication loops in presence of QuantifiedConstraints

[GHC]

#15316: Regarding coherence and implication loops in presence of
QuantifiedConstraints
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        Reporter:  mniip             |                Owner:  (none)
            Type:  bug               |               Status:  merge
        Priority:  high              |            Milestone:  8.6.1
       Component:  Compiler (Type    |              Version:  8.5
  checker)                           |             Keywords:
      Resolution:                    |  QuantifiedConstraints
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  GHC accepts       |            Test Case:  quantified-
  invalid program                    |  constraints/T15316
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by simonpj):

 > Doesn't UndecidableInstances imply the typechecking might not terminate,
 as opposed to a finite typechecking process resulting in a program
 nonterminating due to poor case of context elaboration?

 Usually yes, but in the cases above it can mean the latter too.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15316#comment:11>
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