[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:  new
        Priority:  high              |            Milestone:  8.6.1
       Component:  Compiler (Type    |              Version:  8.5
  checker)                           |             Keywords:
      Resolution:                    |  QuantifiedConstraints
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  GHC accepts       |  Unknown/Multiple
  invalid program                    |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by simonpj):

 I meant:

 * Apply the instance-termination check to every quantified constraint
 `(cxt => constraint)` appearing anywhere in a type.

 So `f :: (Show a => Show a) => blah` would be rejected just as
 {{{
 instance Show a => Show a
 }}}
 is rejected

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15316#comment:5>
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