[GHC] #15231: UndecidableInstances validity checking is wrong in the presence of QuantifiedConstraints

[GHC]

#15231: UndecidableInstances validity checking is wrong in the presence of
QuantifiedConstraints
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        Reporter:  RyanGlScott       |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:  8.6.1
       Component:  Compiler (Type    |              Version:  8.5
  checker)                           |             Keywords:
      Resolution:                    |  QuantifiedConstraints
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  GHC rejects       |  Unknown/Multiple
  valid program                      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by simonpj):

 GHC is following [https://github.com/Gertjan423/ghc-proposals/blob
 /quantified-constraints/proposals/0000-quantified-constraints.rst the
 proposal (Section 3.7)].  Specifically, in the quantified predicate `c =>
 Z a`, `c` does occur more often in the constraint to the left of the
 arrow, namely `c`, than in the head of the quantified constraint `Z a`.

 The error message is poor.  How about this?
 {{{
     • Variable ‘c’ occurs more often
         in the constraint ‘c’
         than in the instance head `Z a'
       (Use UndecidableInstances to permit this)
     • In the quantified constraint `c => Z a`
     • In the instance declaration for ‘Z (ECC c a)’
 }}}

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15231#comment:1>
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