[GHC] #14832: QuantifiedConstraints: Adding to the context causes failure

[GHC]

#14832: QuantifiedConstraints: Adding to the context causes failure
-------------------------------------+-------------------------------------
        Reporter:  Iceland_jack      |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.5
      Resolution:                    |             Keywords:
                                     |  QuantifiedConstraints wipT2893
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 simonpj):

 > The original program still gives a stack overflow with ​latest changes.

 And so it should.  You have a "given" `(Trans rel)`, where
 {{
 type Trans rel = (forall xx yy zz. (rel xx yy, rel yy zz) => rel xx zz ::
 Constraint)
 }}}
 So if you want to solve `[W] rel t1 t2` (for some type `t1` or `t2`, this
 local instance (quantified constraint) matches.  We can solve it by
 solving
 {{{
    [W] rel t1 alpha
    [W] rel alpha t2
 }}}
 where `alpha` is a fresh unification variable (corresponding to `yy` in
 the definition of `Trans rel`.

 But then each of those two constraints can be solved once more with this
 over-general local instance -- and now we hare four constraints. And so
 on.  You used `UndecidableInstances` and sure enough you wrote an infinite
 loop.

 I say there is no bug here.  The same thing wold happen if you wrote
 {{{
 instance (C a x, C x b) => C a b
 }}}

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14832#comment:10>
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