[GHC] #15359: Quantified constraints do not work with equality constraints

[GHC]

#15359: Quantified constraints do not work with equality constraints
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        Reporter:  goldfire          |                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 simonpj):

 > Redundant in what sense?

 Well, can you show me a quantified constraint with an equality in the head
 that is not useless?

 For example what about this (by analogy with `Coercible`)
 {{{
 f :: forall m.
      (forall a b. a ~ b => m a ~ m b)
    => blah
 }}}
 No, that redundant. If we have `[W] t1 a ~ t2 b` there is a built-in rule
 to decompose it to `[W] t1 ~ t2` and `[W] a ~ b`.  (There is no such rule
 in general for `Coercible` which is why we need to allow it.)   So that
 quantified constraint wasn't useful.

 Can you think of one that is?  I can't.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15359#comment:6>
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