[GHC] #15593: QuantifiedConstraints: trouble with type family

[GHC]

#15593: QuantifiedConstraints: trouble with type family
-------------------------------------+-------------------------------------
        Reporter:  Iceland_jack      |                Owner:  (none)
            Type:  bug               |               Status:  closed
        Priority:  normal            |            Milestone:  8.6.1
       Component:  Compiler          |              Version:  8.5
      Resolution:  duplicate         |             Keywords:
                                     |  QuantifiedConstraints,
                                     |  ConstraintKinds
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:  #14860            |  Differential Rev(s):
       Wiki Page:                    |
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Changes (by RyanGlScott):

 * status:  new => closed
 * resolution:   => duplicate
 * related:   => #14860


Comment:

 Update: GHC HEAD now //does// allow quantified constraints to be headed by
 `(~)`. Unfortunately, the original example still won't compile due to the
 fact that the quantified constraint's head mentions a type family:

 {{{
 Bug.hs:11:1: error:
     • Illegal type synonym family application ‘DeRef
                                                  (tree xx)’ in instance:
         DeRef (tree xx) ~ TreeF xx
     • In the quantified constraint ‘forall xx.
                                     DeRef (tree xx) ~ TreeF xx’
       In the context: forall xx. DeRef (tree xx) ~ TreeF xx
       While checking the super-classes of class ‘MuRef1’
       In the class declaration for ‘MuRef1’
    |
 11 | class    (forall xx. DeRef (tree xx) ~ TreeF xx) => MuRef1 tree
    | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

 Bug.hs:12:10: error:
     • Illegal type synonym family application ‘DeRef
                                                  (tree xx)’ in instance:
         DeRef (tree xx) ~ TreeF xx
     • In the quantified constraint ‘forall xx.
                                     DeRef (tree xx) ~ TreeF xx’
       In the instance declaration for ‘MuRef1 tree’
    |
 12 | instance (forall xx. DeRef (tree xx) ~ TreeF xx) => MuRef1 tree
    |          ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 }}}

 This is the subject of # , so I'm going to close this ticket in favor of
 that one.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15593#comment:4>
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