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

[GHC]

#15593: QuantifiedConstraints: trouble with type family
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        Reporter:  Iceland_jack      |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:  8.6.1
       Component:  Compiler          |              Version:  8.5
      Resolution:                    |             Keywords:
                                     |  QuantifiedConstraints,
                                     |  ConstraintKinds
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 RyanGlScott):

 Unfortunately, this is going to work, since you can't quantify over
 constraints that are headed by `(~)`. See #15359.

 GHC HEAD at least gives you a slightly more informative error message
 about this:

 {{{
 $ /opt/ghc/head/bin/ghci Bug.hs
 GHCi, version 8.7.20180827: http://www.haskell.org/ghc/  :? for help
 Loaded GHCi configuration from /home/rgscott/.ghci
 [1 of 1] Compiling Main             ( Bug.hs, interpreted )

 Bug.hs:11:1: error:
     • Class ‘~’ does not support user-specified instances
     • 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:
     • Class ‘~’ does not support user-specified instances
     • 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
    |          ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 }}}

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15593#comment:1>
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