[GHC] #15639: Surprising failure combining QuantifiedConstraints with Coercible

[GHC]

#15639: Surprising failure combining QuantifiedConstraints with Coercible
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        Reporter:  dfeuer            |                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 malo):

 I'm having a similar issue using GHC 8.6.2. Posting as a comment on this
 ticket since I expect it's related. Here's a minimal example:

 {{{
 -- test.hs
 {-# LANGUAGE RankNTypes, QuantifiedConstraints #-}

 import Data.Coerce

 data Foo i = Foo i
 data Bar f i = Bar (f i)

 -- When I ask that we can lift coercions through the functor...
 class (forall i j. Coercible i j => Coercible (f i) (f j)) => MyFunctor f
 where
   -- ...

 -- ... this works fine.
 instance MyFunctor Foo

 -- When I try to ask that we can lift coercions through a higher
 functor...
 class (forall f g. (forall i. Coercible (f i) (g i)) => (forall i.
 Coercible (x f i) (x g i))) => MyHigherFunctor x where
  -- ...

 -- ...this fails
 instance MyHigherFunctor Bar

 -- Output from ghci:
 --
 -- Prelude> :l test.hs
 -- [1 of 1] Compiling Main             ( test.hs, interpreted )
 --
 -- test.hs:21:10: error:
 --     • Couldn't match representation of type ‘f’ with that of ‘g’
 --         arising from the superclasses of an instance declaration
 --       ‘f’ is a rigid type variable bound by
 --         a quantified context
 --         at test.hs:1:1
 --       ‘g’ is a rigid type variable bound by
 --         a quantified context
 --         at test.hs:1:1
 --     • In the instance declaration for ‘MyHigherFunctor Bar’
 --    |
 -- 21 | instance MyHigherFunctor Bar
 --    |          ^^^^^^^^^^^^^^^^^^^
 -- Failed, no modules loaded.

 }}}

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15639#comment:6>
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