[GHC] #14860: QuantifiedConstraints: Can't quantify constraint involving type family

[GHC]

#14860: QuantifiedConstraints: Can't quantify constraint involving type family
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        Reporter:  RyanGlScott       |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler (Type    |              Version:  8.5
  checker)                           |             Keywords:
      Resolution:                    |  QuantifiedConstraints wipT2893
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 RyanGlScott):

 Indeed, GHC appears to have no trouble with this:

 {{{#!hs
 {-# LANGUAGE ConstraintKinds #-}
 {-# LANGUAGE QuantifiedConstraints #-}
 {-# LANGUAGE RankNTypes #-}
 {-# LANGUAGE UndecidableInstances #-}
 module Bug where

 import Control.Monad

 newtype Free cls a = Free (forall xx. cls xx => (a -> xx) -> xx)

 instance Functor (Free cls)
 instance Applicative (Free cls)

 instance (forall xx. cls (Free cls xx)) => Monad (Free cls) where
   Free f >>= g = f g

 f :: Free Show (Free Show a) -> Free Show a
 f = join
 }}}

 {{{
 Bug.hs:18:5: error:
     • No instance for (Show (Free Show xx))
         arising from a use of ‘join’
     • In the expression: join
       In an equation for ‘f’: f = join
    |
 18 | f = join
    |     ^^^^
 }}}

 Here, we reject the use site, not the instance.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14860#comment:8>
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