QuantifiedConstraints: Can't quantify constraint involving type family

This program fails to typecheck using the wip/T2893 branch, to my surprise:

{-# LANGUAGE AllowAmbiguousTypes #-}
{-# LANGUAGE GADTs #-}
{-# LANGUAGE QuantifiedConstraints #-}
{-# LANGUAGE TypeFamilies #-}
{-# LANGUAGE UndecidableInstances #-}
module Bug where

import Data.Proxy

type family   Apply (f :: * -> *) a
type instance Apply Proxy a = Proxy a

data ExTyFam f where
  MkExTyFam :: Apply f a -> ExTyFam f

instance (forall a. Show (Apply f a)) => Show (ExTyFam f) where
  show (MkExTyFam x) = show x
  showsPrec = undefined -- Not defining these leads to the oddities observed in
  showList = undefined  -- https://ghc.haskell.org/trac/ghc/ticket/5927#comment:32

test :: ExTyFam Proxy -> String
test = show

This fails with:

$ ghc-cq/inplace/bin/ghc-stage2 --interactive Bug.hs
GHCi, version 8.5.20180128: http://www.haskell.org/ghc/  :? for help
Loaded GHCi configuration from /home/rgscott/.ghci
[1 of 1] Compiling Bug              ( Bug.hs, interpreted )

Bug.hs:17:24: error:
    • Could not deduce (Show (Apply f a)) arising from a use of ‘show’
      from the context: forall a. Show (Apply f a)
        bound by the instance declaration at Bug.hs:16:10-57
    • In the expression: show x
      In an equation for ‘show’: show (MkExTyFam x) = show x
      In the instance declaration for ‘Show (ExTyFam f)’
   |
17 |   show (MkExTyFam x) = show x
   |                        ^^^^^^

I would have expected the (forall a. Show (Apply f a)) quantified constraint to kick in, but it didn't.

Trac metadata

Trac field	Value
Version	8.5
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler (Type checker)
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