Won't use (forall xx. f xx) with -XQuantifiedConstraints

{-# Language QuantifiedConstraints #-}
{-# Language GADTs #-}
{-# Language ConstraintKinds #-}

data D c where
  D :: c => D c

proof :: (forall xx. f xx) => D (f a)
proof = D

Running this program with wip/T2893 gives

GHCi, version 8.5.20180128: http://www.haskell.org/ghc/  :? for help
[1 of 1] Compiling Main             ( 174-quantifiedconstraints.hs, interpreted )

174-quantifiedconstraints.hs:9:9: error:
    • Could not deduce: f a arising from a use of ‘D’
      from the context: forall xx. f xx
        bound by the type signature for:
                   proof :: forall (f :: * -> Constraint) a.
                            (forall xx. f xx) =>
                            D (f a)
        at 174-quantifiedconstraints.hs:8:1-37
    • In the expression: D
      In an equation for ‘proof’: proof = D
    • Relevant bindings include
        proof :: D (f a) (bound at 174-quantifiedconstraints.hs:9:1)
  |
9 | proof = D
  |         ^

How can I instantiate xx to a?

Trac metadata

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