QuantifiedConstraints: Can't deduce "(a, b)" from "a" and "b"

{-# Language QuantifiedConstraints, RankNTypes, PolyKinds, ConstraintKinds, UndecidableInstances, GADTs, LiberalTypeSynonyms, TypeOperators, MultiParamTypeClasses, FlexibleInstances #-}

data Dict c where
  Dict :: c => Dict c

infixr |-
class    (a => b) => (a |- b)
instance (a => b) => (a |- b)

type a :- b = Dict (a |- b)

class    ((a, b) => c) => Curried a b c
instance ((a, b) => c) => Curried a b c

curryC :: forall a b c. ((a, b) |- c) :- (a |- b |- c)
curryC = Dict

This fails with

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

/tmp/Test.hs:16:10: error:
    • Could not deduce (a, b) arising from a use of ‘Dict’
      from the context: (a, b) |- c
        bound by a quantified context at /tmp/Test.hs:1:1
      or from: a bound by a quantified context at /tmp/Test.hs:1:1
      or from: b bound by a quantified context at /tmp/Test.hs:1:1
      Possible fix:
        add (a, b) to the context of
          the type signature for:
            curryC :: forall (a :: Constraint) (b :: Constraint) (c :: Constraint).
                      ((a, b) |- c) :- (a |- (b |- c))
    • In the expression: Dict
      In an equation for ‘curryC’: curryC = Dict
   |
16 | curryC = Dict
   |          ^^^^
Failed, no modules loaded.

but it works by changing the type of curryC:

curryC :: forall a b c. Curried a b c :- (a |- b |- c)

Trac metadata

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