Cannot satisfy QuantifiedConstraints
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## Motivation
I cannot satisfy a quantified constraint, even if I can satisfy the constraint for every value of the type variable.
Example:
```haskell
{-# LANGUAGE QuantifiedConstraints, UndecidableInstances, RankNTypes,
KindSignatures, TypeApplications, ScopedTypeVariables, GADTs #-}
module QuantifiedConstraints where
import Data.Kind
class IsUnit (a :: Type) where
isUnit :: forall r. (a ~ () => r) -> r
class C (a :: Type)
instance C ()
-- requires quantified constraint I want to satisfy
f1 :: (forall a. IsUnit a => C a) => Int
f1 = 3
-- can satisfy C for any particular a (that has IsUnit)
f2 :: forall a r. IsUnit a => (C a => r) -> r
f2 cr = isUnit @a cr
f :: Int
f = f2 f1 -- GHC (correctly) rejects this, but no other option
```
## Proposal
Provide some way of satisfying quantified constraints in this manner.
Sorry I don't have a concrete proposal, nor is this strictly a defect in GHC, I just want to raise the issue.
issue