:set -XHaskell2010
:set -XRankNTypes -XConstraintKinds -XTypeApplications -XTypeOperators

import Data.Typeable

class Deferrable p where deferEither :: proxy p -> (p => r) -> Either String r

instance (Typeable a, Typeable b) => Deferrable (a ~ b) where deferEither _ _ = undefined

:t deferEither @(_ ~ _)