content: {-# OPTIONS_GHC -fno-warn-redundant-constraints #-}
  {-# LANGUAGE RankNTypes, ConstraintKinds, KindSignatures, DataKinds, TypeFamilies #-}
  module T9569 where
  
  import GHC.Exts
  
  data Proxy (c :: Constraint)
  
  class Deferrable (c :: Constraint) where
    defer :: Proxy c -> (c => a) -> a
  
  deferPair :: (Deferrable c1, Deferrable c2)
            => Proxy (c1,c2) -> (((c1,c2) :: Constraint) => a) -> a
               -- NB: ((c1,c2) :: Constraint) => blah
               -- is different form
               --     (c1,c2) => blah
               -- The former has dict, the latter has two
  deferPair _ _ = undefined
  
  instance (Deferrable c1, Deferrable c2) => Deferrable (c1,c2) where
      -- defer p f = deferPair p f     -- Succeeds
      defer = deferPair                -- Fails
  
  {- Notes Apr 2020.
  ~~~~~~~~~~~~~~~~~
  Note the careful type for deferPair!  You can also say
  
  deferPair :: (Deferrable c1, Deferrable c2, d ~ (c1,c2))
            => Proxy (c1,c2) -> (d => a) -> a
  
  but NOT
  
  deferPair :: (Deferrable c1, Deferrable c2)
            => Proxy (c1,c2) -> ((c1,c2) => a) -> a
  
  The point is that
    (c1,c2) => a
  is short for
    c1 => c2 => a
  -}
  
  {-
    [G] Deferrable c1, Deferrable c2
  
    [W] Proxy (c1,c2)   -> ((c1,c2) => a) -> a
          ~
        Proxy (c1x,c2x) -> ((c1x,c2x) => ax) -> ax
    [w] Deferrable c1x
    [w] Deferrable c2x
  -}
  