type Foo :: forall k. k -> k
type family Foo a where
  forall k (a :: k). Foo a = a
  	-- Defined at T15341.hs:5:1
type Foo :: forall k. k -> k
type family Foo @k a where
  forall k (a :: k). Foo @k a = a
  	-- Defined at T15341.hs:5:1