+ PostTcType
+ -- You might think that the PostTcType was redundant, but it's essential
+ -- data T a where
+ -- T1 :: Int -> T Int
+ -- f :: (T a, a) -> Int
+ -- f (T1 x, z) = z
+ -- When desugaring, we must generate
+ -- f = /\a. \v::a. case v of (t::T a, w::a) ->
+ -- case t of (T1 (x::Int)) ->
+ -- Note the (w::a), NOT (w::Int), because we have not yet
+ -- refined 'a' to Int. So we must know that the second component
+ -- of the tuple is of type 'a' not Int. See selectMatchVar