Hover shows ?_ for arguments of function declaration

[benrbray]

Consider the following code:

eval : (a -> b) -> a -> b
eval f x = f x
--   1 2   3 4

-- (hover @ 1)  f : ?_ -> ?_
-- (hover @ 2)  x : ?_ [no locals in scope]
-- (hover @ 3)  f : a -> b
-- (hover @ 4)  x : a

The ?_ holes are not too helpful, rather I would expect hovering 1 to show the same result as 3 and hovering 2 to show the same result as 4.