type-expression crashes on function applications involving local types

[ncik-roberts]

Here is a test demonstrating the crash: https://github.com/ncik-roberts/merlin/tree/demonstrate-type-expression-bug

I'll reproduce an example in this issue as well. If you have a program like this:

(* In test.ml *)
let go (type a) () =
  let f x : a -> int = assert false in
  let g y = f y in
  ()

and then ask for the type of the f y application expression:

$ merlin single type-expression -expression "f y" -position 4:13 -filename test.ml < test.ml

you get an assertion failure:

"File \"src/ocaml/typing/typecore.ml\", line 3144, characters 31-37: Assertion failed",

If you replace let f x : a -> int = ... with let f x : int -> int = ..., merlin correctly reports int as the type. That is, this issue appears to be related to local types. You can also reproduce the issue by making a a local type introduced by an existential unpack or by locally declaring a new type with let open struct type a = A end in ....