The following example leads to an assertion from printtyp.ml when loaded in
utop.  I made the example as small as I could, but unfortunately it still
depends on a package called Future which in turn depends on Core.

To run it you'll have to do:

$ opam pin add future https://github.com/agarwal/future
Then hit Y to install the package.

$ cat tmp.ml
open Future_std
let f items =
  Pipe.peek_deferred items >>= function
  | `Comment _ -> return ()


#require "future.std";;

# #use "tmp.ml";;
Fatal error: exception "Assert_failure typing/printtyp.ml:583:12"
Raised at file "bytes.ml", line 220, characters 25-34


Doing the equivalent in the standard ocaml toploop or compiling with
ocamlc/ocamlopt doesn't exhibit this assertion. You correctly get the real
error reported:

$ ocamlfind ocamlc -package future.std -thread tmp.ml
File "tmp.ml", line 5, characters 4-14:
Error: This pattern matches values of type [? `Comment of 'a ]
       but a pattern was expected which matches values of type
         ('b Future_std.Deferred.t as 'b) Future_std.Deferred.t
       The second variant type does not allow tag(s) `Comment


The assertion is also avoided if you replace `open Future_std` with a
copy/paste of the relevant part of Future_std.