To add info, I'm getting this error in the context of a larger program with
ocamlc, so it is not a utop specific issue. Unfortunately, I still can't
produce it in this smaller example outside of utop.

On Mon, Sep 15, 2014 at 11:20 AM, Ashish Agarwal <agarwal1975@gmail.com>
wrote:

> 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.
>
>