Typing without -principal is broken in 4.11 and trunk #9759
Description
Noticed while adding support for 4.11 in merlin, reproducible with:
$ ./runtime/ocamlrun ./ocamlc -nostdlib -I stdlib -I utils -I parsing -I typing -principal -c ./typing/typedtree.ml
$ ./runtime/ocamlrun ./ocamlc -nostdlib -I stdlib -I utils -I parsing -I typing -c ./typing/typedtree.ml
File "./typing/typedtree.ml", lines 793-808, characters 17-22:
793 | .................match p.pat_desc with
794 | | Tpat_var (id, s) -> (* note the ``Not_found'' case *)
795 | {p with pat_desc =
796 | try Tpat_var (alpha_var env id, s) with
797 | | Not_found -> Tpat_any}
...
805 | | d ->
806 | let pat_desc =
807 | shallow_map_pattern_desc { f = fun p -> alpha_pat env p } d in
808 | {p with pat_desc}
Error: This expression has type k pattern_desc pattern_data
but an expression was expected of type 'a
This instance of k is ambiguous:
it would escape the scope of its equation
The code in question:
let rec alpha_pat
: type k . _ -> k general_pattern -> k general_pattern
= fun env p -> match p.pat_desc with
| Tpat_var (id, s) -> (* note the ``Not_found'' case *)
{p with pat_desc =
try Tpat_var (alpha_var env id, s) with
| Not_found -> Tpat_any}
| Tpat_alias (p1, id, s) ->
let new_p = alpha_pat env p1 in
begin try
{p with pat_desc = Tpat_alias (new_p, alpha_var env id, s)}
with
| Not_found -> new_p
end
| d ->
let pat_desc =
shallow_map_pattern_desc { f = fun p -> alpha_pat env p } d in
{p with pat_desc}Given the error message, it looks like the type annotation on the function is ignored.
This doesn't appear to be due to #1132 (I confirm that the bug also exists before that PR).
It could also be that this is just an issue with the printing of the error, that the expected type is in fact not ignored but that the level handling is incorrect in some place(s).
Note: I'm going on vacation tonight, so I won't be able to look at this before the end of next week. Anyone else should feel free to have a look.