ppxlib#trunk-support on 5.1.0~alpha1 breaks let f: type a. syntax

[copy]

Consider:

let f: type a. a option -> _ = assert false

(executable
 (name test)
 (flags :standard -dsource)
 (preprocess (pps ppx_here)))

On OCaml 5.1.0~alpha1 it results in the following error:

File "dune", line 1, characters 0-82:  
1 | (executable
2 |  (name test)
3 |  (flags :standard -dsource)
4 |  (preprocess (pps ppx_here)))
[@@@ocaml.ppx.context
  {
    tool_name = "ppx_driver";
    include_dirs = [];
    load_path = [];
    open_modules = [];
    for_package = None;
    debug = false;
    use_threads = false;
    use_vmthreads = false;
    recursive_types = false;
    principal = false;
    transparent_modules = false;
    unboxed_types = false;
    unsafe_string = false;
    cookies = []
  }]
let (f : 'a . a option -> _) = fun (type a) -> (assert false : a option -> _)
File "test.ml", line 1, characters 15-16:
1 | let f: type a. a option -> _ = assert false
                   ^
Error: Unbound type constructor a

While on 5.0:

let f : type a. a option -> _ = assert false

(any ppx that uses ppxlib demonstrates this issue, ppx_here is just a small example)

[copy]

Here's a simpler one:

File "test.ml", line 1, characters 4-10:
1 | let (x, y) : (int * int) = assert false
        ^^^^^^
Error: This pattern matches values of type 'a * 'b
       but a pattern was expected which matches values of type int * int

[panglesd]

Thanks for the report. This is related to the parsetree migration (see #407). I'll have a look.

[panglesd]

Should be fixed by #420.

[pitag-ha]

Thanks again for reporting, @copy! It's fixed now.