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Proto/Alpha: OCaml5: add annotation for parse_instr to typecheck

Pierrick Coudercrequested to merge
picdc@proto-annotate-parse-instr into master

What & why

There is a regression with OCaml 5.0+ that makes some GADT not typecheck anymore without a bit of help.

How

Simply annotate the right function with the return type of parse_instr, as it seems the type equality is lost somewhere in the process.

It unfortunately applies to all the protocols starting proto_010, for which we will also need to find a solution.

Manually testing the MR

From the ocaml5 branch (and switch), checkout on 2e35c5b4998 that is the top branch without this commit and try to typecheck proto_alpha:

dune @src/proto_alpha/lib_protocol/check

which will result in a cryptic typechecking error:

File "src/proto_alpha/lib_protocol/script_ir_translator.ml", lines 3027-3034, characters 10-55:
3027 | ..........record_trace_eval
3028 |             invalid_map_body
3029 |             (let* Eq, ctxt = stack_eq loc ctxt 1 aft_rest rest in
3030 |              let* opt_ty = option_t loc ret in
3031 |              let final_stack = Item_t (opt_ty, rest) in
3032 |              let body = kibody.instr.apply (IHalt loc) in
3033 |              let apply k = IOpt_map {loc; body; k} in
3034 |              typed_no_lwt ctxt loc {apply} final_stack)
Error: This expression has type
         "((a, s) judgement * context, error trace) result"
       but an expression was expected of type "'a" 
       This instance of "$0 * $1" is ambiguous:
       it would escape the scope of its equation
       Type "result" is abstract because no corresponding cmi file was found
       in path.

CHerry-pick the commit, run it again, and it should work properly.

Checklist

  • Document the interface of any function added or modified (see the coding guidelines)
  • Document any change to the user interface, including configuration parameters (see node configuration)
  • Provide automatic testing (see the testing guide).
  • For new features and bug fixes, add an item in the appropriate changelog (docs/protocols/alpha.rst for the protocol and the environment, CHANGES.rst at the root of the repository for everything else).
  • Select suitable reviewers using the Reviewers field below.
  • Select as Assignee the next person who should take action on that MR

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