Merlin infers wrong types for labelled optional arguments in 5.2

[jchavarri]

@anmonteiro and myself have been debugging some issues that surfaced in reason-react test suite when upgrading to 5.2: reasonml/reason-react#833.

It seems that when using 3.11.0 with OCaml 5.2, the type signature inferred from a function like:

let t = (~foo=0, ()) => foo;

The inferred type signature in merlin / lsp will be:

(~foo: int, unit) => int

when it should be:

(~foo: int=?, unit) => int

Some of the things we've gathered:

• The error affects the editor tooling, but not actual compilation
• It only happens when using 3.11.0 with 5.2, but the behavior is right when using it with 5.1
• The issue is not reproducible when using OCaml syntax