Use type annotations from arguments in let rec

[stedolan]

When typing let rec expressions, the compiler 'approximates' the type of each binding, allowing it to use information about each binding during typechecking. For instance, in a let rec of functions, it knows the arity of each definition early, allowing it to produce partial application warnings for mutually recursive functions.

Currently, type annotations on function parameters are ignored during this approximation. This delays typechecking errors, often in a confusing way. This patch makes approximation use these type annotations. As well as improving errors, this also makes type-based disambiguation work in more cases, for instance:

module M = struct type t = A | B end

let rec f () = g A
and g (x : M.t) = f ()

With this patch, the above typechecks, because the compiler knows that g accepts M.t before typing the body of f.

[gasche]

The patch looks correct to me, and I agree that it is a reasonable extension of the current behavior.

(Is the absence of a Changes entry intentional?)

[gasche]

Could you include a case of optional argument, to make sure that this is handled correctly?

[stedolan]

Done

[stedolan]

(Is the absence of a Changes entry intentional?)

Nope, just hadn't written it yet.

[Octachron]

I fear that the merge of #12210 broke the new test. Sorry about this collision.

[gasche]

I rebased the PR (by force-pushing to the remote branch) to the new testsuite output.

[stedolan]

Thanks for rebasing!