let-rec wellformedness check is more restrictive than expected

[vicuna]

Original bug ID: 7767
Reporter: @ivg
Assigned to: @yallop
Status: resolved (set by @yallop on 2018-04-11T07:50:00Z)
Resolution: fixed
Priority: normal
Severity: minor
Version: 4.06.0
Fixed in version: 4.07.0+dev/beta2/rc1/rc2
Category: typing

Bug description

I have an example of a definition that was accepted by previous versions of OCaml and is rejected after #556 1. A read carefully through all comments in that GPR and I didn't find anything that will mention that such definitions are no longer accepted. So this is why I'm marking this as "not expected". And I let you judge whether it is a regression or not and issuing this report to show an example of a real-world program, that is no longer accepted.

Here is an example (I'm sorry I don't like to reduces examples, so I minimized it while still keeping some amount of sense in it, the example is taken from the real code).

The minimized example:

type prog = Seq of prog list | Exp of string;;
let concat_map xs ~f = List.map f xs |> List.concat;;
let rec concat_prog = concat_map ~f:(function Seq xs -> concat_prog xs | x -> [x]);;
Error: This kind of expression is not allowed as right-hand side of `let rec'

[vicuna]

Comment author: @yallop

Thanks for the report. There's a fix here:

#1712

[vicuna]

Comment author: @yallop

Fixed in trunk