Original bug ID: 7103
Reporter: @hhugo
Assigned to: @garrigue
Status: closed (set by @xavierleroy on 2017-02-16T14:18:06Z)
Resolution: fixed
Priority: normal
Severity: minor
Version: 4.02.3
Fixed in version: 4.03.0+dev / +beta1
Category: typing
Monitored by: @hhugo @gasche

Bug description

the following type error

"Type a is not compatible with type 'a"

should contain the constraint on 'a.

"Type a is not compatible with type 'a = < .. >"
"Type a is not compatible with type 'a = [> `b]"

and eventually not talk about 'a is it's not needed.

Steps to reproduce

OCaml version 4.02.3

type 'a t
type a

let f : < .. > t -> unit = fun _ -> ();;

val f : < .. > t -> unit =

let g : [< `b] t -> unit = fun _ -> ();;

val g : [< `b ] t -> unit =

let h : [> `b] t -> unit = fun _ -> ();;

val h : [> `b ] t -> unit =

let _ = fun (x : a t) -> f x;;

Characters 27-28:
let _ = fun (x : a t) -> f x;;
^
Error: This expression has type a t but an expression was expected of type
(< .. > as 'a) t
Type a is not compatible with type 'a

let _ = fun (x : a t) -> g x;;

Characters 27-28:
let _ = fun (x : a t) -> g x;;
^
Error: This expression has type a t but an expression was expected of type
([< `b ] as 'a) t
Type a is not compatible with type 'a

let _ = fun (x : a t) -> h x;;

Characters 27-28:
let _ = fun (x : a t) -> h x;;
^
Error: This expression has type a t but an expression was expected of type
([> `b ] as 'a) t
Type a is not compatible with type 'a