Non-exhaustive matching warning message for open types is confusing

[vicuna]

Original bug ID: 6466
Reporter: @whitequark
Assigned to: @garrigue
Status: closed (set by @xavierleroy on 2016-12-07T10:37:16Z)
Resolution: fixed
Priority: normal
Severity: text
Version: 4.02.0+beta1 / +rc1
Target version: 4.02.2+dev / +rc1
Fixed in version: 4.02.1+dev
Category: ~DO NOT USE (was: OCaml general)
Monitored by: @gasche @hcarty

Bug description

Here it is:

Warning 8: this pattern-matching is not exhaustive.
Here is an example of a value that is not matched:
*extension*

I scratched my head for quite a while before I got it.

Steps to reproduce

function Not_found -> ()

File attachments

• open-types-msg.patch

[vicuna]

Comment author: @damiendoligez

Let's have a discussion of what the error message should look like.

[vicuna]

Comment author: @whitequark

Warning 8: this pattern-matching is not exhaustive.
Matching over values of open types must include
a wildcard pattern in order to be exhaustive.

[vicuna]

Comment author: @garrigue

Fixed as suggested.
The warning is now:

Warning 8: this pattern-matching is not exhaustive.
Here is an example of a value that is not matched:
_
Matching over values of open types must include
a wild card pattern in order to be exhaustive.

Removing the 2nd and 3rd line would require changing the warning definition.

[vicuna]

Comment author: @lpw25

For consistency with the documentation, could we change:

Matching over values of open types must include
a wild card pattern in order to be exhaustive.

to

Matching over values of extensible variant types must include
a wild card pattern in order to be exhaustive.

[vicuna]

Comment author: @damiendoligez

Reopened: let us discuss (or implement) Leo's proposed change to the warning message.

[vicuna]

Comment author: @whitequark

gasche, can you please apply the trivial fix listed in the issue?

[vicuna]

Comment author: @gasche

Merged in trunk and 4.02.