PR#7315: refine some error locations

[gasche]

Fixes MPR#7315 and some related imprecise messages -- I suspect that more remain, and started building a testsuite.

[alainfrisch]

Ah, I was also working on it.

trunk...alainfrisch:better_loc_unbound_constructor

I'd propose to change the interface of Typetexp.find_type (and then other Typetexp.find_* functions as well) to take a longident Location.loc instead of two separate arguments. This encourage proper use on the call site, passing the exact location corresponding to the item being looked up. This would catch, for instance, the weird location on:

# type exn += X = Y;;
Characters 12-17:
  type exn += X = Y;;
              ^^^^^
Error: Unbound constructor Y

[gasche]

That sounds like a good idea. Feel free to integrate my tests, add some, and send a pull-request with your implementation.

[alainfrisch]

I'm not sure I'll have time to finalize that quickly, so it's perhaps better to merge this PR and keep the API change in mind for later.

Do you think the two failing tests are related to this PR?

    tests/typing-misc/polyvars.ml
    tests/typing-objects/pr6383.ml.reference

[gasche]

Yes, the tests were related to changed location information. I fixed the tests and integrated your own changes to extension redefinitions (plus lookup of the extensible type's name). I'll wait to leave some opportunity for review, then merge in trunk. Thanks for the advice!

[alainfrisch]

One could argue that the pattern here is #ab, not ab, so the old location might be better.

[gasche]

Note: it's not possible to refine the location here to M only without adding more location information inside Longident.t, which would result in a painful change to the interface used by everyone manipulating the parsetree -- not a good idea.

[ghost]

The original behavior:

# type foo = (unit,unit,unit,unit) bar;;
Error: Unbound type constructor bar
# type foo = unit bar;;
Error: Unbound type constructor bar

The new behavior:

# type foo = (unit,unit,unit,unit) bar;;
Error: Unbound type constructor bar
# type foo = unit bar;;
Error: Unbound type constructor bar

It is not a big thing, but nevertheless a strict improvement, in my opinion.