[m-rev.] for review: improve messages for some mode errors

[Zoltan Somogyi]

2020-01-24 14:07 GMT+11:00 Julien Fischer<jfischer at opturion.com>:
> That looks fine; 

Thanks.

> did you intend this for the release branch as well?

I am in two minds about that. In the vast majority of cases,
identifying the arg(s) responsible for the mode error is definitely
a help. On the other hand, in some rare cases, the error message
is slightly misleading. In one test case, the error message is for
a recursive call. One argument is the same in both the head and
the call, so their modes match exactly, yet there is an error in the
mode anyway: the mode is "in(pred(in, in, out) is det)", which is
wrong *only* because the *type* of the argument is a predicate
of arity five, not three. Both the old error message, and the new
addition, say that the problem is with the inst, but in this case,
it is actually with the inst/type combination. A later diff could
point this out. That diff will happen on the development branch,
but probably not in the near future, so it probably won't make it
into the release.

So the question for everybody is: should we include this diff
in the release because it is useful in the vast majority of cases,
or should we exclude it because of the potential for adding
a slight increment of confusion in a small minority of cases?

Zoltan.