Hi Tobias,

> On 10.06.21 20:52, Harald Anlauf via Fortran wrote:
> > +static bool
> > +substring_has_constant_len (gfc_expr *e)
> > +{
> > +  ptrdiff_t istart, iend;
> > +  size_t length;
> > +  bool equal_length = false;
> > +
> > +  if (e->ts.type != BT_CHARACTER
> > +      || !e->ref
> > +      || e->ref->type != REF_SUBSTRING
> 
> Is there a reason why you do not handle:
> 
> type t
>    character(len=5) :: str1
>    character(len=:), allocatable :: str2
> end type
> type(t) :: x
> 
> allocate(x%str2, source="abd")
> if (len (x%str)) /= 1) ...
> if (len (x%str2(1:2) /= 2) ...
> etc.
> 
> Namely: Search the last_ref = expr->ref->next->next ...?
> and then check that lastref?

I was assuming that the argument passed to LEN() is already the ultimate
component for the case of substrings, and I was unable to find a case which
requires implementing that iteration.  The cases you provided do not seem
to apply here:

- derived type component str1, which is a string of given length, poses no
  problem.  I added a case to the testcase, see attached updated patch.

- derived type component str2 has deferred length.  I do not see that the
  simplification can be applied here, as the allocation could lead to str2
  being too short, and we do not want to simplify invalid code, such as:

type t
   character(len=:), allocatable :: str2
end type
type(t) :: x

allocate(x%str2, source="z")
if (len (x%str2(1:2)) /= 2) stop 1
end

If we want this to be catchable by bounds checking, we need to punt
at simplification of this.  The updated patch skips deferred strings.

>    * * *
> 
> Slightly unrelated: I think the following does not violate
> F2018's R916 / C923 – but is rejected, namely:
>    R916  type-param-inquiry  is  designator % type-param-name
> the latter is 'len' or 'kind' for intrinsic types. And:
>    R901  designator is ...
>                     or substring
> But
> 
> character(len=5) :: str
> print *, str(1:3)%len
> end
> 
> fails with
> 
>      2 | print *, str(1:3)%len
>        |                  1
> Error: Syntax error in PRINT statement at (1)
> 
> 
> Assuming you don't want to handle it, can you open a new PR?
> Thanks!

Good point.  I'd rather open a separate PR for this, though.

> > +  istart = gfc_mpz_get_hwi (e->ref->u.ss.start->value.integer);
> > +  iend = gfc_mpz_get_hwi (e->ref->u.ss.end->value.integer);
> > +  length = gfc_mpz_get_hwi (e->ref->u.ss.length->length->value.integer);
> > +
> > +  if (istart <= iend)
> > +    {
> > +      if (istart < 1)
> > +     {
> > +       gfc_error ("Substring start index (%ld) at %L below 1",
> > +                  (long) istart, &e->ref->u.ss.start->where);
> 
> As mentioned by Bernhard, you could use HOST_WIDE_INT_PRINT_DEC.
> 
> (It probably only matters on Windows which uses long == int = 32bit for
> strings longer than INT_MAX.)

I am not familiar enough with Windows.  What is HOST_WIDE_INT
on that system?  (As compared to e.g. size_t, ptrdiff_t).

The (slightly) updated patch regtests fine.

Thanks,
Harald