[Bug fortran/61933] Inquire on internal units

["Joost.VandeVondele at mat dot ethz.ch" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=61933

--- Comment #9 from Joost VandeVondele <Joost.VandeVondele at mat dot ethz.ch> ---
(In reply to Jerry DeLisle from comment #8)
> You might notice that we redefined existence to be whether or not it is
> connected.  Units get connected when opened so your sample code needs only
> ask:
> 
> IF ((.NOT.is_open).AND.(istat == 0)) RETURN
> 
> Whether this is what we really want to do of course is open to discussion.
> 
> The other definition for existence is .true. for all units except -1 which
> is moot because -1 will give an error and the test for existence is always
> .true. and not needed.  Also unit existence is processor dependent.
> 
> In your opinion, should we change it to the other definition?  Unit
> existence is sort of a nebulous situation.  Will your code be more portable
> without the test for existence?

The code in comment #7 worked on all compilers we had access to (and is part of
our released code since ages), so this change of behaviour would be a problem.

I think unless gfortran has a maximum for the allowed unit numbers, any postive
unit number should exist (i.e. can possibly be used in an open statement). 

Since:
> cat test.f90
open(UNIT=HUGE(1_16))
END
yields:
> ./a.out
At line 1 of file test.f90
Fortran runtime error: Unit number in I/O statement too large
that would be a unit that doesn't exist.