Re-examining PR61933 negative unit numbers

[Jerry DeLisle <jvdelisle at charter dot net>]

I found some time during the Holidays to get back to this PR.

I think the solution depends on how we define whether or not a UNIT exists. All 
the back and forth emails and c.l.f discussions did not help me much and were 
leaning toward just letting any UNIT exist regardless except -1.

Combing through the standard I find the following:

"The unit identified by a NEWUNIT value shall not be preconnected."

To me this means I can do write(10,*) without explicitly connecting UNIT 10 with 
an open statement, but I can not do write(-10,*).

Likewise, if iunit = -10, I can not do write(iunit, *) unless UNIT -10 was 
previously connected with an OPEN(NEWUNIT=someunit, ...). I disregard whether or 
not the programmer "knows" the value of the unit number somehow.  I think that 
if one tries to write to a negative unit that was not established with a NEWUNIT 
specifier, then this is an error.

Elsewhere in the standard I find the following:

"At any given time, there is a processor-dependent set of external units that 
exist for a program."

This is talking about UNIT existence being processor-dependent.  To me this 
means we have some flexibility regarding what we define as existence for a UNIT.

Proposed:

I would like to recommend that we define for gfortran that an external UNIT 
exists if it is preconnected or if it does not require an explicit OPEN to be 
connected, or if it has been connected via an OPEN (with or without NEWUNIT 
being used).

With this suggested definition INQUIRE(UNIT=someunit, EXISTS=unit_exists) has 
some meaningful usefulness. Further, this would allow a programmer to find out 
what negative units have been connected or are still connected.  I am not 
suggesting that "usefulness" is some sort of universally understood thing or has 
any implied correctness.  I am just asking myself what is the point of testing 
for existence of a UNIT if one does not define existence.  And...I am trying to 
get an agreed to definition at least for gfortran.

Is this definition OK with the gfortran team?

Regards,

Jerry