[Bug libfortran/48618] New: Negative unit number in OPEN(...) is sometimes allowed

[jb at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=48618

           Summary: Negative unit number in OPEN(...) is sometimes allowed
           Product: gcc
           Version: unknown
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: libfortran
        AssignedTo: unassig...@gcc.gnu.org
        ReportedBy: j...@gcc.gnu.org


From F2008 9.5.1 paragraph 2: 

"A unit is either an external unit or an internal unit. An external unit is
used to refer to an external file and
is specified by an asterisk or a file-unit-number . The value of file-unit-number
shall be nonnegative, equal to
one of the named constants INPUT UNIT, OUTPUT UNIT, or ERROR UNIT of the
intrinsic module ISO -
FORTRAN ENV (13.8.2), or a NEWUNIT value (9.5.6.12)."

That is, consider:

program unittest
  implicit none
  open(10, file="foo.txt")
  open(10, file="bar.txt")
end program unittest

The second open will close the file foo.txt and open bar.txt on the existing
unit. Similarly,

program nutest
  implicit none
  integer :: id
  open(newunit=id, file="foo.txt")
  open(id, file="bar.txt")
end program nutest

should work ("...or a NEWUNIT value" in 9.5.1), but

$ ./a.out 
At line 5 of file nutest.f90
Fortran runtime error: Bad unit number in OPEN statement

However for

program negunit
  implicit none
  open(-10, file="foo.txt")
end program negunit

we are correct to generate the error

$ ./negunit 
At line 3 of file negunit.f90
Fortran runtime error: Bad unit number in OPEN statement

as the unit number does not refer to an existing unit. That is, we must change
the logic such that we generate an error for negative unit number in the OPEN
statement only after we make sure that the unit doesn't exist.