https://gcc.gnu.org/bugzilla/show_bug.cgi?id=125383
Bug ID: 125383
Summary: Optional dummy procedure called from contained
subprogram results in linking error
Product: gcc
Version: 16.1.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: fortran
Assignee: unassigned at gcc dot gnu.org
Reporter: ripero84 at gmail dot com
Target Milestone: ---
Compilation of the following file
===========
module m
implicit none
contains
subroutine work(parg)
interface
subroutine subint
end subroutine
end interface
procedure(subint), optional :: parg
call setup
contains
subroutine setup
if (present(parg)) call parg
end subroutine setup
end subroutine work
subroutine sub0
end subroutine sub0
end module m
program p
use m
implicit none
call work(parg=sub0)
end program p
===========
using gfortran 16.1.0 leads to a linking error:
$ gfortran p.f90 -o p
/usr/bin/ld: /tmp/cc8p8UEW.o: in function `setup.0':
p.f90:(.text+0x24): undefined reference to `parg_'
collect2: error: ld returned 1 exit status
This suggests an issue with the generated object code. The error appears if an
optional dummy procedure is called from within a contained subprogram. If the
optional dummy procedure is called also from the host subroutine, or if it is
passed to the contained subroutine as an argument, the linking error
disappears.
While the error happens in the linking stage, I think the error is not related
to the linker, since the same linker will work for other versions of the
compiler.
The issue also affects the current (as of today) gcc-14 and gcc-15 branches.
This issue was not present in any previous 14.x and 15.x releases, so it looks
like a regression.
I have not bisected from the source code when the issue was introduced, but
based on the Debian changelog and on the Debian package versions with and
without the problem it looks like the regressions would have been introduced in
gcc-14 and gcc-15 at some point between early September and late October 2025.
