Since ivec is not passed to sub, I think the two calls
to set_optional should be equivalent, but gfortran crashes
upon the second call. 

u:\vrao\fortran type xopt_bug.f90 
module sub_mod
contains
elemental subroutine set_optional(i,idef,iopt)
! set i to (iopt,idef) if iopt (is,is not) PRESENT
integer, intent(out)          :: i
integer, intent(in)           :: idef
integer, intent(in), optional :: iopt
if (present(iopt)) then
   i = iopt
else
   i = idef
end if
end subroutine set_optional
!
subroutine sub(ivec)
integer          , intent(in), optional :: ivec(:)
integer                                 :: ivec_(2)
call set_optional(ivec_,(/1,2/))
print*,"ivec_=",ivec_
call set_optional(ivec_,(/1,2/),ivec)
print*,"ivec_=",ivec_
end subroutine sub
end module sub_mod

program main
use sub_mod, only: sub
call sub()
end program main

U:\vrao\fortran>gfortran -v 
Using built-in specs.
Target: i386-pc-mingw32
Configured with: ../trunk/configure --prefix=/mingw --enable-languages=c,fortran --with-gmp=/home/coudert/local --disable-nls --with-ld=/mingw/bin/ld --with-as=/mingw/bin/as --disable-werror --enable-bootstrap --enable-threads --host=i386-pc-mingw32 --disable-shared --enable-libgomp
Thread model: win32
gcc version 4.3.0 20061021 (experimental)

U:\vrao\fortran>gfortran -W -Wall -pedantic-errors -std=f2003 -fbounds-check -Werror xopt_bug.f90 

U:\vrao\fortran>a.exe
Fortran runtime error: Array bound mismatch, size mismatch for dimension 1 of array 'ivec' (in file 'xopt_bug.f90', at line 20)
 ivec_=           1           2