https://gcc.gnu.org/bugzilla/show_bug.cgi?id=119118
Bug ID: 119118
Summary: substring with negative start index
Product: gcc
Version: 15.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: fortran
Assignee: unassigned at gcc dot gnu.org
Reporter: kargls at comcast dot net
Target Milestone: ---
All,
I found the following issue when perusing the lfortran bug list.
program main
implicit none
integer n
character(len=30) str
n = 3
str = "123456789012345678901234567890"
print *, str(-1:n), len(str(-1:n)) ! Whoops
print *, str(-1:-2), len(str(-1:-2)) ! OK? zero-length substring
! print*, str(-1:3), len(str(-1:3)) ! gfortran issues error.
print *, myshow(n, n, str) ! OK
print *, myshow(-1, -2, str) ! OK? zero-length substring
! print *, myshow(-1, n, str) ! gfortran issues a run-time with -fcheck=all
! print *, myshow(-1, 3, str) ! gfortran issues a run-time with -fcheck=all
contains
function myshow(i, j, s) result(t)
character(len=:), allocatable :: t
character(len=*), intent(in) :: s
integer, intent(in) :: i, j
t = s(i:j)
end function myshow
end program main
% gfcx -o z -fcheck=all -O3 -Wall -Wextra a.f90
% ./z
123 5
0
3
As you can see, the line marked with 'Whoops' yields a rather odd result.
Not only is the line not diagnosed with a run-time error, it produces a
string with 3 characters but with a reported length of 5.
