[Bug fortran/119118] substring with negative start index

[anlauf at gcc dot gnu.org via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=119118

anlauf at gcc dot gnu.org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
     Ever confirmed|0                           |1
             Status|UNCONFIRMED                 |NEW
   Last reconfirmed|                            |2025-03-05
                 CC|                            |anlauf at gcc dot gnu.org

--- Comment #3 from anlauf at gcc dot gnu.org ---
The following adjustment to the logic around the bounds-checking code
fixes the issue:

diff --git a/gcc/fortran/trans-expr.cc b/gcc/fortran/trans-expr.cc
index fbe7333fd71..ed29623fcdf 100644
--- a/gcc/fortran/trans-expr.cc
+++ b/gcc/fortran/trans-expr.cc
@@ -2814,8 +2813,8 @@ gfc_conv_substring (gfc_se * se, gfc_ref * ref, int kind,
     end.expr = gfc_evaluate_now (end.expr, &se->pre);

   if ((gfc_option.rtcheck & GFC_RTCHECK_BOUNDS)
-      && (ref->u.ss.start->symtree
-         && !ref->u.ss.start->symtree->n.sym->attr.implied_index))
+      && !(ref->u.ss.start->symtree
+         && ref->u.ss.start->symtree->n.sym->attr.implied_index))
     {
       tree nonempty = fold_build2_loc (input_location, LE_EXPR,
                                       logical_type_node, start.expr,

Needs regtesting.