[Bug c++/80907] New: False positive: "warning: array subscript is above array bounds"

[cas43 at cs dot stanford.edu]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=80907

            Bug ID: 80907
           Summary: False positive: "warning: array subscript is above
                    array bounds"
           Product: gcc
           Version: 8.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: cas43 at cs dot stanford.edu
  Target Milestone: ---

THIS PROGRAM:

int x[3];
int n=2;
void foo() 
{
    for(int i=0;i<n;i++) for(int j=0;j<=i;j++) x[i+j]++;
}


PRODUCES THIS DIAGNOSTIC:

prog.cpp: In function ‘void foo()’:
prog.cpp:5:53: warning: array subscript is above array bounds [-Warray-bounds]
     for(int i=0;i<n;i++) for(int j=0;j<=i;j++) x[i+j]++;
                                                ~~~~~^
prog.cpp:5:53: warning: array subscript is above array bounds [-Warray-bounds]


WHEN COMPILED LIKE THIS:

g++ -c -O3 -Warray-bounds prog.cpp


COMPILER VERSION:

Using built-in specs.
COLLECT_GCC=/home/craig/new-gcc/i-trunk/bin/g++
COLLECT_LTO_WRAPPER=/home/craig/new-gcc/i-trunk/libexec/gcc/x86_64-pc-linux-gnu/8.0.0/lto-wrapper
Target: x86_64-pc-linux-gnu
Configured with: ../s-trunk/configure --disable-linux-futex
--prefix=/home/craig/new-gcc/i-trunk --disable-multilib
Thread model: posix
gcc version 8.0.0 20170526 (experimental) (GCC) 


OTHER NOTES:

In the original code, there are also assertions guarding the access:

T& operator()(int i,int j)
{assert((unsigned)i<2 && (unsigned)j<2);return array[i+j];}