[Bug c++/68531] New: incorrect code for VLA in C++

[msebor at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=68531

            Bug ID: 68531
           Summary: incorrect code for VLA in C++
           Product: gcc
           Version: 6.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: msebor at gcc dot gnu.org
  Target Milestone: ---

G++ accepts variable length arrays as an extension but generates the wrong code
for some basic uses cases of such arrays: it allows changes in the value of a
variable that determines the number of elements in a VLA type to affect the
bound of the VLA after it has been defined.

When compiled with gcc (in C mode), or by Clang in either C or C++ modes, the
following program compiles and runs successfully to completion.  However, when
compiled with g++, either today's trunk or as far back as 4.5.3 (the oldest I
tested), it aborts.

$ cat x.c && /build/gcc-trunk-svn/gcc/xgcc -B /build/gcc-trunk-svn/gcc -Wall
-Wextra -Wpedantic -xc++ x.c && ./a.out
int main ()
{
    int nelems = 7;

    typedef char A [nelems];

    nelems = 2;

    A a;

    if (sizeof (A) != 7 || sizeof a != 7) __builtin_abort ();
}

x.c: In function ‘int main()’:
x.c:5:27: warning: ISO C++ forbids variable length array ‘A’ [-Wvla]
     typedef char A [nelems];
                           ^

Aborted (core dumped)