[Bug c++/70075] New: incorrect initialization of multidimensional VLAs

["msebor at gcc dot gnu.org" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=70075

            Bug ID: 70075
           Summary: incorrect initialization of multidimensional VLAs
           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: ---

Unlike GCC in C mode, G++ accepts explicitly initialized variable length arrays
(as specified in WG21 N3639,
http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2013/n3639.html).  However,
when initializing VLAs with more elements than explicitly specified in the
initializer, G++ fails to zero-initialize the remaining elements as would be
the case for ordinary arrays.

The following test case demonstrates the problem:

 cat z.cpp && /build/gcc-trunk/gcc/xgcc -B /build/gcc-trunk/gcc -Wall -Wextra
-Wpedantic z.cpp && ./a.out
void __attribute__ ((noclone, noinline)) bar (int n)
{
  const int m = 2;
  char a [m][n] = { { 0, 1, 2 } };

  for (int i = 0; i < m; ++i)
    for (int j = 0; j < n; ++j)
      __builtin_printf ("%hhi ", a [i][j]);
  __builtin_printf ("\n");

  if (a [0][n - 1] != 0)
    __builtin_abort ();
}

int main ()
{
    bar (4);
}
z.cpp: In function âvoid bar(int)â:
z.cpp:4:15: warning: ISO C++ forbids variable length array âaâ [-Wvla]
   char a [m][n] = { { 0, 1, 2 } };
               ^
0 1 2 12 -1 127 0 0 
Aborted (core dumped)