[Bug c++/69516] New: [5/6 regression] infinite recursion on a VLA with excess initializer elements in constexpr function

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

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=69516

            Bug ID: 69516
           Summary: [5/6 regression] infinite recursion on a VLA with
                    excess initializer elements in constexpr function
           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: ---

GCC gets itself into infinite recursion compiling the following test case
derived derived from the one shown in the discussion of a patch for bug 69496
(https://gcc.gnu.org/ml/gcc-patches/2016-01/msg02098.html).  See also bug 69509
(which might be the same problem.)

The same problem exists in 5.x.  4.9.3 doesn't suffer from it because it
doesn't implement the more relaxed C++ 14 constexpr rules and rejects constexpr
functions that consist of more than just a return statement.

$ (cat t.c && ulimit -t 10 && /home/msebor/build/gcc-trunk-svn/gcc/xgcc
-B/home/msebor/build/gcc-trunk-svn/gcc  -Wall -Wextra -Wpedantic -std=c++14
-xc++ t.c)
constexpr int foo (int n)
{
     int a[n] = { 1, 2, 3, 4, 5, 6 };
     int z = 0;
     for (unsigned i = 0; i < 3; ++i)
       z += a[i];
     return z;
}

int main ()
{
   constexpr int n = foo (3);
   __builtin_printf ("%d\n", n);
}
t.c: In function âconstexpr int foo(int)â:
t.c:3:13: warning: ISO C++ forbids variable length array âaâ [-Wvla]
      int a[n] = { 1, 2, 3, 4, 5, 6 };
             ^
xgcc: internal compiler error: Killed (program cc1plus)
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