A number of Fedora 24 packages have been observed to fail with the trunk of GCC 6 due to the new -Wplacement-new warning.  At least some of those are due to the invalid but not entirely uncommon "idiom" of using placement to construct a larger object in the last member of a structure, where the member's type is an array of 1 element.  An example of one such error was discussed in the following thread:
https://lists.fedoraproject.org/archives/list/devel@lists.fedoraproject.org/thread/ELWZFEXE72PJWXUU7N5WKYXD4DXEPUFY/

The code representative of this "idiom" was reduced to the following small test case:

#include <stdlib.h>
#include <new>
 
struct X {
  enum Type { Int, Double };
  Type type;
  char data[1];
};
 
int main()
{
  X* p = (X*)malloc(sizeof(X) + sizeof(double) -1);
  double* d = new (p->data) double(1.0);
  p->type = X::Double;
}

Since this code is questionable but not currently (with GCC 6) unsafe we want to avoid diagnosing it by default (-Wplacement-new is enabled by default).  To make it possible to request a warning for such code, -Wplacement-new will be changed analogously to (for example) -Wshift-overflow to optionally take an integer argument: 1 or 2.  With 1 (or without an argument), the diagnostic will not be issued for the code above.  With 2, the code will be diagnosed the same way as it is now.