https://gcc.gnu.org/bugzilla/show_bug.cgi?id=69662
Bug ID: 69662
Summary: -Wplacement-new on allocated one element array members
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: ---
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/[email protected]/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.
