https://gcc.gnu.org/bugzilla/show_bug.cgi?id=100307
Bug ID: 100307
Summary: Wrong placement-new warning
Product: gcc
Version: 11.1.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c++
Assignee: unassigned at gcc dot gnu.org
Reporter: s.rueckerl at tum dot de
Target Milestone: ---
A placement new warning is generated whenever the placement new is used with
in-line pointer arithmetic resulting in a negative offset to the original
pointers address. It does not fail with a positive offset or if the calculation
is done with a temporary variable before.
This did not happen with GCC-10 and can be observed with different GCC-11
versions (and the trunk version as provided on godbolt.org).
The following code replicates this behavior: (https://godbolt.org/z/nPGvEM44e)
```
#include <new>
static char myMemory [128];
int main()
{
//make it fail
char* memoryPtr = myMemory+32;
int* myValue = new (memoryPtr - 1) int {42};
//does not fail with positive offset
char* secondMemoryPtr = myMemory+64;
int* mySecondValue = new (secondMemoryPtr + 1) int {42};
//does not fail with temporary
char* thirdMemoryPtr = myMemory+96;
char* placementNewAddress = thirdMemoryPtr - 1;
int* myThirdValue = new (placementNewAddress) int {1};
// use all variables for return value to avoid unused variable warnings
return *myValue + *mySecondValue + *myThirdValue;
}
```
