Extended Description

gcc 4.1.2 emits the following warning on SmallVector:

llvm/include/llvm/ADT/SmallVector.h:46: warning: dereferencing type-punned
pointer will break strict-aliasing rules

This appears to be due to the use of a union:

template
class SmallVectorImpl {
T *Begin, *End, *Capacity;

// Allocate raw space for N elements of type T. If T has a ctor or dtor, we
// don't want it to be automatically run, so we need to represent the space as
// something else. An array of char would work great, but might not be
// aligned sufficiently. Instead, we either use GCC extensions, or some
// number of union instances for the space, which guarantee maximal alignment.
protected:
union U {
double D;
long double LD;
long long L;
void P;
} FirstEl;
// Space after 'FirstEl' is clobbered, do not add any instance vars after it.
public:
// Default ctor - Initialize to empty.
SmallVectorImpl(unsigned N)
: Begin((T)&FirstEl), End((T*)&FirstEl), Capacity((T*)&FirstEl+N) {
}

Line 46 being the one with where Begin, End and Capacity are being assigned. The
trouble is that you aren't allowed to put a "T" in the union anyways (otherwise,
that would be a neat hack to preserve the type without going through the
constructor).