C++17 std::launder and aliasing

[Jakub Jelinek]

On Mon, Oct 24, 2016 at 01:38:13PM +0200, Richard Biener wrote:
> Certainly a possibility - though points-to information is _not_ affected
> by launder semantics.  std::launder only is a memory optimization
> barrier for aliasing accesses (I believe even TBAA is valid as it
> constrains the types that can be instantiated at the place).  For that
> to work you'd have to instead make it have a VDEF and (optionally)
> add special code to the stmt_may_use/clobber alias helpers.

I guess the question is if std::launder affects just the returned pointer,
or something else too.

struct A {
  virtual int f();
  virtual int g() { return 2; }
};
struct B : A {
  virtual int f() { new (this) A; return 1; }
  virtual int g() { return 1; }  
};
int A::f() { new (this) B; return 2; }
static_assert(sizeof(B) == sizeof(A), "");

int main() {
  A a;
  int b = a.f();
  int c = std::launder(&a)->g();
  int d = a.g(); // Is this UB?
  int e = std::launder(&a)->f();
  if(b != 2 || c != 1 || d != 1 || f != 1)
    std::abort();
}

	Jakub