Move or Named Return Value Optimization (NRVO)?

Lets say we have the following code:

std::vector<int> f()
{
  std::vector<int> y;
  ...
  return y;
} 

std::vector<int> x = ...
x = f();

It seems the compiler has two approaches here:

(a) NRVO: Destruct x, then construct f() in place of x.
(b) Move: Construct f() in temp space, move f() into x, destruct f().

Is the compiler free to use either approach, according to the standard?