Return type for *this in a builder-class method - lvalue vs rvalue refence

Suppose I have a "builder" class B which builds a class C, and looks somewhat like the following:

class B {public:  // ...  B& set_foo(Foo a_foo) { foo_ = std::move(a_foo); return *this; }  C build() const { /* ... */ }};

This is all nice and good ... until I think about what happens when my B instance is a temporary. How should I design B to work also as temporary/rvalue, not just as an lvalue?

• Should I write a set_foo() && variant, returning a B&&? That would mean replicating lots of methods, and well, DRY.
• Should I just return B&& to begin with?
• Should I do something else?