Which of these conversions should be ambiguous?

I have some code like the following:

class bar;

class foo
{
public:
    operator bar() const;
};

class bar
{
public:
    bar(const foo& foo);
};

void baz() {
    foo f;
    bar b = f;   // [1]

    const foo f2;
    bar b2 = f2; // [2]
}

GCC gives an error at [2] but not [1]. Clang gives an error on both, and apparently MSVC gives an error on neither. Who's right?