Why std::void_t is required to detect ill-formed types in SFINAE context?

Question

I have is_callable trait as below:

template<class T, class = void>
struct is_callable: std::false_type {};

template<class T>
struct is_callable<T, std::void_t<decltype(&T::operator())>>: std::true_type {};  // (A)

There I have to wrap decltype(&T::operator()) with std::void_t rather than use just the decltype itself like this:

template<class T>
struct is_callable<T, decltype(&T::operator())>: std::true_type {};  // (B)

If I use just the decltype itself (B) then I got no compilation error thus I assume that decltype is still in SFINAE context but the detection does not work - the trait value is always false. Can someone explain how exactly void_t helps here.