Why is_default_constructible

Following works fine:

struct X { };  // OK
static_assert(std::is_default_constructible<X>::value, "Error"); 

Following assert fails to compile:

struct X { static_assert(std::is_default_constructible<X>::value, "Error"); };  // Fails

Why does the static_assert inside the class fail?

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Side Qn: Is std::is_default_constructible supposed to fail for the classes with private constructors as discussed in:
std::is_default_constructible<T> error, if constructor is private