Why is_default_constructible<Class>::value fails within the same class scope

Question

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

Answer 1

The documentation page says that for std::is_default_constructible<T> :

T shall be a complete type, (possibly cv-qualified) void, or an array of unknown bound. Otherwise, the behavior is undefined.

Since you are within your class, the type is not completely defined yet, i guess that's the reason for the difference.

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As for the side question, this trait seems to be based on std::is_constructible which seems to mean that if the variable definition

T obj();

is well formed the member constant value equal to true . In all other cases, value is false .

So my understanding of this and my candid name based semantical instinct would say that it should fail if the default constructor is private.