Why strict_not_null<> assignment needs exact same pointer types?

[shrinktofit]

struct A {
};

struct B: A {
};

// true
std::is_assignable_v<gsl::not_null<std::shared_ptr<A>>, gsl::not_null<std::shared_ptr<B>>>;

// false
std::is_assignable_v<gsl::strict_not_null<std::shared_ptr<A>>, gsl::strict_not_null<std::shared_ptr<B>>>;

// true
std::is_constructible_v<gsl::not_null<std::shared_ptr<A>>, gsl::not_null<std::shared_ptr<B>>>;

// true
std::is_constructible_v<gsl::strict_not_null<std::shared_ptr<A>>, gsl::strict_not_null<std::shared_ptr<B>>>;

[ArtemSarmini]

Because is_constructible can call explicit ctors, while is_assignable cannot. I guess it's by design and wontfix, unless std::tuple-like conditional explicit is introduced