not_null conversion operator for non-copy constructible types

[matt77hias]

gsl::not_null< T > has the following conversion operator:

constexpr operator T() const
{
    return get();
}

which is deleted for non-copy constructible types such as std::unique_ptr due to gsl::not_null< T >'s conditional return type mapping to const T&:

constexpr std::conditional_t<std::is_copy_constructible<T>::value, T, const T&> get() const
{
    Ensures(ptr_ != nullptr);
    return ptr_;
}

Therefore, it is not possible to extract (the potentially expensive) ptr_ anymore for non-copy constructible types once a gsl::not_null< T > is constructed around it. Wouldn't it make sense to use ref-qualified member methods for both methods (i.e. conversion operator and get)?

// For non-copy constructible types:

constexpr operator const T&() const &
{
    return ptr_;
}

constexpr operator T() &&
{
    return std::move(ptr_);
}

[matt77hias]

Note that it could make sense to use ref-qualified member methods for copy constructible types as well.

E.g.,

gsl::not_null< std::shared_ptr< Widget > > nptr = ...;
...
std::shared_ptr< Widget > ptr = std::move(nptr).get(); // Avoids copying the std::shared_ptr.

[sunnychatterjee]

Thanks for the feedback. We'll follow-up with the editors and circle back.

[JordanMaples]

We didn't have time to discuss this during today's sync. This will be the first item on our agenda next time we meet.

[ujos]

Is there an update for this issue?

[carsonRadtke]

@gdr-at-ms Can you investigate the decision made by gsl-lite to see if there is any way we can add constraints to add a sufficient conversion operator while not introducing any ambiguities?