Compilation error with std::hash<gsl::not_null<std::shared_ptr<T>>> in GSL 4.2.0

[lm1458777]

Describe the bug
The specialization of std::hash for gsl::not_null<std::shared_ptr<T>> does not compile in GSL version 4.2.0.

To Reproduce

#include <gsl/pointers>

void test()
{
	std::hash<gsl::not_null<std::shared_ptr<int>>> hasher{};
}

Compilation Error

error C2280: 'gsl::not_null_hash<gsl::not_null<std::shared_ptr<int>>,const T &,false>::not_null_hash(void)': attempting to reference a deleted function
error C2280:         with
error C2280:         [
error C2280:             T=std::shared_ptr<int>
error C2280:         ]
    GSL-4.2.0\include\gsl\pointers(244,5):
    see declaration of 'gsl::not_null_hash<gsl::not_null<std::shared_ptr<int>>,const T &,false>::not_null_hash'
        with
        [
            T=std::shared_ptr<int>
        ]
    GSL-4.2.0\include\gsl\pointers(244,5):
    'gsl::not_null_hash<gsl::not_null<std::shared_ptr<int>>,const T &,false>::not_null_hash(void)': function was explicitly deleted
        with
        [
            T=std::shared_ptr<int>
        ]

Expected behavior
The specialization of std::hash for gsl::not_null<std::shared_ptr<T>> should compile successfully.

Spec:

• OS: Windows
• Compiler: MSVC 17.13.6
• C++ Version: C++20

Additional context
In the following template from <gsl/pointers>:

template <class T, class U = decltype(std::declval<const T&>().get()), bool = std::is_default_constructible<std::hash<U>>::value>
struct not_null_hash
{
    std::size_t operator()(const T& value) const { return std::hash<U>{}(value.get()); }
};

it might be necessary to apply std::remove_cv_t<std::remove_reference_t<...>> to the type U to properly deduce the underlying type and avoid issues with const/reference qualifiers.

[carsonRadtke]

Thanks for reporting this issue! I'll investigate and post an update when I know more.

[carsonRadtke]

it might be necessary to apply std::remove_cv_t<std::remove_reference_t<...>> to the type U to properly deduce the underlying type and avoid issues with const/reference qualifiers.

Yes, U was resolving to const std::shared_ptr<int> & instead of std::shared_ptr<int> which was causing further failures to the template instantiations for std::hash<T>. Your solution of applying the nested remove_* would have worked, but I think we can just use gsl::not_null<T>::element_type here instead.

[carsonRadtke]

Let's update the template for not_null_hash to grab T::element_type and use that to construct the hasher.