GSL/include/gsl/span

///////////////////////////////////////////////////////////////////////////////
//
// Copyright (c) 2015 Microsoft Corporation. All rights reserved.
//
// This code is licensed under the MIT License (MIT).
//
// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
// IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
// FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
// AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
// LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
// OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
// THE SOFTWARE.
//
///////////////////////////////////////////////////////////////////////////////

#ifndef GSL_SPAN_H
#define GSL_SPAN_H

#include <gsl/gsl_assert> // for Expects
#include <gsl/gsl_byte>   // for byte
#include <gsl/gsl_util>   // for narrow_cast, narrow

#include <algorithm> // for lexicographical_compare
#include <array>     // for array
#include <cstddef>   // for ptrdiff_t, size_t, nullptr_t
#include <iterator>  // for reverse_iterator, distance, random_access_...
#include <limits>
#include <stdexcept>
#include <type_traits> // for enable_if_t, declval, is_convertible, inte...
#include <utility>
#include <memory> // for std::addressof

#if defined(_MSC_VER) && !defined(__clang__)
#pragma warning(push)

// turn off some warnings that are noisy about our Expects statements
#pragma warning(disable : 4127) // conditional expression is constant
#pragma warning(disable : 4702) // unreachable code

// Turn MSVC /analyze rules that generate too much noise. TODO: fix in the tool.
#pragma warning(disable : 26495) // uninitalized member when constructor calls constructor
#pragma warning(disable : 26446) // parser bug does not allow attributes on some templates

#if _MSC_VER < 1910
#pragma push_macro("constexpr")
#define constexpr /*constexpr*/
#define GSL_USE_STATIC_CONSTEXPR_WORKAROUND

#endif // _MSC_VER < 1910
#endif // _MSC_VER

// See if we have enough C++17 power to use a static constexpr data member
// without needing an out-of-line definition
#if !(defined(__cplusplus) && (__cplusplus >= 201703L))
#define GSL_USE_STATIC_CONSTEXPR_WORKAROUND
#endif // !(defined(__cplusplus) && (__cplusplus >= 201703L))

// GCC 7 does not like the signed unsigned missmatch (size_t ptrdiff_t)
// While there is a conversion from signed to unsigned, it happens at
// compiletime, so the compiler wouldn't have to warn indiscriminently, but
// could check if the source value actually doesn't fit into the target type
// and only warn in those cases.
#if defined(__GNUC__) && __GNUC__ > 6
#pragma GCC diagnostic push
#pragma GCC diagnostic ignored "-Wsign-conversion"
#endif

namespace gsl
{

// [views.constants], constants
constexpr const std::ptrdiff_t dynamic_extent = -1;

template <class ElementType, std::ptrdiff_t Extent = dynamic_extent>
class span;

// implementation details
namespace details
{
    template <class T>
    struct is_span_oracle : std::false_type
    {
    };

    template <class ElementType, std::ptrdiff_t Extent>
    struct is_span_oracle<gsl::span<ElementType, Extent>> : std::true_type
    {
    };

    template <class T>
    struct is_span : public is_span_oracle<std::remove_cv_t<T>>
    {
    };

    template <class T>
    struct is_std_array_oracle : std::false_type
    {
    };

    template <class ElementType, std::size_t Extent>
    struct is_std_array_oracle<std::array<ElementType, Extent>> : std::true_type
    {
    };

    template <class T>
    struct is_std_array : public is_std_array_oracle<std::remove_cv_t<T>>
    {
    };

    template <std::ptrdiff_t From, std::ptrdiff_t To>
    struct is_allowed_extent_conversion
        : public std::integral_constant<bool, From == To || From == gsl::dynamic_extent ||
                                                  To == gsl::dynamic_extent>
    {
    };

    template <class From, class To>
    struct is_allowed_element_type_conversion
        : public std::integral_constant<bool, std::is_convertible<From (*)[], To (*)[]>::value>
    {
    };

    template <class Span, bool IsConst>
    class span_iterator
    {
        using element_type_ = typename Span::element_type;

    public:
#ifdef _MSC_VER
        // Tell Microsoft standard library that span_iterators are checked.
        using _Unchecked_type = typename Span::pointer;
#endif

        using iterator_category = std::random_access_iterator_tag;
        using value_type = std::remove_cv_t<element_type_>;
        using difference_type = typename Span::index_type;

        using reference = std::conditional_t<IsConst, const element_type_, element_type_>&;
        using pointer = std::add_pointer_t<reference>;

        span_iterator() = default;

        constexpr span_iterator(const Span* span, difference_type idx) noexcept
            : span_(span), index_(idx)
        {}

        friend span_iterator<Span, true>;
        template <bool B, std::enable_if_t<!B && IsConst>* = nullptr>
        constexpr span_iterator(const span_iterator<Span, B>& other) noexcept
            : span_iterator(other.span_, other.index_)
        {}

        GSL_SUPPRESS(bounds.1) // NO-FORMAT: attribute
        constexpr reference operator*() const
        {
            Expects(index_ != span_->size());
            return *(span_->data() + index_);
        }

        constexpr pointer operator->() const
        {
            Expects(index_ != span_->size());
            return span_->data() + index_;
        }

        constexpr span_iterator& operator++() noexcept
        {
            Expects(0 <= index_ && index_ != span_->size());
            ++index_;
            return *this;
        }

        constexpr span_iterator operator++(int) noexcept
        {
            auto ret = *this;
            ++(*this);
            return ret;
        }

        constexpr span_iterator& operator--() noexcept
        {
            Expects(index_ != 0 && index_ <= span_->size());
            --index_;
            return *this;
        }

        constexpr span_iterator operator--(int) noexcept
        {
            auto ret = *this;
            --(*this);
            return ret;
        }

        constexpr span_iterator operator+(difference_type n) const noexcept
        {
            auto ret = *this;
            return ret += n;
        }

        friend constexpr span_iterator operator+(difference_type n, span_iterator const& rhs) noexcept
        {
            return rhs + n;
        }

        constexpr span_iterator& operator+=(difference_type n) noexcept
        {
            Expects((index_ + n) >= 0 && (index_ + n) <= span_->size());
            index_ += n;
            return *this;
        }

        constexpr span_iterator operator-(difference_type n) const noexcept
        {
            auto ret = *this;
            return ret -= n;
        }

        constexpr span_iterator& operator-=(difference_type n) noexcept { return *this += -n; }

        constexpr difference_type operator-(span_iterator rhs) const noexcept
        {
            Expects(span_ == rhs.span_);
            return index_ - rhs.index_;
        }

        constexpr reference operator[](difference_type n) const noexcept { return *(*this + n); }

        constexpr friend bool operator==(span_iterator lhs, span_iterator rhs) noexcept
        {
            return lhs.span_ == rhs.span_ && lhs.index_ == rhs.index_;
        }

        constexpr friend bool operator!=(span_iterator lhs, span_iterator rhs) noexcept
        {
            return !(lhs == rhs);
        }

        constexpr friend bool operator<(span_iterator lhs, span_iterator rhs) noexcept
        {
            return lhs.index_ < rhs.index_;
        }

        constexpr friend bool operator<=(span_iterator lhs, span_iterator rhs) noexcept
        {
            return !(rhs < lhs);
        }

        constexpr friend bool operator>(span_iterator lhs, span_iterator rhs) noexcept
        {
            return rhs < lhs;
        }

        constexpr friend bool operator>=(span_iterator lhs, span_iterator rhs) noexcept
        {
            return !(rhs > lhs);
        }

#ifdef _MSC_VER
        // MSVC++ iterator debugging support; allows STL algorithms in 15.8+
        // to unwrap span_iterator to a pointer type after a range check in STL
        // algorithm calls
        friend constexpr void _Verify_range(span_iterator lhs, span_iterator rhs) noexcept
        { // test that [lhs, rhs) forms a valid range inside an STL algorithm
            Expects(lhs.span_ == rhs.span_        // range spans have to match
                    && lhs.index_ <= rhs.index_); // range must not be transposed
        }

        constexpr void _Verify_offset(const difference_type n) const noexcept
        { // test that the iterator *this + n is a valid range in an STL
            // algorithm call
            Expects((index_ + n) >= 0 && (index_ + n) <= span_->size());
        }

        GSL_SUPPRESS(bounds.1) // NO-FORMAT: attribute
        constexpr pointer _Unwrapped() const noexcept
        { // after seeking *this to a high water mark, or using one of the
            // _Verify_xxx functions above, unwrap this span_iterator to a raw
            // pointer
            return span_->data() + index_;
        }

        // Tell the STL that span_iterator should not be unwrapped if it can't
        // validate in advance, even in release / optimized builds:
#if defined(GSL_USE_STATIC_CONSTEXPR_WORKAROUND)
        static constexpr const bool _Unwrap_when_unverified = false;
#else
        static constexpr bool _Unwrap_when_unverified = false;
#endif
        GSL_SUPPRESS(con.3) // NO-FORMAT: attribute // TODO: false positive
        constexpr void _Seek_to(const pointer p) noexcept
        { // adjust the position of *this to previously verified location p
            // after _Unwrapped
            index_ = p - span_->data();
        }
#endif

    protected:
        const Span* span_ = nullptr;
        std::ptrdiff_t index_ = 0;
    };

    template <std::ptrdiff_t Ext>
    class extent_type
    {
    public:
        using index_type = std::ptrdiff_t;

        static_assert(Ext >= 0, "A fixed-size span must be >= 0 in size.");

        constexpr extent_type() noexcept {}

        template <index_type Other>
        constexpr extent_type(extent_type<Other> ext)
        {
            static_assert(Other == Ext || Other == dynamic_extent,
                          "Mismatch between fixed-size extent and size of initializing data.");
            Expects(ext.size() == Ext);
        }

        constexpr extent_type(index_type size) { Expects(size == Ext); }

        constexpr index_type size() const noexcept { return Ext; }
    };

    template <>
    class extent_type<dynamic_extent>
    {
    public:
        using index_type = std::ptrdiff_t;

        template <index_type Other>
        explicit constexpr extent_type(extent_type<Other> ext) : size_(ext.size())
        {}

        explicit constexpr extent_type(index_type size) : size_(size) { Expects(size >= 0); }

        constexpr index_type size() const noexcept { return size_; }

    private:
        index_type size_;
    };

    template <class ElementType, std::ptrdiff_t Extent, std::ptrdiff_t Offset, std::ptrdiff_t Count>
    struct calculate_subspan_type
    {
        using type = span<ElementType, Count != dynamic_extent
                                           ? Count
                                           : (Extent != dynamic_extent ? Extent - Offset : Extent)>;
    };
} // namespace details

// [span], class template span
template <class ElementType, std::ptrdiff_t Extent>
class span
{
public:
    // constants and types
    using element_type = ElementType;
    using value_type = std::remove_cv_t<ElementType>;
    using index_type = std::ptrdiff_t;
    using pointer = element_type*;
    using reference = element_type&;

    using iterator = details::span_iterator<span<ElementType, Extent>, false>;
    using const_iterator = details::span_iterator<span<ElementType, Extent>, true>;
    using reverse_iterator = std::reverse_iterator<iterator>;
    using const_reverse_iterator = std::reverse_iterator<const_iterator>;

    using size_type = index_type;

#if defined(GSL_USE_STATIC_CONSTEXPR_WORKAROUND)
    static constexpr const index_type extent{Extent};
#else
    static constexpr index_type extent{Extent};
#endif

    // [span.cons], span constructors, copy, assignment, and destructor
    template <bool Dependent = false,
              // "Dependent" is needed to make "std::enable_if_t<Dependent || Extent <= 0>" SFINAE,
              // since "std::enable_if_t<Extent <= 0>" is ill-formed when Extent is greater than 0.
              class = std::enable_if_t<(Dependent || Extent <= 0)>>
    constexpr span() noexcept : storage_(nullptr, details::extent_type<0>())
    {}

    constexpr span(pointer ptr, index_type count) noexcept: storage_(ptr, count) {}

    constexpr span(pointer firstElem, pointer lastElem) noexcept
        : storage_(firstElem, std::distance(firstElem, lastElem))
    {}

    template <std::size_t N>
    constexpr span(element_type (&arr)[N]) noexcept
        : storage_(KnownNotNull{std::addressof(arr[0])}, details::extent_type<N>())
    {}

    template <std::size_t N, class = std::enable_if_t<(N > 0)>>
    constexpr span(std::array<std::remove_const_t<element_type>, N>& arr) noexcept
        : storage_(KnownNotNull{arr.data()}, details::extent_type<N>())
    {
    }

    constexpr span(std::array<std::remove_const_t<element_type>, 0>&) noexcept
        : storage_(static_cast<pointer>(nullptr), details::extent_type<0>())
    {
    }

    template <std::size_t N, class = std::enable_if_t<(N > 0)>>
    constexpr span(const std::array<std::remove_const_t<element_type>, N>& arr) noexcept
        : storage_(KnownNotNull{arr.data()}, details::extent_type<N>())
    {
    }

    constexpr span(const std::array<std::remove_const_t<element_type>, 0>&) noexcept
        : storage_(static_cast<pointer>(nullptr), details::extent_type<0>())
    {
    }

    // NB: the SFINAE here uses .data() as a incomplete/imperfect proxy for the requirement
    // on Container to be a contiguous sequence container.
    template <class Container,
              class = std::enable_if_t<
                  !details::is_span<Container>::value && !details::is_std_array<Container>::value &&
                  std::is_convertible<typename Container::pointer, pointer>::value &&
                  std::is_convertible<typename Container::pointer,
                                      decltype(std::declval<Container>().data())>::value>>
    constexpr span(Container& cont) noexcept : span(cont.data(), narrow<index_type>(cont.size()))
    {}

    template <class Container,
              class = std::enable_if_t<
                  std::is_const<element_type>::value && !details::is_span<Container>::value &&
                  std::is_convertible<typename Container::pointer, pointer>::value &&
                  std::is_convertible<typename Container::pointer,
                                      decltype(std::declval<Container>().data())>::value>>
    constexpr span(const Container& cont) noexcept : span(cont.data(), narrow<index_type>(cont.size()))
    {}

    constexpr span(const span& other) noexcept = default;

    template <
        class OtherElementType, std::ptrdiff_t OtherExtent,
        class = std::enable_if_t<
            details::is_allowed_extent_conversion<OtherExtent, Extent>::value &&
            details::is_allowed_element_type_conversion<OtherElementType, element_type>::value>>
    constexpr span(const span<OtherElementType, OtherExtent>& other) noexcept
        : storage_(other.data(), details::extent_type<OtherExtent>(other.size()))
    {}

    ~span() noexcept = default;
    constexpr span& operator=(const span& other) noexcept = default;

    // [span.sub], span subviews
    template <std::ptrdiff_t Count>
    constexpr span<element_type, Count> first() const noexcept
    {
        Expects(Count >= 0 && Count <= size());
        return {data(), Count};
    }

    template <std::ptrdiff_t Count>
    GSL_SUPPRESS(bounds.1) // NO-FORMAT: attribute
    constexpr span<element_type, Count> last() const noexcept
    {
        Expects(Count >= 0 && size() - Count >= 0);
        return {data() + (size() - Count), Count};
    }

    template <std::ptrdiff_t Offset, std::ptrdiff_t Count = dynamic_extent>
    GSL_SUPPRESS(bounds.1) // NO-FORMAT: attribute
    constexpr auto subspan() const noexcept ->
        typename details::calculate_subspan_type<ElementType, Extent, Offset, Count>::type
    {
        Expects((Offset >= 0 && size() - Offset >= 0) &&
                (Count == dynamic_extent || (Count >= 0 && Offset + Count <= size())));

        return {data() + Offset, Count == dynamic_extent ? size() - Offset : Count};
    }

    constexpr span<element_type, dynamic_extent> first(index_type count) const noexcept
    {
        Expects(count >= 0 && count <= size());
        return {data(), count};
    }

    constexpr span<element_type, dynamic_extent> last(index_type count) const noexcept
    {
        return make_subspan(size() - count, dynamic_extent, subspan_selector<Extent>{});
    }

    constexpr span<element_type, dynamic_extent> subspan(index_type offset,
                                                         index_type count = dynamic_extent) const noexcept
    {
        return make_subspan(offset, count, subspan_selector<Extent>{});
    }

    // [span.obs], span observers
    constexpr index_type size() const noexcept { return storage_.size(); }
    constexpr index_type size_bytes() const noexcept
    {
        return size() * narrow_cast<index_type>(sizeof(element_type));
    }
    constexpr bool empty() const noexcept { return size() == 0; }

    // [span.elem], span element access
    GSL_SUPPRESS(bounds.1) // NO-FORMAT: attribute
    constexpr reference operator[](index_type idx) const noexcept
    {
        Expects(CheckRange(idx, storage_.size()));
        return data()[idx];
    }

    constexpr reference front() const noexcept
    {
        Expects(size() > 0);
        return data()[0];
    }

    constexpr reference back() const noexcept
    {
        Expects(size() > 0);
        return data()[size() - 1];
    }


    constexpr reference at(index_type idx) const noexcept{ return this->operator[](idx); }
    constexpr reference operator()(index_type idx) const noexcept{ return this->operator[](idx); }

    constexpr pointer data() const noexcept { return storage_.data(); }

    // [span.iter], span iterator support
    constexpr iterator begin() const noexcept { return {this, 0}; }
    constexpr iterator end() const noexcept { return {this, size()}; }

    constexpr const_iterator cbegin() const noexcept { return {this, 0}; }
    constexpr const_iterator cend() const noexcept { return {this, size()}; }

    constexpr reverse_iterator rbegin() const noexcept { return reverse_iterator{end()}; }
    constexpr reverse_iterator rend() const noexcept { return reverse_iterator{begin()}; }

    constexpr const_reverse_iterator crbegin() const noexcept
    {
        return const_reverse_iterator{cend()};
    }
    constexpr const_reverse_iterator crend() const noexcept
    {
        return const_reverse_iterator{cbegin()};
    }

#ifdef _MSC_VER
    // Tell MSVC how to unwrap spans in range-based-for
    constexpr pointer _Unchecked_begin() const noexcept { return data(); }
    constexpr pointer _Unchecked_end() const noexcept
    {
        GSL_SUPPRESS(bounds.1) // NO-FORMAT: attribute
        return data() + size();
    }
#endif // _MSC_VER

private:
    static constexpr bool CheckRange(index_type idx, index_type size) noexcept
    {
        // Optimization:
        //
        // idx >= 0 && idx < size
        // =>
        // static_cast<size_t>(idx) < static_cast<size_t>(size)
        //
        // because size >=0 by span construction, and negative idx will
        // wrap around to a value always greater than size when casted.

        // check if we have enough space to wrap around
#if defined(__cpp_if_constexpr)
        if constexpr (sizeof(index_type) <= sizeof(size_t))
#else
        if (sizeof(index_type) <= sizeof(size_t))
#endif
        {
            return narrow_cast<size_t>(idx) < narrow_cast<size_t>(size);
        }
        else
        {
            return idx >= 0 && idx < size;
        }
    }

    // Needed to remove unnecessary null check in subspans
    struct KnownNotNull
    {
        pointer p;
    };

    // this implementation detail class lets us take advantage of the
    // empty base class optimization to pay for only storage of a single
    // pointer in the case of fixed-size spans
    template <class ExtentType>
    class storage_type : public ExtentType
    {
    public:
        // KnownNotNull parameter is needed to remove unnecessary null check
        // in subspans and constructors from arrays
        template <class OtherExtentType>
        constexpr storage_type(KnownNotNull data, OtherExtentType ext)
            : ExtentType(ext), data_(data.p)
        {
            Expects(ExtentType::size() >= 0);
        }

        template <class OtherExtentType>
        constexpr storage_type(pointer data, OtherExtentType ext) : ExtentType(ext), data_(data)
        {
            Expects(ExtentType::size() >= 0);
            Expects(data || ExtentType::size() == 0);
        }

        constexpr pointer data() const noexcept { return data_; }

    private:
        pointer data_;
    };

    storage_type<details::extent_type<Extent>> storage_;

    // The rest is needed to remove unnecessary null check
    // in subspans and constructors from arrays
    constexpr span(KnownNotNull ptr, index_type count) : storage_(ptr, count) {}

    template <std::ptrdiff_t CallerExtent>
    class subspan_selector
    {
    };

    template <std::ptrdiff_t CallerExtent>
    span<element_type, dynamic_extent> make_subspan(index_type offset, index_type count,
                                                    subspan_selector<CallerExtent>) const
    {
        const span<element_type, dynamic_extent> tmp(*this);
        return tmp.subspan(offset, count);
    }

    GSL_SUPPRESS(bounds.1) // NO-FORMAT: attribute
    span<element_type, dynamic_extent> make_subspan(index_type offset, index_type count,
                                                    subspan_selector<dynamic_extent>) const
    {
        Expects(offset >= 0 && size() - offset >= 0);

        if (count == dynamic_extent) { return {KnownNotNull{data() + offset}, size() - offset}; }

        Expects(count >= 0 && size() - offset >= count);
        return {KnownNotNull{data() + offset}, count};
    }
};

#if defined(GSL_USE_STATIC_CONSTEXPR_WORKAROUND)
template <class ElementType, std::ptrdiff_t Extent>
constexpr const typename span<ElementType, Extent>::index_type span<ElementType, Extent>::extent;
#endif

// [span.comparison], span comparison operators
template <class ElementType, std::ptrdiff_t FirstExtent, std::ptrdiff_t SecondExtent>
constexpr bool operator==(span<ElementType, FirstExtent> l, span<ElementType, SecondExtent> r)
{
    return std::equal(l.begin(), l.end(), r.begin(), r.end());
}

template <class ElementType, std::ptrdiff_t Extent>
constexpr bool operator!=(span<ElementType, Extent> l, span<ElementType, Extent> r)
{
    return !(l == r);
}

template <class ElementType, std::ptrdiff_t Extent>
constexpr bool operator<(span<ElementType, Extent> l, span<ElementType, Extent> r)
{
    return std::lexicographical_compare(l.begin(), l.end(), r.begin(), r.end());
}

template <class ElementType, std::ptrdiff_t Extent>
constexpr bool operator<=(span<ElementType, Extent> l, span<ElementType, Extent> r)
{
    return !(l > r);
}

template <class ElementType, std::ptrdiff_t Extent>
constexpr bool operator>(span<ElementType, Extent> l, span<ElementType, Extent> r)
{
    return r < l;
}

template <class ElementType, std::ptrdiff_t Extent>
constexpr bool operator>=(span<ElementType, Extent> l, span<ElementType, Extent> r)
{
    return !(l < r);
}

namespace details
{
    // if we only supported compilers with good constexpr support then
    // this pair of classes could collapse down to a constexpr function

    // we should use a narrow_cast<> to go to std::size_t, but older compilers may not see it as
    // constexpr
    // and so will fail compilation of the template
    template <class ElementType, std::ptrdiff_t Extent>
    struct calculate_byte_size
        : std::integral_constant<std::ptrdiff_t,
                                 static_cast<std::ptrdiff_t>(sizeof(ElementType) *
                                                             static_cast<std::size_t>(Extent))>
    {
    };

    template <class ElementType>
    struct calculate_byte_size<ElementType, dynamic_extent>
        : std::integral_constant<std::ptrdiff_t, dynamic_extent>
    {
    };
} // namespace details

// [span.objectrep], views of object representation
template <class ElementType, std::ptrdiff_t Extent>
span<const byte, details::calculate_byte_size<ElementType, Extent>::value>
as_bytes(span<ElementType, Extent> s) noexcept
{
    GSL_SUPPRESS(type.1) // NO-FORMAT: attribute
    return {reinterpret_cast<const byte*>(s.data()), s.size_bytes()};
}

template <class ElementType, std::ptrdiff_t Extent,
          class = std::enable_if_t<!std::is_const<ElementType>::value>>
span<byte, details::calculate_byte_size<ElementType, Extent>::value>
as_writeable_bytes(span<ElementType, Extent> s) noexcept
{
    GSL_SUPPRESS(type.1) // NO-FORMAT: attribute
    return {reinterpret_cast<byte*>(s.data()), s.size_bytes()};
}

//
// make_span() - Utility functions for creating spans
//
template <class ElementType>
constexpr span<ElementType> make_span(ElementType* ptr,
                                      typename span<ElementType>::index_type count)
{
    return span<ElementType>(ptr, count);
}

template <class ElementType>
constexpr span<ElementType> make_span(ElementType* firstElem, ElementType* lastElem)
{
    return span<ElementType>(firstElem, lastElem);
}

template <class ElementType, std::size_t N>
constexpr span<ElementType, N> make_span(ElementType (&arr)[N]) noexcept
{
    return span<ElementType, N>(arr);
}

template <class Container>
constexpr span<typename Container::value_type> make_span(Container& cont)
{
    return span<typename Container::value_type>(cont);
}

template <class Container>
constexpr span<const typename Container::value_type> make_span(const Container& cont)
{
    return span<const typename Container::value_type>(cont);
}

template <class Ptr>
constexpr span<typename Ptr::element_type> make_span(Ptr& cont, std::ptrdiff_t count)
{
    return span<typename Ptr::element_type>(cont, count);
}

template <class Ptr>
constexpr span<typename Ptr::element_type> make_span(Ptr& cont)
{
    return span<typename Ptr::element_type>(cont);
}

// Specialization of gsl::at for span
template <class ElementType, std::ptrdiff_t Extent>
constexpr ElementType& at(span<ElementType, Extent> s, index i)
{
    // No bounds checking here because it is done in span::operator[] called below
    return s[i];
}

// [span.obs] Free observer functions
template <class ElementType, std::ptrdiff_t Extent>
constexpr typename span<ElementType, Extent>::index_type ssize(const span<ElementType, Extent> &span) noexcept
{
    return span.size();
}

// [span.iter] Free functions for begin/end functions
template <class ElementType, std::ptrdiff_t Extent>
constexpr typename span<ElementType, Extent>::iterator begin(const span<ElementType, Extent> &span) noexcept
{
    return span.begin();
}

template <class ElementType, std::ptrdiff_t Extent = dynamic_extent>
constexpr typename span<ElementType, Extent>::iterator end(const span<ElementType, Extent> &span) noexcept
{
    return span.end();
}

template <class ElementType, std::ptrdiff_t Extent>
constexpr typename span<ElementType, Extent>::const_iterator cbegin(const span<ElementType, Extent> &span) noexcept
{
    return span.cbegin();
}

template <class ElementType, std::ptrdiff_t Extent>
constexpr typename span<ElementType, Extent>::const_iterator cend(const span<ElementType, Extent> &span) noexcept
{
    return span.cend();
}

template <class ElementType, std::ptrdiff_t Extent>
constexpr typename span<ElementType, Extent>::reverse_iterator rbegin(const span<ElementType, Extent> &span) noexcept
{
    return span.rbegin();
}

template <class ElementType, std::ptrdiff_t Extent>
constexpr typename span<ElementType, Extent>::reverse_iterator rend(const span<ElementType, Extent> &span) noexcept
{
    return span.rend();
}

template <class ElementType, std::ptrdiff_t Extent>
constexpr typename span<ElementType, Extent>::const_reverse_iterator crbegin(const span<ElementType, Extent> &span) noexcept
{
    return span.crbegin();
}

template <class ElementType, std::ptrdiff_t Extent>
constexpr typename span<ElementType, Extent>::const_reverse_iterator crend(const span<ElementType, Extent> &span) noexcept
{
    return span.crend();
}

} // namespace gsl

#if defined(_MSC_VER) && !defined(__clang__)
#if _MSC_VER < 1910
#undef constexpr
#pragma pop_macro("constexpr")

#endif // _MSC_VER < 1910

#pragma warning(pop)
#endif // _MSC_VER

#if defined(__GNUC__) && __GNUC__ > 6
#pragma GCC diagnostic pop
#endif // __GNUC__ > 6

#endif // GSL_SPAN_H