Why are span's array and std::array constructors different from its container constructors

Question

I have been playing around with the latest specification for std::span using the clang trunk and libc++ on Godbolt and find some of the constructors confusing.

In particular I find the constructors from a plain old array and and a std::array to be different from other containers.

For example the following code appears to compile:

std::vector<int*> v = {nullptr, nullptr};
std::span<const int* const> s{v};

However this does not:

std::array<int*, 2> a = {nullptr, nullptr}; 
std::span<const int* const> s{a};

This seems to be in keeping with the way the constructors are described on cppreference.com, I am just struggling to understand why this is the case. Can anybody shed any light?

Answer 1

This seems like an oversight. The array constructors are currently specified as:

template<size_t N> constexpr span(array<value_type, N>& arr) noexcept;
template<size_t N> constexpr span(const array<value_type, N>& arr) noexcept;

But should probably be specified as:

template<class T, size_t N>
    requires std::convertible_to<T(*)[], ElementType(*)[]>
  constexpr span(array<T, N>& arr) noexcept;
template<class T, size_t N>
    requires std::convertible_to<const T(*)[], ElementType(*)[]>
  constexpr span(const array<T, N>& arr) noexcept;

Which would make your example compile, as it is safe to do. I submitted an LWG issue. This is now LWG 3255.

---

The wording already has this constraint specified in [span.cons]/11:

template<size_t N> constexpr span(element_type (&arr)[N]) noexcept;
template<size_t N> constexpr span(array<value_type, N>& arr) noexcept;
template<size_t N> constexpr span(const array<value_type, N>& arr) noexcept;

Constraints:

• extent == dynamic_­extent || N == extent is true, and
• remove_­pointer_­t<decltype(data(arr))>(*)[] is convertible to ElementType(*)[].

So we already have the right constraint. It's just that data(arr) isn't actually dependent in any of these cases, so the constraint is trivially satisfied. We just need to make these templates.