message: Support conversion between Spans of compatible types
diff: @@ -25,6 +25,23 @@ public:
  constexpr Span(C* data, std::ptrdiff_t size) noexcept : m_data(data), m_size(size) {}
  constexpr Span(C* data, C* end) noexcept : m_data(data), m_size(end - data) {}
  + /** Implicit conversion of spans between compatible types.
  + *
  + * Specifically, if a pointer to an array of type O can be implicitly converted to a pointer to an array of type
  + * C, then permit implicit conversion of Span<O> to Span<C>. This matches the behavior of the corresponding
  + * C++20 std::span constructor.
  + *
  + * For example this means that a Span<T> can be converted into a Span<const T>.
  + */
  + template <typename O, typename std::enable_if<std::is_convertible<O (*)[], C (*)[]>::value, int>::type = 0>
  + constexpr Span(const Span<O>& other) noexcept : m_data(other.m_data), m_size(other.m_size) {}
  +
  + /** Default copy constructor. */
  + constexpr Span(const Span&) noexcept = default;
  +
  + /** Default assignment operator. */
  + Span& operator=(const Span& other) noexcept = default;
  +
  constexpr C* data() const noexcept { return m_data; }
  constexpr C* begin() const noexcept { return m_data; }
  constexpr C* end() const noexcept { return m_data + m_size; }
  @@ -44,6 +61,8 @@ public:
  friend constexpr bool operator<=(const Span& a, const Span& b) noexcept { return !(b < a); }
  friend constexpr bool operator>(const Span& a, const Span& b) noexcept { return (b < a); }
  friend constexpr bool operator>=(const Span& a, const Span& b) noexcept { return !(a < b); }
  +
  + template <typename O> friend class Span;
  };
  /** Create a span to a container exposing data() and size().
  