span: add lifetimebound attribute

See http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2018/p0936r0.pdf for
reference.

This helps to guard against dangling references caused by construction from
temporaries such as:

    Span<const int> sp(std::vector<int>{1,2,3});

src/span.h

@@ -18,6 +18,16 @@
 #define ASSERT_IF_DEBUG(x)
 #endif
 
+#if defined(__clang__)
+#if __has_attribute(lifetimebound)
+#define SPAN_ATTR_LIFETIMEBOUND [[clang::lifetimebound]]
+#else
+#define SPAN_ATTR_LIFETIMEBOUND
+#endif
+#else
+#define SPAN_ATTR_LIFETIMEBOUND
+#endif
+
 /** A Span is an object that can refer to a contiguous sequence of objects.
  *
  * It implements a subset of C++20's std::span.
@@ -87,14 +97,14 @@ public:
      * Note that this restriction does not exist when converting arrays or other Spans (see above).
      */
     template <typename V>
-    constexpr Span(V& other,
+    constexpr Span(V& other SPAN_ATTR_LIFETIMEBOUND,
         typename std::enable_if<!is_Span<V>::value &&
                                 std::is_convertible<typename std::remove_pointer<decltype(std::declval<V&>().data())>::type (*)[], C (*)[]>::value &&
                                 std::is_convertible<decltype(std::declval<V&>().size()), std::size_t>::value, std::nullptr_t>::type = nullptr)
         : m_data(other.data()), m_size(other.size()){}
 
     template <typename V>
-    constexpr Span(const V& other,
+    constexpr Span(const V& other SPAN_ATTR_LIFETIMEBOUND,
         typename std::enable_if<!is_Span<V>::value &&
                                 std::is_convertible<typename std::remove_pointer<decltype(std::declval<const V&>().data())>::type (*)[], C (*)[]>::value &&
                                 std::is_convertible<decltype(std::declval<const V&>().size()), std::size_t>::value, std::nullptr_t>::type = nullptr)
@@ -154,9 +164,9 @@ public:
 /** MakeSpan for arrays: */
 template <typename A, int N> Span<A> constexpr MakeSpan(A (&a)[N]) { return Span<A>(a, N); }
 /** MakeSpan for temporaries / rvalue references, only supporting const output. */
-template <typename V> constexpr auto MakeSpan(V&& v) -> typename std::enable_if<!std::is_lvalue_reference<V>::value, Span<const typename std::remove_pointer<decltype(v.data())>::type>>::type { return std::forward<V>(v); }
+template <typename V> constexpr auto MakeSpan(V&& v SPAN_ATTR_LIFETIMEBOUND) -> typename std::enable_if<!std::is_lvalue_reference<V>::value, Span<const typename std::remove_pointer<decltype(v.data())>::type>>::type { return std::forward<V>(v); }
 /** MakeSpan for (lvalue) references, supporting mutable output. */
-template <typename V> constexpr auto MakeSpan(V& v) -> Span<typename std::remove_pointer<decltype(v.data())>::type> { return v; }
+template <typename V> constexpr auto MakeSpan(V& v SPAN_ATTR_LIFETIMEBOUND) -> Span<typename std::remove_pointer<decltype(v.data())>::type> { return v; }
 
 /** Pop the last element off a span, and return a reference to that element. */
 template <typename T>