Merge bitcoin#19387: (almost) match std::span's constructor behavior

bitcoin@62733fe

src/span.h

@@ -28,6 +28,14 @@ class Span
     C* m_data;
     std::size_t m_size;
 
+    template <class T>
+    struct is_Span_int : public std::false_type {};
+    template <class T>
+    struct is_Span_int<Span<T>> : public std::true_type {};
+    template <class T>
+    struct is_Span : public is_Span_int<typename std::remove_cv<T>::type>{};
+
+
 public:
     constexpr Span() noexcept : m_data(nullptr), m_size(0) {}
 
@@ -78,8 +86,19 @@ class Span
      * To prevent surprises, only Spans for constant value types are supported when passing in temporaries.
      * Note that this restriction does not exist when converting arrays or other Spans (see above).
      */
-    template <typename V, typename std::enable_if<(std::is_const<C>::value || std::is_lvalue_reference<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, int>::type = 0>
-    constexpr Span(V&& v) noexcept : m_data(v.data()), m_size(v.size()) {}
+    template <typename V>
+    constexpr Span(V& other,
+        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,
+        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)
+        : m_data(other.data()), m_size(other.size()){}
 
     constexpr C* data() const noexcept { return m_data; }
     constexpr C* begin() const noexcept { return m_data; }