util/StringView: prepare the transition to std::string_view

src/util/StringView.hxx

@@ -32,9 +32,14 @@
 
 #include "ConstBuffer.hxx"
 #include "StringAPI.hxx"
+#include "Compiler.h"
 
 #include <utility>
 
+#if !GCC_OLDER_THAN(7,0)
+#include <string_view>
+#endif
+
 template<typename T>
 struct BasicStringView : ConstBuffer<T> {
 	typedef typename ConstBuffer<T>::size_type size_type;
@@ -66,6 +71,15 @@ struct BasicStringView : ConstBuffer<T> {
 	constexpr BasicStringView(std::nullptr_t n) noexcept
 		:ConstBuffer<T>(n) {}
 
+#if !GCC_OLDER_THAN(7,0)
+	constexpr BasicStringView(std::basic_string_view<T> src) noexcept
+		:ConstBuffer<T>(src.data(), src.size()) {}
+
+	constexpr operator std::basic_string_view<T>() const noexcept {
+		return {data, size};
+	}
+#endif
+
 	using ConstBuffer<T>::empty;
 	using ConstBuffer<T>::begin;
 	using ConstBuffer<T>::end;