string_view interoperability

fix boostorg#26

include/boost/static_string/config.hpp

@@ -147,15 +147,34 @@
 #endif
 
 #ifndef BOOST_STATIC_STRING_STANDALONE
+#include <boost/config.hpp>
 #include <boost/assert.hpp>
 #include <boost/container_hash/hash.hpp>
 #include <boost/static_assert.hpp>
 #include <boost/utility/string_view.hpp>
+#include <boost/core/detail/string_view.hpp>
 #include <boost/throw_exception.hpp>
+#if !defined(BOOST_NO_CXX17_HDR_STRING_VIEW) && \
+    !defined(BOOST_STATIC_STRING_CXX17_STRING_VIEW)
+#define BOOST_STATIC_STRING_CXX17_STRING_VIEW
+#endif
 #else
 #include <cassert>
 #include <stdexcept>
+#endif
+
+// Include std::string_view
+#if defined(BOOST_STATIC_STRING_CXX17_STRING_VIEW) || __cplusplus >= 201703L
 #include <string_view>
+#elif defined(__has_include)
+#if __has_include(<string_view>)
+#include <string_view>
+#endif
+#endif
+
+// Determine if the class (and not only the header) is available
+#if __cpp_lib_string_view >= 201606L
+#define BOOST_STATIC_STRING_HAS_STD_STRING_VIEW
 #endif
 
 // Compiler bug prevents constexpr from working with clang 4.x and 5.x
@@ -192,6 +211,12 @@ defined(BOOST_STATIC_STRING_CPP14)
 #define BOOST_STATIC_STRING_GCC5_BAD_CONSTEXPR
 #endif
 
+// Define the basic string_view type used by the library
+// Conversions to and from other available string_view types
+// are still defined.
+#if !defined(BOOST_STATIC_STRING_STANDALONE) || \
+    defined(BOOST_STATIC_STRING_HAS_STD_STRING_VIEW)
+#define BOOST_STATIC_STRING_HAS_STRING_VIEW
 namespace boost {
 namespace static_strings {
 
@@ -205,4 +230,6 @@ using basic_string_view =
 #endif
 } // static_strings
 } // boost
+#endif
+
 #endif