Clean up header detection for more headers + add optional

include/boost/container_hash/hash/hash.hpp

@@ -62,21 +62,28 @@
 #   define BOOST_FUNCTIONAL_HASH_ROTL32(x, r) (x << r) | (x >> (32 - r))
 #endif
 
-// Detect whether standard library has <string_view>.
+// Detect whether standard library has C++17 headers
 
-#if !defined(BOOST_HASH_HAS_STRING_VIEW) && defined(__has_include)
+#if !defined(BOOST_HASH_CXX17)
-#   if __has_include(<string_view>)
+#   if defined(BOOST_MSVC)
-#       if defined(BOOST_MSVC)
+#       if defined(_HAS_CXX17) && _HAS_CXX17
-            // On Visual C++ the header exists, but causes an
+#           define BOOST_HASH_CXX17 1
-            // error if it isn't in C++17 mode.
-#           if defined(_HAS_CXX17) && _HAS_CXX17
-#               define BOOST_HASH_HAS_STRING_VIEW 1
-#               include <string_view>
-#           endif
-#       elif defined(__cplusplus) && __cplusplus >= 201703
-#           include <string_view>
-#           define BOOST_HASH_HAS_STRING_VIEW 1
 #       endif
+#   elif defined(__cplusplus) && __cplusplus >= 201703
+#       define BOOST_HASH_CXX17 1
+#   endif
+#endif
+
+#if !defined(BOOST_HASH_CXX17)
+#   define BOOST_HASH_CXX17 0
+#endif
+
+#if BOOST_HASH_CXX17 && defined(__has_include)
+#   if !defined(BOOST_HASH_HAS_STRING_VIEW) && __has_include(<string_view>)
+#       define BOOST_HASH_HAS_STRING_VIEW 1
+#   endif
+#   if !defined(BOOST_HASH_HAS_OPTIONAL) && __has_include(<optional>)
+#       define BOOST_HASH_HAS_OPTIONAL 1
 #   endif
 #endif
 
@@ -84,6 +91,18 @@
 #   define BOOST_HASH_HAS_STRING_VIEW 0
 #endif
 
+#if !defined(BOOST_HASH_HAS_OPTIONAL)
+#   define BOOST_HASH_HAS_OPTIONAL 0
+#endif
+
+#if BOOST_HASH_HAS_STRING_VIEW
+#   include <string_view>
+#endif
+
+#if BOOST_HASH_HAS_OPTIONAL
+#   include <optional>
+#endif
+
 namespace boost
 {
     namespace hash_detail

test/hash_info.cpp

@@ -55,9 +55,6 @@ void write_compiler_info() {
     if (difference) {
         std::cout << "+" << difference << std::endl;
     }
-#if defined(_HAS_CXX17) && _HAS_CXX17
-    std::cout << "C++17 mode" << std::endl;
-#endif
 }
 
 #else
@@ -75,10 +72,19 @@ int main() {
         << __cplusplus
         << std::endl;
 #endif
+
+    std::cout << "BOOST_HASH_CXX17: "
+        << BOOST_HASH_CXX17
+        << std::endl;
+
     std::cout << "BOOST_HASH_HAS_STRING_VIEW: "
         << BOOST_HASH_HAS_STRING_VIEW
         << std::endl;
 
+    std::cout << "BOOST_HASH_HAS_OPTIONAL: "
+        << BOOST_HASH_HAS_OPTIONAL
+        << std::endl;
+
 #if defined(BOOST_NO_CXX11_HDR_TYPEINDEX)
     std::cout << "No <typeindex>" << std::endl;
 #else