support C++20

Makefile.linux

@@ -20,9 +20,12 @@ else ifeq ($(shell expr match ${CXXVER} "4\.[8-9]"),3) # gcc 4.8 - 4.9
 else ifeq ($(shell expr match ${CXXVER} "[5-6]"),1) # gcc 5 - 6
 	NEEDED_CXXFLAGS += -std=c++11
 	LDLIBS = -latomic
-else ifeq ($(shell expr match ${CXXVER} "[1,7-9]"),1) # gcc >= 7
+else ifeq ($(shell expr match ${CXXVER} "[7-9]"),1) # gcc 7 - 9 
 	NEEDED_CXXFLAGS += -std=c++17
 	LDLIBS = -latomic
+else ifeq ($(shell expr match ${CXXVER} "1[0-9]"),2) # gcc 10 - 19
+	NEEDED_CXXFLAGS += -std=c++20
+	LDLIBS = -latomic
 else # not supported
 $(error Compiler too old)
 endif

libi2pd/SSU2Session.cpp

@@ -2273,7 +2273,11 @@ namespace transport
 	{	
 		if (!msg) return;
 		int32_t msgID = msg->GetMsgID ();
+#if __cplusplus >= 202002L // C++ 20 or higher
+		if (!m_ReceivedI2NPMsgIDs.contains (msgID))
+#else
 		if (!m_ReceivedI2NPMsgIDs.count (msgID))
+#endif		
 		{	
 			if (!msg->IsExpired ())
 			{