Make use of C++17 std::optional<> instead of boost::optional<>.

liblangutil/EVMVersion.h

@@ -22,9 +22,9 @@
 
 #include <libevmasm/Instruction.h>
 
+#include <optional>
 #include <string>
 
-#include <boost/optional.hpp>
 #include <boost/operators.hpp>
 
 
@@ -51,12 +51,12 @@ public:
 	static EVMVersion istanbul() { return {Version::Istanbul}; }
 	static EVMVersion berlin() { return {Version::Berlin}; }
 
-	static boost::optional<EVMVersion> fromString(std::string const& _version)
+	static std::optional<EVMVersion> fromString(std::string const& _version)
 	{
 		for (auto const& v: {homestead(), tangerineWhistle(), spuriousDragon(), byzantium(), constantinople(), petersburg(), istanbul(), berlin()})
 			if (_version == v.name())
 				return v;
-		return {};
+		return std::nullopt;
 	}
 
 	bool operator==(EVMVersion const& _other) const { return m_version == _other.m_version; }

liblangutil/Scanner.cpp

@@ -53,8 +53,9 @@
 #include <liblangutil/Common.h>
 #include <liblangutil/Exceptions.h>
 #include <liblangutil/Scanner.h>
-#include <boost/optional.hpp>
+
 #include <algorithm>
+#include <optional>
 #include <ostream>
 #include <tuple>
 
@@ -187,7 +188,7 @@ bool Scanner::scanHexByte(char& o_scannedByte)
 	return true;
 }
 
-boost::optional<unsigned> Scanner::scanUnicode()
+std::optional<unsigned> Scanner::scanUnicode()
 {
 	unsigned x = 0;
 	for (int i = 0; i < 4; i++)
@@ -718,7 +719,7 @@ bool Scanner::scanEscape()
 		break;
 	case 'u':
 	{
-		if (boost::optional<unsigned> codepoint = scanUnicode())
+		if (auto const codepoint = scanUnicode(); codepoint.has_value())
 			addUnicodeAsUTF8(*codepoint);
 		else
 			return false;

liblangutil/Scanner.h

@@ -57,6 +57,8 @@
 #include <liblangutil/SourceLocation.h>
 #include <libdevcore/Common.h>
 #include <libdevcore/CommonData.h>
+
+#include <optional>
 #include <iosfwd>
 
 namespace langutil
@@ -207,7 +209,7 @@ private:
 	inline Token selectToken(char _next, Token _then, Token _else);
 
 	bool scanHexByte(char& o_scannedByte);
-	boost::optional<unsigned> scanUnicode();
+	std::optional<unsigned> scanUnicode();
 
 	/// Scans a single Solidity token.
 	void scanToken();