add support for std::variant to hashlib

kernel/hashlib.h

@@ -186,6 +186,25 @@ inline unsigned int mkhash(const T &v) {
 	return hash_ops<T>().hash(v);
 }
 
+template<> struct hash_ops<std::monostate> {
+	static inline bool cmp(std::monostate a, std::monostate b) {
+		return a == b;
+	}
+	static inline unsigned int hash(std::monostate) {
+		return mkhash_init;
+	}
+};
+
+template<typename... T> struct hash_ops<std::variant<T...>> {
+	static inline bool cmp(std::variant<T...> a, std::variant<T...> b) {
+		return a == b;
+	}
+	static inline unsigned int hash(std::variant<T...> a) {
+		unsigned int h = std::visit([](const auto &v) { return mkhash(v); }, a);
+		return mkhash(a.index(), h);
+	}
+};
+
 inline int hashtable_size(int min_size)
 {
 	// Primes as generated by https://oeis.org/A175953

kernel/yosys_common.h

@@ -30,6 +30,7 @@
 #include <unordered_map>
 #include <unordered_set>
 #include <initializer_list>
+#include <variant>
 #include <stdexcept>
 #include <memory>
 #include <cmath>