Implement GenTxid as a variant

Reimplements the GenTxid class as a variant for better type safety.
Also adds two temporary functions to the old GenTxid class that
convert to and from the new variant.

src/primitives/transaction.h

@@ -437,6 +437,20 @@ public:
     const uint256& GetHash() const LIFETIMEBOUND { return m_hash; }
     friend bool operator==(const GenTxid& a, const GenTxid& b) { return a.m_is_wtxid == b.m_is_wtxid && a.m_hash == b.m_hash; }
     friend bool operator<(const GenTxid& a, const GenTxid& b) { return std::tie(a.m_is_wtxid, a.m_hash) < std::tie(b.m_is_wtxid, b.m_hash); }
+
+    GenTxidVariant ToVariant() const
+    {
+        return m_is_wtxid ?
+                   GenTxidVariant{Wtxid::FromUint256(m_hash)} :
+                   GenTxidVariant{Txid::FromUint256(m_hash)};
+    }
+
+    static GenTxid FromVariant(const GenTxidVariant& variant)
+    {
+        return GenTxid{
+            std::holds_alternative<::Wtxid>(variant),
+            variant.ToUint256()};
+    }
 };
 
 #endif // BITCOIN_PRIMITIVES_TRANSACTION_H

src/util/transaction_identifier.h

@@ -9,6 +9,10 @@
 #include <uint256.h>
 #include <util/types.h>
 
+#include <compare>
+#include <tuple>
+#include <variant>
+
 /** transaction_identifier represents the two canonical transaction identifier
  * types (txid, wtxid).*/
 template <bool has_witness>
@@ -76,4 +80,22 @@ using Txid = transaction_identifier<false>;
 /** Wtxid commits to all transaction fields including the witness. */
 using Wtxid = transaction_identifier<true>;
 
+class GenTxidVariant : public std::variant<Txid, Wtxid>
+{
+public:
+    using variant::variant;
+
+    bool IsWtxid() const { return std::holds_alternative<Wtxid>(*this); }
+
+    const uint256& ToUint256() const LIFETIMEBOUND
+    {
+        return std::visit([](const auto& id) -> const uint256& { return id.ToUint256(); }, *this);
+    }
+
+    friend auto operator<=>(const GenTxidVariant& a, const GenTxidVariant& b)
+    {
+        return std::tuple(a.IsWtxid(), a.ToUint256()) <=> std::tuple(b.IsWtxid(), b.ToUint256());
+    }
+};
+
 #endif // BITCOIN_UTIL_TRANSACTION_IDENTIFIER_H