Replace READWRITEAS macro with AsBase wrapping function

Co-authored-by: Pieter Wuille <pieter@wuille.net>

src/addrman_impl.h

@@ -65,8 +65,7 @@ public:
 
     SERIALIZE_METHODS(AddrInfo, obj)
     {
-        READWRITEAS(CAddress, obj);
-        READWRITE(obj.source, Using<ChronoFormatter<int64_t>>(obj.m_last_success), obj.nAttempts);
+        READWRITE(AsBase<CAddress>(obj), obj.source, Using<ChronoFormatter<int64_t>>(obj.m_last_success), obj.nAttempts);
     }
 
     AddrInfo(const CAddress &addrIn, const CNetAddr &addrSource) : CAddress(addrIn), source(addrSource)

src/index/disktxpos.h

@@ -14,8 +14,7 @@ struct CDiskTxPos : public FlatFilePos
 
     SERIALIZE_METHODS(CDiskTxPos, obj)
     {
-        READWRITEAS(FlatFilePos, obj);
-        READWRITE(VARINT(obj.nTxOffset));
+        READWRITE(AsBase<FlatFilePos>(obj), VARINT(obj.nTxOffset));
     }
 
     CDiskTxPos(const FlatFilePos &blockIn, unsigned int nTxOffsetIn) : FlatFilePos(blockIn.nFile, blockIn.nPos), nTxOffset(nTxOffsetIn) {

src/netaddress.h

@@ -540,8 +540,7 @@ public:
 
     SERIALIZE_METHODS(CService, obj)
     {
-        READWRITEAS(CNetAddr, obj);
-        READWRITE(Using<BigEndianFormatter<2>>(obj.port));
+        READWRITE(AsBase<CNetAddr>(obj), Using<BigEndianFormatter<2>>(obj.port));
     }
 
     friend class CServiceHash;

src/primitives/block.h

@@ -87,8 +87,7 @@ public:
 
     SERIALIZE_METHODS(CBlock, obj)
     {
-        READWRITEAS(CBlockHeader, obj);
-        READWRITE(obj.vtx);
+        READWRITE(AsBase<CBlockHeader>(obj), obj.vtx);
     }
 
     void SetNull()

src/protocol.h

@@ -433,7 +433,7 @@ public:
         }
         // Invoke V1/V2 serializer for CService parent object.
         OverrideStream<Stream> os(&s, s.GetType(), use_v2 ? ADDRV2_FORMAT : 0);
-        SerReadWriteMany(os, ser_action, ReadWriteAsHelper<CService>(obj));
+        SerReadWriteMany(os, ser_action, AsBase<CService>(obj));
     }
 
     //! Always included in serialization. The behavior is unspecified if the value is not representable as uint32_t.

src/script/script.h

@@ -434,7 +434,7 @@ public:
     CScript(std::vector<unsigned char>::const_iterator pbegin, std::vector<unsigned char>::const_iterator pend) : CScriptBase(pbegin, pend) { }
     CScript(const unsigned char* pbegin, const unsigned char* pend) : CScriptBase(pbegin, pend) { }
 
-    SERIALIZE_METHODS(CScript, obj) { READWRITEAS(CScriptBase, obj); }
+    SERIALIZE_METHODS(CScript, obj) { READWRITE(AsBase<CScriptBase>(obj)); }
 
     explicit CScript(int64_t b) { operator<<(b); }
     explicit CScript(opcodetype b)     { operator<<(b); }

src/serialize.h

@@ -133,12 +133,40 @@ enum
     SER_GETHASH         = (1 << 2),
 };
 
-//! Convert the reference base type to X, without changing constness or reference type.
-template<typename X> X& ReadWriteAsHelper(X& x) { return x; }
-template<typename X> const X& ReadWriteAsHelper(const X& x) { return x; }
+/**
+ * Convert any argument to a reference to X, maintaining constness.
+ *
+ * This can be used in serialization code to invoke a base class's
+ * serialization routines.
+ *
+ * Example use:
+ *   class Base { ... };
+ *   class Child : public Base {
+ *     int m_data;
+ *   public:
+ *     SERIALIZE_METHODS(Child, obj) {
+ *       READWRITE(AsBase<Base>(obj), obj.m_data);
+ *     }
+ *   };
+ *
+ * static_cast cannot easily be used here, as the type of Obj will be const Child&
+ * during serialization and Child& during deserialization. AsBase will convert to
+ * const Base& and Base& appropriately.
+ */
+template <class Out, class In>
+Out& AsBase(In& x)
+{
+    static_assert(std::is_base_of_v<Out, In>);
+    return x;
+}
+template <class Out, class In>
+const Out& AsBase(const In& x)
+{
+    static_assert(std::is_base_of_v<Out, In>);
+    return x;
+}
 
 #define READWRITE(...) (::SerReadWriteMany(s, ser_action, __VA_ARGS__))
-#define READWRITEAS(type, obj) (::SerReadWriteMany(s, ser_action, ReadWriteAsHelper<type>(obj)))
 #define SER_READ(obj, code) ::SerRead(s, ser_action, obj, [&](Stream& s, typename std::remove_const<Type>::type& obj) { code; })
 #define SER_WRITE(obj, code) ::SerWrite(s, ser_action, obj, [&](Stream& s, const Type& obj) { code; })