rpc: Check default value type againts argument type

src/rpc/util.cpp

@@ -498,6 +498,33 @@ RPCHelpMan::RPCHelpMan(std::string name, std::string description, std::vector<RP
         for (const std::string& name : names) {
             CHECK_NONFATAL(named_args.insert(name).second);
         }
+        // Default value type should match argument type only when defined
+        if (arg.m_fallback.index() == 2) {
+            const RPCArg::Type type = arg.m_type;
+            switch (std::get<RPCArg::Default>(arg.m_fallback).getType()) {
+            case UniValue::VOBJ:
+                CHECK_NONFATAL(type == RPCArg::Type::OBJ);
+                break;
+            case UniValue::VARR:
+                CHECK_NONFATAL(type == RPCArg::Type::ARR);
+                break;
+            case UniValue::VSTR:
+                CHECK_NONFATAL(type == RPCArg::Type::STR || type == RPCArg::Type::STR_HEX || type == RPCArg::Type::AMOUNT);
+                break;
+            case UniValue::VNUM:
+                CHECK_NONFATAL(type == RPCArg::Type::NUM || type == RPCArg::Type::AMOUNT || type == RPCArg::Type::RANGE);
+                break;
+            case UniValue::VBOOL:
+                CHECK_NONFATAL(type == RPCArg::Type::BOOL);
+                break;
+            case UniValue::VNULL:
+                // Null values are accepted in all arguments
+                break;
+            default:
+                CHECK_NONFATAL(false);
+                break;
+            }
+        }
     }
 }