Merge #21679: rpc: Keep default argument value in correct type

bee56c78e94417f89b1f48682404e2821b57bdec rpc: Check default value type againts argument type (João Barbosa)
f81ef4303e057e85aa24772c865287c17ffa4350 rpc: Keep default argument value in correct type (João Barbosa)

Pull request description:

  Store default values of RPC arguments in the corresponding type instead of a string. The value is then serialized when the help output is needed. This change simplifies #20017.

  The following examples illustrates how to use the new `RPCArg::Default` and `RPCArg::DefaultHint`:

  ```diff
  - {"verbose", RPCArg::Type::BOOL, /* default */ "false", "True for a json object, false for array of transaction ids"}
  + {"verbose", RPCArg::Type::BOOL, RPCArg::Default(false), "True for a json object, false for array of transaction ids"}
  ```

  ```diff
  - {"nblocks", RPCArg::Type::NUM, /* default */ "one month", "Size of the window in number of blocks"}
  + {"nblocks", RPCArg::Type::NUM, RPCArg::DefaultHint("one month"), "Size of the window in number of blocks"}
  ```

  No behavior change is expected.

ACKs for top commit:
  LarryRuane:
    ACK bee56c78e94417f89b1f48682404e2821b57bdec
  MarcoFalke:
    ACK bee56c78e94417f89b1f48682404e2821b57bdec 🦅

Tree-SHA512: c47d78c918e996d36631d4ad3c933b270a34c5b446b8d736be94cf4a0a7b8c0e33d954149ec786cf9550639865b79deb6a130ad044de6030f95aac33f524293a

1 files changed, 3 insertions, 1 deletions

diff --git a/src/rpc/util.h b/src/rpc/util.h
index 6e783e19fb..8ec18b2f35 100644
--- a/src/rpc/util.h
+++ b/src/rpc/util.h
@@ -145,7 +145,9 @@ struct RPCArg {
          */
         OMITTED,
     };
-    using Fallback = std::variant<Optional, /* default value for optional args */ std::string>;
+    using DefaultHint = std::string;
+    using Default = UniValue;
+    using Fallback = std::variant<Optional, /* hint for default value */ DefaultHint, /* default constant value */ Default>;
     const std::string m_names; //!< The name of the arg (can be empty for inner args, can contain multiple aliases separated by | for named request arguments)
     const Type m_type;
     const bool m_hidden;