nit: If touching this file again, may want to consider defining a default argument for cookie_perms. Since std::optional is used, I'm assuming the intent is not to force the caller to provide an optional object.
diff --git a/src/rpc/request.h b/src/rpc/request.h
index b40df16631..c7e723d962 100644
--- a/src/rpc/request.h
+++ b/src/rpc/request.h
@@ -19,7 +19,7 @@ std::string JSONRPCReply(const UniValue& result, const UniValue& error, const Un
UniValue JSONRPCError(int code, const std::string& message);
/** Generate a new RPC authentication cookie and write it to disk */
-bool GenerateAuthCookie(std::string* cookie_out, std::optional<fs::perms> cookie_perms);
+bool GenerateAuthCookie(std::string* cookie_out, std::optional<fs::perms> cookie_perms=std::nullopt);
/** Read the RPC authentication cookie from disk */
bool GetAuthCookie(std::string *cookie_out);
/** Delete RPC authentication cookie from disk */
GenerateAuthCookie() is currently used with the argument provided, so not a big deal either way.