From 574189fb7d3297851a5d6e706ad8e58c1e536b60 Mon Sep 17 00:00:00 2001
From: Lixin Wei <lixin.wei@metabit-trading.com>
Date: Sun, 22 Mar 2026 23:28:32 +0800
Subject: [PATCH] Fix Cap'n Proto serialization of unsigned integer types

The Writer was casting all integral types to std::int64_t, which causes
uint64_t values like UINT64_MAX to overflow to -1 and trigger a Cap'n
Proto assertion: "expected value >= 0". This adds a std::is_unsigned
branch (before the std::is_integral fallback) in add_value_to_array,
add_value_to_object, and add_value_to_union so unsigned types are cast
to std::uint64_t instead.

Made-with: Cursor
---
 include/rfl/capnproto/Writer.hpp | 10 ++++++++++
 tests/capnproto/test_uint64.cpp  | 34 ++++++++++++++++++++++++++++++++
 2 files changed, 44 insertions(+)
 create mode 100644 tests/capnproto/test_uint64.cpp

diff --git a/include/rfl/capnproto/Writer.hpp b/include/rfl/capnproto/Writer.hpp
index c93f9621b..8d34f6b9c 100644
--- a/include/rfl/capnproto/Writer.hpp
+++ b/include/rfl/capnproto/Writer.hpp
@@ -171,6 +171,9 @@ class RFL_API Writer {
                          std::is_same<std::remove_cvref_t<T>, bool>()) {
       _parent->val_.set(_parent->ix_++, _var);
 
+    } else if constexpr (std::is_unsigned<std::remove_cvref_t<T>>()) {
+      _parent->val_.set(_parent->ix_++, static_cast<std::uint64_t>(_var));
+
     } else if constexpr (std::is_integral<std::remove_cvref_t<T>>()) {
       _parent->val_.set(_parent->ix_++, static_cast<std::int64_t>(_var));
 
@@ -211,6 +214,10 @@ class RFL_API Writer {
                          std::is_same<std::remove_cvref_t<T>, bool>()) {
       _parent->val_.set(to_kj_string_ptr(_name), _var);
 
+    } else if constexpr (std::is_unsigned<std::remove_cvref_t<T>>()) {
+      _parent->val_.set(to_kj_string_ptr(_name),
+                        static_cast<std::uint64_t>(_var));
+
     } else if constexpr (std::is_integral<std::remove_cvref_t<T>>()) {
       _parent->val_.set(to_kj_string_ptr(_name),
                         static_cast<std::int64_t>(_var));
@@ -242,6 +249,9 @@ class RFL_API Writer {
                          std::is_same<std::remove_cvref_t<T>, bool>()) {
       _parent->val_.set(field, _var);
 
+    } else if constexpr (std::is_unsigned<std::remove_cvref_t<T>>()) {
+      _parent->val_.set(field, static_cast<std::uint64_t>(_var));
+
     } else if constexpr (std::is_integral<std::remove_cvref_t<T>>()) {
       _parent->val_.set(field, static_cast<std::int64_t>(_var));
 
diff --git a/tests/capnproto/test_uint64.cpp b/tests/capnproto/test_uint64.cpp
new file mode 100644
index 000000000..df43c478f
--- /dev/null
+++ b/tests/capnproto/test_uint64.cpp
@@ -0,0 +1,34 @@
+#include <rfl.hpp>
+#include <cstdint>
+#include <climits>
+#include <string>
+
+#include "write_and_read.hpp"
+
+namespace test_uint64 {
+
+struct UnsignedStruct {
+  uint64_t x = UINT64_MAX;
+  uint32_t y = UINT32_MAX;
+  uint16_t z = UINT16_MAX;
+  uint8_t w = UINT8_MAX;
+};
+
+TEST(capnproto, test_uint64) {
+  const auto s = UnsignedStruct{};
+  write_and_read(s);
+}
+
+TEST(capnproto, test_uint64_specific_values) {
+  const auto s = UnsignedStruct{
+      .x = 0, .y = 0, .z = 0, .w = 0};
+  write_and_read(s);
+}
+
+TEST(capnproto, test_uint64_max_values) {
+  const auto s = UnsignedStruct{
+      .x = UINT64_MAX, .y = UINT32_MAX, .z = UINT16_MAX, .w = UINT8_MAX};
+  write_and_read(s);
+}
+
+}  // namespace test_uint64