From dc44aad346720c143874ad45741df083e181ee6e Mon Sep 17 00:00:00 2001
From: V <sodexisback@gmail.com>
Date: Tue, 13 Jan 2026 20:07:05 +0300
Subject: [PATCH 1/2] Add uint<N> ctor from builtin_uint128 to avoid truncation

Add an implicit converting constructor for uint<N> (N >= 128) from builtin_uint128 so that uint256{__uint128_t} / static_cast<uint256>(__uint128_t) correctly zero-extend and preserve the high 64 bits instead of going through the variadic uint64_t-convertible ctor and truncating to 64 bits
---
 include/intx/intx.hpp | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/include/intx/intx.hpp b/include/intx/intx.hpp
index eb53d119..f33a7a14 100644
--- a/include/intx/intx.hpp
+++ b/include/intx/intx.hpp
@@ -891,6 +891,12 @@ struct uint
             words_[i] = x[i];
     }
 
+    #if INTX_HAS_BUILTIN_INT128
+    constexpr explicit(false) uint(builtin_uint128 x) noexcept
+      : words_{uint64_t(x), uint64_t(x >> 64)}
+    {}
+    #endif
+
     template <typename... T>
     constexpr explicit(false) uint(T... v) noexcept
         requires std::conjunction_v<std::is_convertible<T, uint64_t>...>

From 33fdc132f16ce4a25c372246852e4ea3df7a93ff Mon Sep 17 00:00:00 2001
From: Valeriy <sodexisback@gmail.com>
Date: Sat, 17 Jan 2026 20:15:07 +0300
Subject: [PATCH 2/2] Add test for uint256 conversion from builtin_uint128

Add test for uint256 conversion from builtin_uint128
---
 test/unittests/test_uint256.cpp | 14 ++++++++++++++
 1 file changed, 14 insertions(+)

diff --git a/test/unittests/test_uint256.cpp b/test/unittests/test_uint256.cpp
index f2bec334..1679fe34 100644
--- a/test/unittests/test_uint256.cpp
+++ b/test/unittests/test_uint256.cpp
@@ -226,3 +226,17 @@ TEST(uint256, mulmod)
     const auto b = 0x8c9f09b6227ba6542a97343c679e1d11d8bfa29228c18615c2_u256;
     EXPECT_EQ(mulmod(a, b, mod), 0xca283039a2ad0dbd3d60fbadb29e9c7a_u128);
 }
+
+#if INTX_HAS_BUILTIN_INT128
+TEST(uint256, conversion_from_builtin_uint128)
+{
+    const builtin_uint128 x = (builtin_uint128{1} << 64) | builtin_uint128{2};
+
+    const auto a = uint256{x};
+    const auto b = static_cast<uint256>(x);
+    const auto expected = uint256{uint64_t{2}, uint64_t{1}};
+
+    EXPECT_EQ(a, expected);
+    EXPECT_EQ(b, expected);
+}
+#endif