From 685ca67c64a1d75d6faef6aba2620d0a6bb25c7f Mon Sep 17 00:00:00 2001
From: "copilot-swe-agent[bot]" <198982749+Copilot@users.noreply.github.com>
Date: Wed, 15 Jul 2026 05:12:57 +0000
Subject: [PATCH] Fix Windows ARM64 overflow detection build

---
 src/util/mpz.cpp | 25 +++++++++++++++++++++++--
 1 file changed, 23 insertions(+), 2 deletions(-)

diff --git a/src/util/mpz.cpp b/src/util/mpz.cpp
index 2d3dd17640..f2e6f20604 100644
--- a/src/util/mpz.cpp
+++ b/src/util/mpz.cpp
@@ -31,12 +31,33 @@ static bool mul_overflows(int64_t a, int64_t b, int64_t & result) {
     return std::ckd_mul(&result, a, b);
 #elif defined(__GNUC__)
     return __builtin_mul_overflow(a, b, &result);
-#elif defined(_MSC_VER)
-    // MSVC _mul128 intrinsic
+#elif defined(_MSC_VER) && !defined(_M_ARM64)
+    // MSVC _mul128 intrinsic is unavailable when targeting ARM64.
     __int64 high;
     result = _mul128(a, b, &high);
     // Overflow if high bits are not the sign extension of result
     return high != (result >> 63);
+#elif defined(_MSC_VER)
+    if (a > 0) {
+        if (b > 0) {
+            if (a > INT64_MAX / b)
+                return true;
+        }
+        else if (b < INT64_MIN / a) {
+            return true;
+        }
+    }
+    else if (a < 0) {
+        if (b > 0) {
+            if (a < INT64_MIN / b)
+                return true;
+        }
+        else if (b < 0 && b < INT64_MAX / a) {
+            return true;
+        }
+    }
+    result = a * b;
+    return false;
 #else
     static_assert(false);
 #endif