Index: include/numeric
===================================================================
--- include/numeric
+++ include/numeric
@@ -532,17 +532,14 @@
 _LIBCPP_DISABLE_UBSAN_UNSIGNED_INTEGER_CHECK
 {
     using _Up = std::make_unsigned_t<_Tp>;
+    constexpr _Up __bitshift = std::numeric_limits<_Up>::digits - 1;
 
-    int __sign = 1;
-    _Up __m = __a;
-    _Up __M = __b;
-    if (__a > __b)
-    {
-        __sign = -1;
-        __m = __b;
-        __M = __a;
-    }
-     return __a + __sign * _Tp(_Up(__M-__m) >> 1);
+    _Up __diff = _Up(__b) - _Up(__a);
+    _Up __sign_bit = __b < __a;
+
+    _Up __half_diff = (__diff / 2) + (__sign_bit << __bitshift) + (__sign_bit & __diff);
+
+    return __a + __half_diff;
 }