Index: libcxx/include/__numeric/gcd_lcm.h
===================================================================
--- libcxx/include/__numeric/gcd_lcm.h
+++ libcxx/include/__numeric/gcd_lcm.h
@@ -12,6 +12,10 @@
 
 #include <__assert>
 #include <__config>
+#if _LIBCPP_STD_VER >= 20
+#  include <__algorithm/min.h>
+#  include <__bit/countr.h>
+#endif
 #include <__type_traits/common_type.h>
 #include <__type_traits/is_integral.h>
 #include <__type_traits/is_same.h>
@@ -49,13 +53,30 @@
     _Result operator()(_Source __t) const noexcept { return __t; }
 };
 
-
-template<class _Tp>
-_LIBCPP_CONSTEXPR _LIBCPP_HIDDEN
-_Tp __gcd(_Tp __m, _Tp __n)
-{
+template <class _Tp>
+_LIBCPP_CONSTEXPR _LIBCPP_HIDDEN _Tp __gcd(_Tp __m, _Tp __n) {
     static_assert((!is_signed<_Tp>::value), "");
+#  if _LIBCPP_STD_VER >= 20
+    if (__m == 0)
+        return __n;
+    if (__n == 0)
+        return __m;
+
+    int __az    = _VSTD::countr_zero(__m);
+    int __bz    = _VSTD::countr_zero(__n);
+    int __shift = _VSTD::min(__az, __bz);
+    __m >>= __az, __n >>= __bz;
+    while (__m != 0) {
+        int __diff    = __m - __n;
+        __n           = _VSTD::min(__m, __n);
+        __m           = __diff >= 0 ? __diff : -__diff;
+        int __m_shift = _VSTD::min(_VSTD::countr_zero(__m), _VSTD::numeric_limits<_Tp>::digits - 1);
+        __m >>= __m_shift;
+    }
+    return __n << __shift;
+#  else
     return __n == 0 ? __m : _VSTD::__gcd<_Tp>(__n, __m % __n);
+#  endif
 }
 
 template<class _Tp, class _Up>
Index: libcxx/test/libcxx/transitive_includes/cxx2b.csv
===================================================================
--- libcxx/test/libcxx/transitive_includes/cxx2b.csv
+++ libcxx/test/libcxx/transitive_includes/cxx2b.csv
@@ -432,6 +432,7 @@
 numbers version
 numeric cmath
 numeric cstddef
+numeric initializer_list
 numeric limits
 numeric version
 optional compare