Index: include/type_traits
===================================================================
--- include/type_traits
+++ include/type_traits
@@ -838,7 +838,7 @@
 
 // is_convertible
 
-#if __has_feature(is_convertible_to)
+#if __has_feature(is_convertible_to) && !defined(_LIBCPP_USE_IS_CONVERTIBLE_FALLBACK)
 
 template <class _T1, class _T2> struct _LIBCPP_TYPE_VIS_ONLY is_convertible
     : public integral_constant<bool, __is_convertible_to(_T1, _T2) &&
@@ -900,6 +900,7 @@
 template <class _T1, class _T2> struct __is_convertible<_T1, _T2, 1, 0> : false_type {};
 
 template <class _T1> struct __is_convertible<_T1, const _T1&, 1, 0> : true_type {};
+template <class _T1> struct __is_convertible<const _T1, const _T1&, 1, 0> : true_type {};
 #ifndef _LIBCPP_HAS_NO_RVALUE_REFERENCES
 template <class _T1> struct __is_convertible<_T1, _T1&&, 1, 0> : true_type {};
 template <class _T1> struct __is_convertible<_T1, const _T1&&, 1, 0> : true_type {};