diff --git a/libcxx/include/iterator b/libcxx/include/iterator
--- a/libcxx/include/iterator
+++ b/libcxx/include/iterator
@@ -538,10 +538,10 @@
 inline _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX14
 void advance(_InputIter& __i, _Distance __orig_n)
 {
-    _LIBCPP_ASSERT(__orig_n >= 0 || __is_cpp17_bidirectional_iterator<_InputIter>::value,
-                   "Attempt to advance(it, n) with negative n on a non-bidirectional iterator");
     typedef decltype(_VSTD::__convert_to_integral(__orig_n)) _IntegralSize;
     _IntegralSize __n = __orig_n;
+    _LIBCPP_ASSERT(__n >= 0 || __is_cpp17_bidirectional_iterator<_InputIter>::value,
+                   "Attempt to advance(it, n) with negative n on a non-bidirectional iterator");
     _VSTD::__advance(__i, __n, typename iterator_traits<_InputIter>::iterator_category());
 }
 
diff --git a/libcxx/test/std/iterators/iterator.primitives/iterator.operations/robust_against_adl.pass.cpp b/libcxx/test/std/iterators/iterator.primitives/iterator.operations/robust_against_adl.pass.cpp
--- a/libcxx/test/std/iterators/iterator.primitives/iterator.operations/robust_against_adl.pass.cpp
+++ b/libcxx/test/std/iterators/iterator.primitives/iterator.operations/robust_against_adl.pass.cpp
@@ -6,8 +6,6 @@
 //
 //===----------------------------------------------------------------------===//
 
-// XFAIL: LIBCXX-DEBUG-FIXME
-
 // <iterator>
 
 #include <iterator>