Revert static_assert

Author: frederick-vs-ja

libcxx/include/__iterator/bounded_iter.h

@@ -52,11 +52,8 @@ _LIBCPP_BEGIN_NAMESPACE_STD
 //    pointer, it is undefined at the language level (see [expr.add]). If
 //    bounded iterators exhibited this undefined behavior, we risk compiler
 //    optimizations deleting non-redundant bounds checks.
-template <class _Iterator>
+template <class _Iterator, class = __enable_if_t< __libcpp_is_contiguous_iterator<_Iterator>::value > >
 struct __bounded_iter {
-  static_assert(__libcpp_is_contiguous_iterator<_Iterator>::value,
-                "Only contiguous iterators can be adapted by __bounded_iter.");
-
   using value_type        = typename iterator_traits<_Iterator>::value_type;
   using difference_type   = typename iterator_traits<_Iterator>::difference_type;
   using pointer           = typename iterator_traits<_Iterator>::pointer;
@@ -261,7 +258,7 @@ struct __bounded_iter {
 private:
   template <class>
   friend struct pointer_traits;
-  template <class>
+  template <class, class>
   friend struct __bounded_iter;
   _Iterator __current_;       // current iterator
   _Iterator __begin_, __end_; // valid range represented as [begin, end]