Index: libcxx/include/__debug
===================================================================
--- libcxx/include/__debug
+++ libcxx/include/__debug
@@ -40,7 +40,11 @@
 #endif
 
 #ifndef _LIBCPP_ASSERT
-#   define _LIBCPP_ASSERT(x, m) ((void)0)
+#   ifdef _LIBCPP_HARDEN
+#      define _LIBCPP_ASSERT(x, m) ((x) ? (void)0 : __builtin_trap())
+#   else
+#      define _LIBCPP_ASSERT(x, m) ((void)0)
+#   endif
 #endif
 #ifndef _LIBCPP_DEBUG_ASSERT
 #   define _LIBCPP_DEBUG_ASSERT(x, m) ((void)0)
Index: libcxx/include/string_view
===================================================================
--- libcxx/include/string_view
+++ libcxx/include/string_view
@@ -278,7 +278,9 @@
 
     // [string.view.access], element access
     _LIBCPP_CONSTEXPR _LIBCPP_INLINE_VISIBILITY
-    const_reference operator[](size_type __pos) const _NOEXCEPT { return __data[__pos]; }
+    const_reference operator[](size_type __pos) const _NOEXCEPT {
+      return _LIBCPP_ASSERT(size() == 0 || __pos < size(), "string_view[] index out of bounds"), __data[__pos];
+    }
 
     _LIBCPP_CONSTEXPR _LIBCPP_INLINE_VISIBILITY
     const_reference at(size_type __pos) const