diff --git a/include/gsl/string_span b/include/gsl/string_span
index b5dfd18..85112e5 100644
--- a/include/gsl/string_span
+++ b/include/gsl/string_span
@@ -175,6 +175,7 @@ class basic_string_span
 {
 public:
     using element_type = CharT;
+    using value_type = std::remove_cv_t<element_type>;
     using pointer = std::add_pointer_t<element_type>;
     using reference = std::add_lvalue_reference_t<element_type>;
     using const_reference = std::add_lvalue_reference_t<std::add_const_t<element_type>>;
@@ -186,6 +187,8 @@ public:
     using reverse_iterator = typename impl_type::reverse_iterator;
     using const_reverse_iterator = typename impl_type::const_reverse_iterator;
 
+    using size_type = index_type;
+
     // default (empty)
     constexpr basic_string_span() noexcept = default;