types/traits: add inner_type query for ::value_type

types/traits.hpp

@@ -328,4 +328,28 @@ struct is_contiguous<std::vector<T,Allocator>>: public std::true_type {};
 template <typename T>
 constexpr auto is_contiguous_v = is_contiguous<T>::value;
 
+
+///////////////////////////////////////////////////////////////////////////////
+/// stores the type of '::value_type' for a given type, or the type itself if
+/// it does not have such a type.
+template <typename ValueT, typename = std::void_t<>>
+struct inner_type {
+    using type = ValueT;
+};
+
+
+//-----------------------------------------------------------------------------
+template <typename ValueT>
+struct inner_type<
+    ValueT,
+    std::void_t<typename ValueT::value_type>
+> {
+    using type = typename ValueT::value_type;
+};
+
+
+//-----------------------------------------------------------------------------
+template <typename ValueT>
+using inner_type_t = typename inner_type<ValueT>::type;
+
 #endif