Improving flat containers interface.

base/template_util.h

Index: base/template_util.h
diff --git a/base/template_util.h b/base/template_util.h
index 536cad8f3a3b6f0d77dc7d28076753470d2e90bc..874b8ea508070d9fb46e64bc178a81fe65bb76df 100644
--- a/base/template_util.h
+++ b/base/template_util.h
@@ -42,6 +42,19 @@ template <class T> struct is_non_const_reference<const T&> : std::false_type {};
 
 namespace internal {
 
+template <typename...>
+struct make_void {
+  using type = void;
+};
+
+// A clone of C++17 std::void_t.
+// Unlike the original version, we need |make_void| as a helper struct to avoid
+// a C++14 defect.
+// ref: http://en.cppreference.com/w/cpp/types/void_t
+// ref: http://open-std.org/JTC1/SC22/WG21/docs/cwg_defects.html#1558
+template <typename... Ts>
+using void_t = typename make_void<Ts...>::type;
+
 // Uses expression SFINAE to detect whether using operator<< would work.
 template <typename T, typename = void>
 struct SupportsOstreamOperator : std::false_type {};