SERVER-28551 Implementation of `void_t` in stdx.

There are a lot of template metaprograms and `std::enable_if` expressions
which we'd like to use in our codebase.  This import of a C++17 future
feature will help make them easier to write.

1 files changed, 26 insertions, 0 deletions

diff --git a/src/mongo/stdx/type_traits.h b/src/mongo/stdx/type_traits.h
index 9fc95279b98..40503382d96 100644
--- a/src/mongo/stdx/type_traits.h
+++ b/src/mongo/stdx/type_traits.h
@@ -52,5 +52,31 @@ using enable_if_t = typename std::enable_if<B, T>::type;
 
 }  // namespace stdx
 }  // namespace mongo
+#endif
+
+#if __cplusplus >= 201703
+
+namespace mongo {
+namespace stdx {
+
+using std::void_t;
+
+}  // namespace stdx
+}  // namespace mongo
 
+#elif __cplusplus >= 201402
+
+namespace mongo {
+namespace stdx {
+
+template <typename...>
+struct make_void {
+    using type = void;
+};
+
+template <typename... Args>
+using void_t = typename make_void<Args...>::type;
+
+}  // namespace stdx
+}  // namespace mongo
 #endif