Index: include/tuple
===================================================================
--- include/tuple
+++ include/tuple
@@ -706,7 +706,7 @@
               typename enable_if
                       <
                          _CheckArgsConstructor<
-                             sizeof...(_Up) <= sizeof...(_Tp)
+                             sizeof...(_Up) == sizeof...(_Tp)
                              && !_PackExpandsToThisTuple<_Up...>::value
                          >::template __enable_implicit<_Up...>(),
                          bool
@@ -735,7 +735,11 @@
                          _CheckArgsConstructor<
                              sizeof...(_Up) <= sizeof...(_Tp)
                              && !_PackExpandsToThisTuple<_Up...>::value
-                         >::template __enable_explicit<_Up...>(),
+                         >::template __enable_explicit<_Up...>() ||
+                         _CheckArgsConstructor<
+                             sizeof...(_Up) < sizeof...(_Tp)
+                             && !_PackExpandsToThisTuple<_Up...>::value
+                         >::template __enable_implicit<_Up...>(),
                          bool
                       >::type = false
              >
Index: test/std/utilities/tuple/tuple.tuple/tuple.cnstr/PR27374_implicit_reduced_arity_extension_non_conforming.pass.cpp
===================================================================
--- /dev/null
+++ test/std/utilities/tuple/tuple.tuple/tuple.cnstr/PR27374_implicit_reduced_arity_extension_non_conforming.pass.cpp
@@ -0,0 +1,53 @@
+//===----------------------------------------------------------------------===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is dual licensed under the MIT and the University of Illinois Open
+// Source Licenses. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+//
+// UNSUPPORTED: c++98, c++03
+
+// <tuple>
+
+// template <class... Types> class tuple;
+
+// Check that the reduced arity constructors are never implicit.
+// See also:
+//  http://llvm.org/PR27374
+//  http://wg21.link/lwg2419
+
+#include <tuple>
+#include <string>
+#include <cassert>
+
+bool eat_tuple(const std::tuple<char, char>& w) { return false; }
+template <class ...Args>
+auto eat_char(Args&&... args) -> decltype(eat_tuple(args...)) { return eat_tuple(args...); }
+bool eat_char(const char&) { return true; }
+
+int count(std::tuple<std::string, std::string>) {
+    return 2;
+}
+
+int count(std::tuple<std::string, std::string, std::string>) {
+   return 3;
+}
+
+using Tup = std::tuple<int, int, int>;
+Tup explicit_still_allowed() { return Tup{1, 2}; }
+
+int main() {
+    {
+        const char ch = 'a';
+        assert(eat_char(ch));
+    }
+    {
+        assert(count({ "hungry", "zombies" }) == 2);
+        assert(count({ "cute", "fluffy", "kittens" }) == 3);
+    }
+    {
+        explicit_still_allowed();
+    }
+}