Features: Add cxx_variadic_templates.

Expect cxx_variadic_templates to implement N2555.

N2555 is essentially a bugfix and predates most compiler releases which
aimed to experimentally support variadic templates.

Tests/CompileFeatures/cxx_variadic_templates.cpp

@@ -0,0 +1,65 @@
+template<int I, int... Is>
+struct Interface;
+
+template<int I>
+struct Interface<I>
+{
+  static int accumulate()
+  {
+    return I;
+  }
+};
+
+template<int I, int... Is>
+struct Interface
+{
+  static int accumulate()
+  {
+    return I + Interface<Is...>::accumulate();
+  }
+};
+
+// Note: split this into a separate test if a
+// cxx_variadic_template_template_parameters feature is added.
+
+template<typename T>
+struct eval {
+  enum {
+    Matched = 0
+  };
+};
+
+template<template<typename...> class T, typename... U>
+struct eval<T<U...> > {
+  enum {
+    Matched = 1
+  };
+};
+
+template<typename...>
+struct A {
+
+};
+template<typename T>
+struct B {
+
+};
+template<typename T, typename U>
+struct C {
+
+};
+template<typename T, typename U, typename...>
+struct D {
+
+};
+
+// Note: This test assumes that a compiler supporting this feature
+// supports static_assert. Add a workaround if that does not hold.
+static_assert(eval<A<> >::Matched, "A Matches");
+static_assert(eval<A<int> >::Matched, "A Matches");
+static_assert(eval<A<int, char> >::Matched, "A Matches");
+static_assert(eval<B<int> >::Matched, "B Matches");
+static_assert(eval<C<int, char> >::Matched, "C Matches");
+static_assert(eval<D<int, char> >::Matched, "D Matches");
+static_assert(eval<D<int, char, bool> >::Matched, "D Matches");
+static_assert(eval<D<int, char, bool, double> >::Matched, "D Matches");