[libcxx] Fix PR24779 -- Prevent evaluation of std::is_default_constructible in clearly ill-formed tuple constructor.

[EricWF]

Summary

This patch fixes PR24779 (https://llvm.org/bugs/show_bug.cgi?id=24779)

[EricWF]

This probably isn't quite right. I need to think about it for a minute.

[EricWF]

This change is garbage. Sorry for the noise.

include/tuple

                          >::value &&
                          __all_default_constructible<
                             typename __make_tuple_types<tuple, sizeof...(_Tp),
-                                sizeof...(_Up) < sizeof...(_Tp) ?
+                                sizeof...(_Up) < sizeof...(_Tp)
+                                && sizeof...(_Up) != 0 ?
                                     sizeof...(_Up) :
                                     sizeof...(_Tp)>::type
                          >::value

test/std/utilities/tuple/tuple.tuple/tuple.cnstr/alloc_UTypes_PR24779.pass.cpp

+//===----------------------------------------------------------------------===//
+//
+//                     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.
+//
+//===----------------------------------------------------------------------===//
+
+// <tuple>
+
+// template <class... Types> class tuple;
+
+// template <class Alloc, class... UTypes>
+//   tuple(allocator_arg_t, const Alloc& a, UTypes&&...);
+
+// UNSUPPORTED: c++98, c++03
+
+// Check that this constructor does not evaluate
+// std::is_default_constructible<T> for any T in Types.
+
+#include <memory>
+#include <tuple>
+
+template <class Tp = void>
+struct BadDefaultCtor {
+    constexpr BadDefaultCtor() {
+        static_assert(!std::is_same<Tp, void>::value, "");
+    }
+
+    explicit BadDefaultCtor(int) {}
+};
+
+void DeleteFn(int*) {}
+
+int main() {
+    typedef BadDefaultCtor<> Type;
+    typedef std::tuple<Type, Type> Tuple;
+    Tuple t(Type(0), Type(0));
+}