Give tuple a constexpr default constructor.

llvm-svn: 159857
diff --git a/libcxx/include/tuple b/libcxx/include/tuple
index 683c9dd..a485d7d 100644
--- a/libcxx/include/tuple
+++ b/libcxx/include/tuple
@@ -227,7 +227,7 @@
 
     __tuple_leaf& operator=(const __tuple_leaf&);
 public:
-    _LIBCPP_INLINE_VISIBILITY __tuple_leaf() : value()
+    _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR __tuple_leaf() : value()
        {static_assert(!is_reference<_Hp>::value,
               "Attempted to default construct a reference element in a tuple");}
 
@@ -347,7 +347,7 @@
 
     __tuple_leaf& operator=(const __tuple_leaf&);
 public:
-    _LIBCPP_INLINE_VISIBILITY __tuple_leaf() {}
+    _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR __tuple_leaf() {}
 
     template <class _Alloc>
         _LIBCPP_INLINE_VISIBILITY
@@ -436,6 +436,9 @@
 struct __tuple_impl<__tuple_indices<_Indx...>, _Tp...>
     : public __tuple_leaf<_Indx, _Tp>...
 {
+    _LIBCPP_INLINE_VISIBILITY
+    _LIBCPP_CONSTEXPR __tuple_impl() {}
+
     template <size_t ..._Uf, class ..._Tf,
               size_t ..._Ul, class ..._Tl, class ..._Up>
         _LIBCPP_INLINE_VISIBILITY
@@ -532,6 +535,9 @@
 public:
 
     _LIBCPP_INLINE_VISIBILITY
+    _LIBCPP_CONSTEXPR tuple() {}
+
+    _LIBCPP_INLINE_VISIBILITY
     explicit tuple(const _Tp& ... __t)
         : base_(typename __make_tuple_indices<sizeof...(_Tp)>::type(),
                 typename __make_tuple_types<tuple, sizeof...(_Tp)>::type(),
@@ -687,7 +693,7 @@
 {
 public:
     _LIBCPP_INLINE_VISIBILITY
-    tuple() {}
+    _LIBCPP_CONSTEXPR tuple() {}
     template <class _Alloc>
     _LIBCPP_INLINE_VISIBILITY
         tuple(allocator_arg_t, const _Alloc&) {}