[libcxx-commits] [libcxx] [libc++] P2255R2: Add deleted tuple constructor overloads (PR #205379)

[Yihan Wang via libcxx-commits]

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@@ -604,12 +604,38 @@ public:
       : __base_(allocator_arg_t(), __a, __forward_args{}, __t...) {}
 
   // tuple(U&& ...) constructors (including allocator_arg_t variants)
+  template <class... _Up>
+  struct _IsThisTuple : false_type {};
+  template <class _Up>
+  struct _IsThisTuple<_Up> : is_same<__remove_cvref_t<_Up>, tuple> {};
+
   template <class... _Up>
   struct _EnableUTypesCtor
-      : _And<_Or<integral_constant<bool, sizeof...(_Up) != 1>,
-                 _IsNotSame<__remove_cvref_t<_Up...[0]>, tuple>>, // extension to allow mis-behaved user constructors
+      : _And<_Not<_IsThisTuple<_Up...> >, // extension to allow mis-behaved user constructors
              is_constructible<_Tp, _Up>... > {};
 
+#    if _LIBCPP_STD_VER >= 23
+  template <class... _Up>
+  static constexpr bool __has_danglings() {
+    if constexpr (sizeof...(_Up) == sizeof...(_Tp))
+      return (__reference_constructs_from_temporary_v<_Tp, _Up&&> || ...);
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yronglin wrote:

Good catch, we expect lazy instantiation, so `_Or` is more appropriate here.

https://github.com/llvm/llvm-project/pull/205379