[libcxx-commits] [libcxx] [libc++] Make `constexpr std::variant`. Implement P2231R1 (PR #83335)

[Louis Dionne via libcxx-commits]

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@@ -798,23 +802,22 @@ public:
   using __base_type::operator=;
 
 protected:
-  template <size_t _Ip, class _Tp, class... _Args>
-  _LIBCPP_HIDE_FROM_ABI static _Tp& __construct_alt(__alt<_Ip, _Tp>& __a, _Args&&... __args) {
-    ::new ((void*)std::addressof(__a)) __alt<_Ip, _Tp>(in_place, std::forward<_Args>(__args)...);
-    return __a.__value;
-  }
-
   template <class _Rhs>
-  _LIBCPP_HIDE_FROM_ABI static void __generic_construct(__ctor& __lhs, _Rhs&& __rhs) {
+  _LIBCPP_HIDE_FROM_ABI static _LIBCPP_CONSTEXPR_SINCE_CXX20 void __generic_construct(__ctor& __lhs, _Rhs&& __rhs) {
     __lhs.__destroy();
     if (!__rhs.valueless_by_exception()) {
+      // We cannot directly construct at the target __alt because its direct enclosing union is not activated yet.
+      // We will get error if we did:
+      // construction of subobject of member '__tail' of union with active member '__dummy' is not allowed in a constant
+      // expression
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ldionne wrote:

This comment is useful within this review, but IMO doesn't need to live in the code.

https://github.com/llvm/llvm-project/pull/83335