Proposed resolution:

This wording is relative to N4958.

1. Modify [tuple.tuple] as indicated:

   namespace std {
     template<class... Types>
     class tuple {
       […]
     };
   
     […]
   }
   

   -?- If a program declares an explicit or partial specialization of tuple, the program is ill-formed, no diagnostic required.

2. Modify [variant.variant.general] as indicated:

   […]

   -3- A program that instantiates the definition of variant with no template arguments is ill-formed.

   -?- If a program declares an explicit or partial specialization of variant, the program is ill-formed, no diagnostic required.

[ 2023-11-11 Approved at November 2023 meeting in Kona. Status changed: Voting → WP. ]

[ 2023-11-02; Reflector poll ]

Set status to Tentatively Ready after five votes in favour during reflector poll.

Currently, program-defined specializations of std::tuple and std::variant are not explicitly disallowed. However, they can't be properly supported by standard library implementations, because the corresponding std::get function templates have to inspect the implementation details of these types, and users have no way to make std::get behave correctly for a program-defined specializations.

Perhaps we should explicitly disallow specializing std::tuple and std::variant.