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3990. Program-defined specializations of std::tuple and std::variant can't be properly supported

Section: 22.4.4 [tuple.tuple], 22.6.3.1 [variant.variant.general] Status: WP Submitter: Jiang An Opened: 2023-08-29 Last modified: 2023-11-22

Priority: Not Prioritized

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Discussion:

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.

[2023-11-02; Reflector poll]

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

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

Proposed resolution:

This wording is relative to N4958.

1. Modify 22.4.4 [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 22.6.3.1 [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.