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4526. std::monostate could be a valid error for task

Section: 33.13.6.5 [task.promise] Status: New Submitter: Dietmar Kühl Opened: 2026-02-21 Last modified: 2026-02-27

Priority: Not Prioritized

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Discussion:

The current specification of the error-variant in 33.13.6.5 [task.promise] p2 uses std::monostate as a placeholder for the absence of an error: variant<monostate, remove_cvref_t<E>...>. However, std::monostate can be an actual error type. It should be replaced by an implementation-defined, default constructible type.

[2026-02-27; Reflector discussion.]

This issue seems to be resolved by 4339(i).

Proposed resolution:

This wording is relative to N5032.

  1. In 33.13.6.5 [task.promise] p2 replace monostate with an implementation-defined, default constructible type:

    -2- error-variant is a variant<monostateempty, remove_cvref_t<E>...>, with duplicate types removed, where empty is an implementation-defined, default constructible type and E... are the parameter types of the template arguments of the specialization of execution::completion_signatures denoted by error_types.