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2958. Moves improperly defined as deleted

Section: 22.3.2 [pairs.pair], 22.4.4.2 [tuple.assign] Status: C++20 Submitter: Casey Carter Opened: 2017-05-05 Last modified: 2021-02-25

Priority: 2

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Discussion:

LWG 2729 constrained many pair & tuple assignment operators to "not participate in overload resolution" and "be defined as deleted if." As discussed when LWG reviewed 2756, it is undesirable to require that a move constructor or assignment operator be "defined as deleted," since it is unclear whether that operation should be implicitly deleted, and therefore not participate in overload resolution, or explicitly deleted and therefore impede overload resolution for usages that would otherwise find copies.

[2017-07 Toronto Wed Issue Prioritization]

Priority 2

[2017-11 Albuquerque Wednesday issue processing]

Move to Immediate

Proposed resolution:

This wording is relative to N4659.

1. Edit 22.3.2 [pairs.pair] as indicated:

   pair& operator=(pair&& p) noexcept(see below);
   

   -21- Effects: Assigns to first with std::forward<first_type>(p.first) and to second with std::forward<second_type>(p.second).

   -22- Remarks: This operator shall be defined as deletednot participate in overload resolution unless is_move_assignable_v<first_type> is true and is_move_assignable_v<second_type> is true.

   […]

2. Edit 22.4.4.2 [tuple.assign] as indicated:

   tuple& operator=(tuple&& u) noexcept(see below);
   

   -5- Effects: For all i, assigns std::forward<T_i>(get<i>(u)) to get<i>(*this).

   -6- Remarks: This operator shall be defined as deletednot participate in overload resolution unless is_move_assignable_v<T_i> is true for all i.

   […]