c++ - Why does C++17's std::any not allow returning a movable value by any_cast? -


while implementing c++17's std::any according specification available in wiki stumbled across seemed nonsensical me:

in definition of free function std::any_cast, used retrieve values std::any instance, overload r-value references supplied (it's third one):

template< class valuetype > valuetype any_cast(any&& operand); // (3)

now, there requirement listed below synopsis applies overloads 2 , 3 (that means including r-value overload):

2-3) returns *any_cast<std::remove_reference_t<valuetype>>(&operand)

the definition does not seem allow moving data!

the function call redirected pointer-based overload; information temporary nature of operand is lost!

is intended can't move out of instance? error in wiki? wrong here?

the issue in wp status @ time of writing this, which means:

wp - (working paper) - proposed resolution has not been accepted technical corrigendum, full wg21/pl22.16 committee has voted apply defect report's proposed resolution working paper.

see lwg here more info: http://wg21.cmeerw.net/lwg/issue2509

a proposed resolution indeed

for third form, if is_move_constructible_v<valuetype> true , is_lvalue_reference_v<valuetype> false, std::move(*any_cast<remove_reference_t<valuetype>>(&operand)), otherwise, *any_cast<remove_reference_t<valuetype>>(&operand)

and defect report list listing wp: http://cplusplus.github.io/lwg/lwg-defects.html#2509


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