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Asked: 2026-05-28T07:33:10+00:00

§20.2.4 [declval] template <class T> typename add_rvalue_reference<T>::type declval() noexcept; // as unevaluated operand Why

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§20.2.4 [declval]

template <class T>
typename add_rvalue_reference<T>::type declval() noexcept; // as unevaluated operand

Why use add_rvalue_reference here?

From §20.9.7.2 [meta.trans.ref] on add_rvalue_reference:

If T names an object or function type then the member typedef type shall name T&&; otherwise, type shall name T. [ Note: This rule reflects the semantics of reference collapsing (8.3.2). For example, when a type T names a type T1&, the type add_rvalue_reference<T>::type is not an rvalue reference. —end note ]

Since add_rvalue_reference is meant to reflect reference collapsing anyways, why not just use T&& like the following?

template<class T>
T&& declval();

What could go wrong? What exactly are the differences between the two versions?

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1 Answer

  1. Editorial Team

    I don’t know if this is the actual reason, but add_rvalue_reference has different behavior for void.

    add_rvalue_reference<void>::type is simply void.

    void&& is an error.

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