Return type of std::declval<cv void> should be (cv-unqualified) void
Jiang An

Created on 2023-03-07.00:00:00 last changed 1 week ago


Date: 2023-03-22.22:40:39

Proposed resolution:

This wording is relative to N4928.

  1. Modify [utility.syn], header <utility> synopsis, and [declval] as indicated:

    template<class T>
      remove_cv_t<add_rvalue_reference_t<T>> declval() noexcept; // as unevaluated operand
Date: 2023-03-15.00:00:00

[ 2023-03-22; Reflector poll ]

Set priority to 4 after reflector poll. "The testcase isn't even valid with the previous 'conforming' libstdc++ implementation." "declval isn't an addressable function, so would prefer if this was ill-formed rather than complicating the definition for this case."

Date: 2023-03-07.00:00:00

Currently libc++ and libstdc++ determine the return type of std::declval like this:

template<class _Tp>
_Tp&& __declval_ret(int); // selected when _Tp is a referenceable type

template<class _Tp>
_Tp __declval_ret(long); // selected when _Tp is cv void

template<class _Tp>
decltype(__declval_ret<_Tp>(0)) declval() noexcept;

This strategy avoids instantiation of class templates. But it also drops cv-qualifiers of the return type when the type is cv void, which is different from the standard requirements. Such difference has no impact in normal use of std::declval, but is observable via decltype(std::declval<const void>) and its friends.

Given maintainers may think it's reasonable to keep the current implementation (see GCC Bugzilla #109049), it may be worthwhile to legitimate such strategy.

Should we make such construction ill-formed?

Date User Action Args
2023-03-22 22:40:39adminsetmessages: + msg13486
2023-03-12 13:25:52adminsetmessages: + msg13452
2023-03-07 00:00:00admincreate