[PATCH] Use _GLIBCXX_NOEXCEPT_IF for std::swap

[Jonathan Wakely]

On 02/05/19 20:11 +0100, Jonathan Wakely wrote:
>On 02/05/19 20:09 +0100, Jonathan Wakely wrote:
>>On 02/05/19 20:34 +0200, Stephan Bergmann wrote:
>>>On 29/04/2019 15:26, Jonathan Wakely wrote:
>>>>Â Â Â Â * include/bits/move.h (swap(T&, T&), swap(T (&)[N], T (&)[N])): Use
>>>>Â Â Â Â _GLIBCXX_NOEXCEPT_IF to simplify declarations.
>>>>
>>>>This just avoids having to repeat the name and parameter-list of the
>>>>functions.
>>>
>>>libstdc++-v3/include/std/type_traits still has
>>>
>>>>template<typename _Tp>
>>>>  inline
>>>>  typename enable_if<__and_<__not_<__is_tuple_like<_Tp>>,
>>>>			      is_move_constructible<_Tp>,
>>>>			      is_move_assignable<_Tp>>::value>::type
>>>>  swap(_Tp&, _Tp&)
>>>>  noexcept(__and_<is_nothrow_move_constructible<_Tp>,
>>>>	            is_nothrow_move_assignable<_Tp>>::value);
>>>
>>>using noexcept instead of _GLIBCXX_NOEXPECT_IF, and at least 
>>>during configure of building LibreOffice with Clang, that causes 
>>>failure
>>>
>>>>.../gcc/trunk/lib/gcc/x86_64-pc-linux-gnu/10.0.0/../../../../include/c++/10.0.0/bits/move.h:185:5: error: exception specification in declaration does not match previous declaration
>>>>  swap(_Tp& __a, _Tp& __b)
>>>>  ^
>>>>.../gcc/trunk/lib/gcc/x86_64-pc-linux-gnu/10.0.0/../../../../include/c++/10.0.0/type_traits:2531:5: note: previous declaration is here
>>>>  swap(_Tp&, _Tp&)
>>>>  ^
>>>
>>>I didn't try to track down under what conditions 
>>>_GLIBCXX_NOEXCEPT_IF would not expand to noexcept, but I assume 
>>>that just type_traits needs adapting, too?
>>
>>That's weird. _GLIBCXX_NOEXCEPT_IF is only empty for C++11, in which
>>case the declaration in <type_traits> isn't seen anyway. And there's
>>no point using _GLIBCXX_NOEXCEPT_IF in <type_traits> because we can
>>(and should) just use noexcept directly in C++11 code. The macro
>>exists for code that needs to be compiled as C++98 too.
>>
>>The only difference is that there's an extra set of parentheses around
>>the NOEXCEPT_IF condition, so the preprocessor doesn't try to eat the
>>comma.
>>
>>So one declaration is:
>>
>>template<typename _Tp>
>>  inline
>>  typename enable_if<__and_<__not_<__is_tuple_like<_Tp>>,
>>			      is_move_constructible<_Tp>,
>>			      is_move_assignable<_Tp>>::value>::type
>>  swap(_Tp&, _Tp&)
>>  noexcept(__and_<is_nothrow_move_constructible<_Tp>,
>>	            is_nothrow_move_assignable<_Tp>>::value);
>>
>>And the other is:
>>
>>template<typename _Tp>
>>  inline
>>  typename enable_if<__and_<__not_<__is_tuple_like<_Tp>>,
>>			      is_move_constructible<_Tp>,
>>			      is_move_assignable<_Tp>>::value>::type
>>  swap(_Tp&, _Tp&)
>>  noexcept((__and_<is_nothrow_move_constructible<_Tp>,
>>	            is_nothrow_move_assignable<_Tp>>::value));
>
>Yep, Clang doesn't like that:
>https://wandbox.org/permlink/clslE9PGCVtKPppz

This was also reported as https://gcc.gnu.org/PR90314