[PATCH] Use _GLIBCXX_NOEXCEPT_IF for std::swap

[Jonathan Wakely]

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));

Does adding the extra parens into type_traits fix it? i.e.

--- a/libstdc++-v3/include/std/type_traits
+++ b/libstdc++-v3/include/std/type_traits
@@ -2529,8 +2529,8 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
                              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);
+    noexcept((__and_<is_nothrow_move_constructible<_Tp>,
+                   is_nothrow_move_assignable<_Tp>>::value));

   template<typename _Tp, size_t _Nm>
     inline