On 5 April 2013 21:12, François Dumont wrote:
>
> In fact my first attempt was a very simple one:
>
> template<typename _From, typename _To>
> class __is_convertible_helper<_From, _To, false>
> {
> template<typename _To1>
> static true_type
> __test(_To1);
>
> template<typename>
> static false_type
> __test(...);
>
> public:
> typedef decltype(__test<_To>(std::declval<_From>())) type;
> };
>
> But some tests failed like:
> In file included from
> /home/fdt/dev/gcc/build/x86_64-unknown-linux-gnu/libstdc++-v3/include/bits/move.h:57:0,
> from
> /home/fdt/dev/gcc/build/x86_64-unknown-linux-gnu/libstdc++-v3/include/bits/stl_pair.h:59,
> from
> /home/fdt/dev/gcc/build/x86_64-unknown-linux-gnu/libstdc++-v3/include/utility:70,
> from
> /home/fdt/dev/gcc/build/x86_64-unknown-linux-gnu/libstdc++-v3/include/tuple:38,
> from
> /home/fdt/dev/gcc/build/x86_64-unknown-linux-gnu/libstdc++-v3/include/functional:55,
> from
> /home/fdt/dev/gcc/src/libstdc++-v3/testsuite/20_util/bind/38889.cc:23:
> /home/fdt/dev/gcc/build/x86_64-unknown-linux-gnu/libstdc++-v3/include/type_traits:
> In instantiation of 'struct std::__is_convertible_helper<const
> std::tuple<std::_Placeholder<1> >&, std::_Placeholder<1>, false>':
> /home/fdt/dev/gcc/build/x86_64-unknown-linux-gnu/libstdc++-v3/include/type_traits:1321:12:
> required from 'struct std::is_convertible<const
> std::tuple<std::_Placeholder<1> >&, std::_Placeholder<1> >'
> /home/fdt/dev/gcc/build/x86_64-unknown-linux-gnu/libstdc++-v3/include/type_traits:111:12:
> required from 'struct std::__and_<std::is_convertible<const
> std::tuple<std::_Placeholder<1> >&, std::_Placeholder<1> > >'
> /home/fdt/dev/gcc/build/x86_64-unknown-linux-gnu/libstdc++-v3/include/tuple:400:40:
> required from 'struct std::_Bind<void (*(std::_Placeholder<1>))(int)>'
> /home/fdt/dev/gcc/src/libstdc++-v3/testsuite/20_util/bind/38889.cc:28:41:
> required from here
> /home/fdt/dev/gcc/build/x86_64-unknown-linux-gnu/libstdc++-v3/include/type_traits:1316:30:
> error: 'std::_Placeholder<1>' is an inaccessible base of
> 'std::tuple<std::_Placeholder<1> >'
> typedef decltype(__test<_To>(std::declval<_From>())) type;
> ^
> /home/fdt/dev/gcc/build/x86_64-unknown-linux-gnu/libstdc++-v3/include/type_traits:1309:2:
> error: initializing argument 1 of 'static std::true_type
> std::__is_convertible_helper<_From, _To, false>::__test(_To1) [with _To1 =
> std::_Placeholder<1>; _From = const std::tuple<std::_Placeholder<1> >&; _To
> = std::_Placeholder<1>; std::true_type = std::integral_constant<bool,
> true>]'
> __test(_To1);
> ^
>
> From my point of view this is an other example of use case for which gcc is
> not SFINAE friendly enough, no ?
No, I don't think this is a GCC problem. In this code the
derived-to-base conversion does not happen in the context of the
function template argument deduction but happens afterwards, so the
access failure is not part of argument deduction and so is a hard
error not a substitution failure.
> But the version with the default template parameter is fine and more
> consistent with the other helpers implementation so, adopted! Here is an
> other version of the patch for validation.
>
> Daniel, I agree that inheritance with integral_constant is not as
> obvious as before but it is still there and it is just what the compiler
> need.
I assume Daniel's reply was an HTML mail and didn't make it to the
list, was there an objection to the change or a general comment?
> I even hope that it also simplified a (very) little bit the job for
> the compiler.
I don't know if the compiler's job is easier or not, but I think with
your change the template instantiation depth is increased by one, with
your change we get false_type instantiated by the instantiation of
is_convertible, rather than being done after it using its value
member.
> Ok to commit ?
I'd like to hear Daniel's comment first, but if we don't hear from him
please commit it in 24 hours. Thanks.
