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Re: functional and type_traits cleanup

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/
> /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/
> 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

> 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.

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