[GSoC] Conflicted Built-in Trait Name

[François Dumont]

On 26/03/2023 04:01, Ken Matsui via Libstdc++ wrote:
> On Sat, Mar 25, 2023 at 5:38 AM Marc Glisse <marc.glisse@inria.fr> wrote:
>> On Sat, 25 Mar 2023, Ken Matsui via Gcc wrote:
>>
>>> Built-in trait naming simply adds two underscores (__) to the original
>>> trait name. However, the same names are already in use for some
>>> built-in traits, such as is_void, is_pointer, and is_signed.
>>>
>>> For example, __is_void is used in the following files:
>>>
>>> * gcc/testsuite/g++.dg/tm/pr46567.C
>> This is a testcase, you can rename __is_void to whatever in there, it
>> doesn't matter.
>>
>>> * libstdc++-v3/include/bits/cpp_type_traits.h
>> This __is_void seems to be used in a single place in
>> include/debug/helper_functions.h, couldn't we tweak that code so __is_void
>> becomes unused and can be removed?
> That worked. Thank you!
What worked ?
>
> So, we can remove a code in a header as long as it is not standard and
> is not used elsewhere, can't we?

You can do anything you like as long as you run the testsuite before 
presenting your patch. Here note that you'll need to run:

make check-debug

to run tests in _GLIBCXX_DEBUG mode which is making use of the code in 
helper_functions.h.

Clearly this usage of std::__is_void could be replaced with your builtin 
by reimplementing _Distance_traits like this:

   template<typename _Iterator,
        typename = typename std::__is_integer<_Iterator>::__type>
     struct _Distance_traits
     {
     private:
       typedef
     typename std::iterator_traits<_Iterator>::difference_type _ItDiffType;

       typedef
     typename std::conditional<__is_void<_ItDiffType>,
                   std::ptrdiff_t, _ItDiffType>::type _DiffType;

     public:
       typedef std::pair<_DiffType, _Distance_precision> __type;
     };

this is untested, just to give you an idea of what your patch could be.

François