Re: [GSoC] Conflicted Built-in Trait Name

[Jonathan Wakely <jwakely.gcc@gmail.com>]

On Mon, 27 Mar 2023 at 22:43, Jonathan Wakely <jwakely.gcc@gmail.com> wrote:
>
> On Mon, 27 Mar 2023 at 21:17, Ken Matsui via Libstdc++
> <libstdc++@gcc.gnu.org> wrote:
> >
> > On Mon, Mar 27, 2023 at 10:33 AM François Dumont <frs.dumont@gmail.com> wrote:
> > >
> > >
> > > 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 ?
> >
> > If you meant about the code, I sent a patch series, as you can see in
> > the following link.
> >
> > https://gcc.gnu.org/pipermail/gcc-patches/2023-March/614620.html
> >
> > * In the following patch, I replaced __is_void with std::is_void in
> > helper_functions.h.
>
> You can't do that. std::is_void is not present in C++98 mode, but the
> code in <debug/helper_functions.h> needs to work for C++98 mode.
>
> >   https://gcc.gnu.org/pipermail/gcc-patches/2023-March/614624.html
> > * And then, I deleted __is_void in cpp_type_traits.h.
> >   https://gcc.gnu.org/pipermail/gcc-patches/2023-March/614623.html
> > * Also, I replaced __is_void with ____is_void in the testcase.
> >   https://gcc.gnu.org/pipermail/gcc-patches/2023-March/614625.html
> >
> > If you meant what tests had been done, I did the following tests
> > because the header is under libstdc++.
> >
> > * make check-gcc-c++ (because the patches involve changes on GCC)
> > * make check-target-libstdc++-v3
> >
> > > > 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
> >
> > Thank you! I thought the tests that I did included the
> > hepler_function.h header, but they don’t...? If not, I must check if
> > std::is_void in the type_traits header was correctly used.

Something like this would work:

--- a/libstdc++-v3/include/debug/helper_functions.h
+++ b/libstdc++-v3/include/debug/helper_functions.h
@@ -66,13 +66,12 @@ namespace __gnu_debug
      typedef
       typename std::iterator_traits<_Iterator>::difference_type _ItDiffType;

-      template<typename _DiffType,
-              typename = typename std::__is_void<_DiffType>::__type>
+      template<typename _DiffType, typename = const _DiffType>
       struct _DiffTraits
       { typedef _DiffType __type; };

      template<typename _DiffType>
-       struct _DiffTraits<_DiffType, std::__true_type>
+       struct _DiffTraits<_DiffType, const void>
       { typedef std::ptrdiff_t __type; };

      typedef typename _DiffTraits<_ItDiffType>::__type _DiffType;