Re: [PATCH] PR78991 make __gnu_cxx::__ops constructors explicit

[Jonathan Wakely <jwakely at redhat dot com>]

On 09/01/17 10:39 +0000, Kyrill Tkachov wrote:
> Hi Jonathan,
>
> On 06/01/17 12:40, Jonathan Wakely wrote:
> > This solves a problem when using libstdc++ with Clang, due to Clang
> > more eagerly instantiating constexpr function templates during
> > argument deduction. G++ has some shortcuts to avoid this problem, but
> > Clang doesn't, and it's not clear that it's strictly speaking a bug in
> > Clang or if it's following the standard. By making these constructors
> > explicit we stop them being considered by overload resolution for
> > copying these functors, which stops us ending up back in the
> > std::function SFINAE checks.
> >
> > I'm also using _GLIBCXX_MOVE to turn some internal copies into moves,
> > because otherwise using something like std::function with <algorithm>
> > results in a number of potentially expensive copies.
> >
> >    PR libstdc++/78991
> >    * include/bits/predefined_ops.h (_Iter_comp_iter, _Iter_comp_val)
> >    (_Val_comp_iter, _Iter_equals_val, _Iter_pred, _Iter_comp_to_val)
> >    (_Iter_comp_to_iter, _Iter_negate): Make constructors explicit and
> >    move function objects.
> >    (__iter_comp_iter, __iter_comp_val, __val_comp_iter, __pred_iter)
> >    (__iter_comp_val, __iter_comp_iter, __negate): Move function objects.
> >    * testsuite/25_algorithms/sort/78991.cc: New test.
> >
> > Tested powerpc64le-linux, committed to trunk.
> >
> > I'll backport  the 'explicit' constructors (but not the _GLIBCXX_MOVE
> > changes) to the branches too.
>
> I see this test fail on the GCC 5 branch on arm and aarch64 (error message pasted below).
> Does the test need a gnu++11 guard or something on the branch?

I thought I'd changed that before committing, I'll fix it.