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From: "ppalka at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug libstdc++/100577] New: [11/12 Regression] Unhelpful diagnostics
 for ill-formed call to partially applied range adaptor object
Date: Thu, 13 May 2021 02:16:25 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D100577

            Bug ID: 100577
           Summary: [11/12 Regression] Unhelpful diagnostics for
                    ill-formed call to partially applied range adaptor
                    object
           Product: gcc
           Version: 11.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: libstdc++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: ppalka at gcc dot gnu.org
  Target Milestone: ---

For the following invalid testcase:

#include <ranges>

int main() {
  extern int x[100];
  std::views::take(nullptr)(x);
}

GCC 10 emits the following errors:

In file included from <stdin>:1:=20=20=20=20=20=20=20=20=20=20=20=20=20=20=
=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=
=20=20=20=20=20=20=20=20=20
/usr/include/c++/10/ranges: In instantiation of
=E2=80=98std::ranges::views::<lambda(_Range&&, _Tp&&)> [with _Range =3D int=
 (&)[100]; _Tp
=3D std::nullptr_t]=E2=80=99:=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=
=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=
=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20
/usr/include/c++/10/ranges:1142:27:   required from
=E2=80=98std::ranges::views::__adaptor::_RangeAdaptor<_Callable>::operator(=
)<{std::nullptr_t}>::<lambda(_Range&&)>
[with _Range =3D int (&)[100]]=E2=80=99=20=20=20=20=20=20=20=20=20=20=20=20=
=20=20=20=20=20=20=20=20=20=20=20
/usr/include/c++/10/ranges:1162:44:   required from =E2=80=98constexpr auto
std::ranges::views::__adaptor::_RangeAdaptorClosure<_Callable>::operator()(=
_Range&&)
const [with _Range =3D int (&)[100]; _Callable =3D std::ranges
::views::__adaptor::_RangeAdaptor<_Callable>::operator()<{std::nullptr_t}>:=
:<lambda(_Range&&)>]=E2=80=99=20
<stdin>:5:30:   required from here=20=20=20=20=20=20=20=20=20=20=20=20=20=
=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=
=20=20=20=20=20=20=20=20
/usr/include/c++/10/ranges:2148:68: error: class template argument deduction
failed:=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=
=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=
=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=
=20
 2148 |  return take_view{std::forward<_Range>(__r), std::forward<_Tp>(__n)=
};=20=20=20
      |                                                                    =
^=20=20=20=20
/usr/include/c++/10/ranges:2148:68: error: no matching function for call to
=E2=80=98take_view(int [100], std::nullptr_t)=E2=80=99=20=20=20=20=20=20=20=
=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=
=20=20=20=20=20=20=20=20=20
                                                       [...]
/usr/include/c++/10/ranges:2140:5: note: candidate:
=E2=80=98std::ranges::take_view(_Range&&, std::ranges::range_difference_t<_=
Range>)->
std::ranges::take_view<std::ranges::views::all_t<_Range> > [with _Range =3D=
 int
(&)[
100]; std::ranges::views::all_t<_Range> =3D std::ranges::ref_view<int [100]=
>;
std::ranges::range_difference_t<_Range> =3D long int]=E2=80=99=20=20=20=20=
=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20
 2140 |     take_view(_Range&&, range_difference_t<_Range>)=20=20=20=20=20=
=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20
      |     ^~~~~~~~~=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=
=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=
=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20


Whereas GCC 11, after r11-8053, emits an almost useless error:

<stdin>:5:28: error: use of deleted function =E2=80=98constexpr auto
std::ranges::views::__adaptor::_Partial<_Adaptor, _Arg>::operator()(_Range&=
&)
const && [with _Range =3D int (&)[100]; _Adaptor =3D std::ranges::views::_T=
ake;
_Arg =3D std::nullptr_t]=E2=80=99
In file included from <stdin>:1:
/home/patrick/code/gcc-master/libstdc++-v3/include/std/ranges:865:9: note:
declared here
  865 |         operator()(_Range&& __r) const && =3D delete;
      |         ^~~~~~~~=