Re: [PATCH] libstdc++: suppress -Wdangling-reference with operator| [PR111410]

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

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On Sat, 20 Jan 2024, 03:47 Marek Polacek, <polacek@redhat.com> wrote:

> Bootstrapped/regtested on x86_64-pc-linux-gnu, ok for trunk?
>

OK, thanks.

The standard ranges have their own protection against dangling via the
opt-in borrowed_range concept, and algorithms that don't allow returning
iterators into rvalue ranges, and the automatic use of ref_view or
owning_view as needed. So I think it's reasonable to assume they are less
prone to the bugs this warning detects, at least when used idiomatically.


> -- >8 --
> It seems to me that we should exclude
> std::ranges::views::__adaptor::operator|
> from the -Wdangling-reference warning.  It's commonly used when handling
> ranges.
>
>         PR c++/111410
>
> libstdc++-v3/ChangeLog:
>
>         * include/std/ranges: Add #pragma to disable -Wdangling-reference
> with
>         std::ranges::views::__adaptor::operator|.
>
> gcc/testsuite/ChangeLog:
>
>         * g++.dg/warn/Wdangling-reference17.C: New test.
> ---
>  gcc/testsuite/g++.dg/warn/Wdangling-reference17.C | 15 +++++++++++++++
>  libstdc++-v3/include/std/ranges                   |  3 +++
>  2 files changed, 18 insertions(+)
>  create mode 100644 gcc/testsuite/g++.dg/warn/Wdangling-reference17.C
>
> diff --git a/gcc/testsuite/g++.dg/warn/Wdangling-reference17.C
> b/gcc/testsuite/g++.dg/warn/Wdangling-reference17.C
> new file mode 100644
> index 00000000000..223698422c2
> --- /dev/null
> +++ b/gcc/testsuite/g++.dg/warn/Wdangling-reference17.C
> @@ -0,0 +1,15 @@
> +// PR c++/111410
> +// { dg-do compile { target c++20 } }
> +// { dg-options "-Wdangling-reference" }
> +
> +#include <vector>
> +#include <ranges>
> +
> +int main()
> +{
> +  std::vector v{1, 2, 3, 4, 5};
> +  for (auto i : std::span{v} | std::views::take(1))
> +    {
> +      (void) i;
> +    }
> +}
> diff --git a/libstdc++-v3/include/std/ranges
> b/libstdc++-v3/include/std/ranges
> index 7ef835f486a..f2413badd9c 100644
> --- a/libstdc++-v3/include/std/ranges
> +++ b/libstdc++-v3/include/std/ranges
> @@ -942,6 +942,8 @@ namespace views::__adaptor
>      concept __is_range_adaptor_closure
>        = requires (_Tp __t) {
> __adaptor::__is_range_adaptor_closure_fn(__t, __t); };
>
> +#pragma GCC diagnostic push
> +#pragma GCC diagnostic ignored "-Wdangling-reference"
>    // range | adaptor is equivalent to adaptor(range).
>    template<typename _Self, typename _Range>
>      requires __is_range_adaptor_closure<_Self>
> @@ -961,6 +963,7 @@ namespace views::__adaptor
>        return _Pipe<decay_t<_Lhs>,
> decay_t<_Rhs>>{std::forward<_Lhs>(__lhs),
>
>  std::forward<_Rhs>(__rhs)};
>      }
> +#pragma GCC diagnostic pop
>
>    // The base class of every range adaptor non-closure.
>    //
>
> base-commit: 615e25c82de97acc17ab438f88d6788cf7ffe1d6
> --
> 2.43.0
>
>