[PATCH] libstdc++: Do not use list-initialization in std::span members [PR120997]

[Tomasz Kaminski]

On Tue, Jul 8, 2025 at 11:46 PM Jonathan Wakely <jwakely@redhat.com> wrote:

> As the bug report shows, for span<const bool> the return statements of
> the form `return {data(), count};` will use the new C++26 constructor,
> span(initializer_list<element_type>).
>
> Although the conversions from data() to bool and count to bool are
> narrowing and should be ill-formed, in system headers the narrowing
> diagnostics are suppressed. In any case, even if the compiler diagnosed
> them as ill-formed, we still don't want the initializer_list constructor
> to be used. We want to use the span(element_type*, size_t) constructor
> instead.
>
> Replace the braced-init-list uses with S(data(), count) where S is the
> correct return type. We need to make similar changes in the C++26
> working draft, which will be taken care of via an LWG issue.
>
> libstdc++-v3/ChangeLog:
>
>         PR libstdc++/120997
>         * include/std/span (span::first, span::last, span::subspan): Do
>         not use braced-init-list for return statements.
>         * testsuite/23_containers/span/120997.cc: New test.
> ---
>
> Tested x86_64-linux.
>
LGTM.

>
>  libstdc++-v3/include/std/span                 | 15 +++---
>  .../testsuite/23_containers/span/120997.cc    | 46 +++++++++++++++++++
>  2 files changed, 54 insertions(+), 7 deletions(-)
>  create mode 100644 libstdc++-v3/testsuite/23_containers/span/120997.cc
>
> diff --git a/libstdc++-v3/include/std/span b/libstdc++-v3/include/std/span
> index 5629a71b9bd2..44f9b36a7efe 100644
> --- a/libstdc++-v3/include/std/span
> +++ b/libstdc++-v3/include/std/span
> @@ -376,7 +376,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>           else
>             static_assert(_Count <= extent);
>           using _Sp = span<element_type, _Count>;
> -         return _Sp{ _SizedPtr{this->data()} };
> +         return _Sp(_SizedPtr{this->data()});
>         }
>
>        [[nodiscard]]
> @@ -384,7 +384,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>        first(size_type __count) const noexcept
>        {
>         __glibcxx_assert(__count <= size());
> -       return { this->data(), __count };
> +       return span<element_type>(this->data(), __count);
>        }
>
>        template<size_t _Count>
> @@ -397,7 +397,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>           else
>             static_assert(_Count <= extent);
>           using _Sp = span<element_type, _Count>;
> -         return _Sp{ _SizedPtr{this->data() + (this->size() - _Count)} };
> +         return _Sp(_SizedPtr{this->data() + (this->size() - _Count)});
>         }
>
>        [[nodiscard]]
> @@ -405,7 +405,8 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>        last(size_type __count) const noexcept
>        {
>         __glibcxx_assert(__count <= size());
> -       return { this->data() + (this->size() - __count), __count };
> +       return span<element_type>(this->data() + (this->size() - __count),
> +                                 __count);
>        }
>
>        template<size_t _Offset, size_t _Count = dynamic_extent>
> @@ -424,7 +425,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>           using _Sp = span<element_type, _S_subspan_extent<_Offset,
> _Count>()>;
>
>           if constexpr (_Count == dynamic_extent)
> -           return _Sp{ this->data() + _Offset, this->size() - _Offset };
> +           return _Sp(this->data() + _Offset, this->size() - _Offset);
>           else
>             {
>               if constexpr (_Extent == dynamic_extent)
> @@ -437,7 +438,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>                   static_assert(_Count <= extent);
>                   static_assert(_Count <= (extent - _Offset));
>                 }
> -             return _Sp{ _SizedPtr{this->data() + _Offset} };
> +             return _Sp(_SizedPtr{this->data() + _Offset});
>             }
>         }
>
> @@ -454,7 +455,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>             __glibcxx_assert(__count <= size());
>             __glibcxx_assert(__offset + __count <= size());
>           }
> -       return {this->data() + __offset, __count};
> +       return span<element_type>(this->data() + __offset, __count);
>        }
>
>      private:
> diff --git a/libstdc++-v3/testsuite/23_containers/span/120997.cc
> b/libstdc++-v3/testsuite/23_containers/span/120997.cc
> new file mode 100644
> index 000000000000..fbf194c87388
> --- /dev/null
> +++ b/libstdc++-v3/testsuite/23_containers/span/120997.cc
> @@ -0,0 +1,46 @@
> +// { dg-do run { target c++26 } }
> +
> +#include <span>
> +#include <testsuite_hooks.h>
> +
> +void
> +test_first()
> +{
> +  bool arr[5];
> +  std::span<const bool> s(arr);
> +  std::span<const bool> s2 = s.first(5);
> +  VERIFY( s2.data() == s.data() );
> +  std::span<const bool> s3 = s.first<5>();
> +  VERIFY( s3.data() == s.data() );
> +}
> +
> +void
> +test_last()
> +{
> +  bool arr[5];
> +  std::span<const bool> s(arr);
> +  std::span<const bool> s2 = s.last(5);
> +  VERIFY( s2.data() == s.data() );
> +  std::span<const bool> s3 = s.last<5>();
> +  VERIFY( s3.data() == s.data() );
> +}
> +
> +void
> +test_subspan()
> +{
> +  bool arr[5];
> +  std::span<const bool> s(arr);
> +  std::span<const bool> s2 = s.subspan(0, 5);
> +  VERIFY( s2.data() == s.data() );
> +  std::span<const bool> s3 = s.subspan<0>();
> +  VERIFY( s3.data() == s.data() );
> +  std::span<const bool> s4 = s.subspan<0, 5>();
> +  VERIFY( s4.data() == s.data() );
> +}
> +
> +int main()
> +{
> +  test_first();
> +  test_last();
> +  test_subspan();
> +}
> --
> 2.50.0
>
>
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