As PR c++/117966 shows, the Debug Mode checks cause a compilation error for a global constexpr std::span. Those checks are redundant when constructing from an array or a range, because we already know we have a valid range and we know its size. Instead of delegating to the std::span(contiguous_iterator, contiguous_iterator) constructor, just initialize the data members directly.
libstdc++-v3/ChangeLog: PR libstdc++/117966 * include/std/span (span(T (&)[N])): Do not delegate to constructor that preforms redundant checks. (span(array<T, N>&), span(const array<T, N>&)): Likewise. (span(Range&&), span(const span<T, N>&)): Likewise. * testsuite/23_containers/span/117966.cc: New test. --- Tested x86_64-linux. This needs to be backported all the way to gcc-12 as well. libstdc++-v3/include/std/span | 10 +++++----- libstdc++-v3/testsuite/23_containers/span/117966.cc | 13 +++++++++++++ 2 files changed, 18 insertions(+), 5 deletions(-) create mode 100644 libstdc++-v3/testsuite/23_containers/span/117966.cc diff --git a/libstdc++-v3/include/std/span b/libstdc++-v3/include/std/span index f1c19b58737..67227348c90 100644 --- a/libstdc++-v3/include/std/span +++ b/libstdc++-v3/include/std/span @@ -190,21 +190,21 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION requires (_Extent == dynamic_extent || _ArrayExtent == _Extent) constexpr span(type_identity_t<element_type> (&__arr)[_ArrayExtent]) noexcept - : span(static_cast<pointer>(__arr), _ArrayExtent) + : _M_ptr(__arr), _M_extent(_ArrayExtent) { } template<typename _Tp, size_t _ArrayExtent> requires __is_compatible_array<_Tp, _ArrayExtent>::value constexpr span(array<_Tp, _ArrayExtent>& __arr) noexcept - : span(static_cast<pointer>(__arr.data()), _ArrayExtent) + : _M_ptr(__arr.data()), _M_extent(_ArrayExtent) { } template<typename _Tp, size_t _ArrayExtent> requires __is_compatible_array<const _Tp, _ArrayExtent>::value constexpr span(const array<_Tp, _ArrayExtent>& __arr) noexcept - : span(static_cast<pointer>(__arr.data()), _ArrayExtent) + : _M_ptr(__arr.data()), _M_extent(_ArrayExtent) { } template<typename _Range> @@ -218,7 +218,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION span(_Range&& __range) noexcept(noexcept(ranges::data(__range)) && noexcept(ranges::size(__range))) - : span(ranges::data(__range), ranges::size(__range)) + : _M_ptr(ranges::data(__range)), _M_extent(ranges::size(__range)) { if constexpr (extent != dynamic_extent) { @@ -236,7 +236,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION constexpr explicit(extent != dynamic_extent && _OExtent == dynamic_extent) span(const span<_OType, _OExtent>& __s) noexcept - : _M_extent(__s.size()), _M_ptr(__s.data()) + : _M_ptr(__s.data()), _M_extent(__s.size()) { if constexpr (extent != dynamic_extent) { diff --git a/libstdc++-v3/testsuite/23_containers/span/117966.cc b/libstdc++-v3/testsuite/23_containers/span/117966.cc new file mode 100644 index 00000000000..8bbb5ca1e07 --- /dev/null +++ b/libstdc++-v3/testsuite/23_containers/span/117966.cc @@ -0,0 +1,13 @@ +// { dg-options "-D_GLIBCXX_DEBUG" } +// { dg-do compile { target c++20 } } + +// Bug 117966 +// constexpr std::span construction fails to compile with D_GLIBCXX_DEBUG + +#include <array> +#include <span> + +struct A { + constexpr A(std::span<const unsigned char>) {} +}; +constexpr A val{std::array<unsigned char, 2>{0x11, 0x22}}; -- 2.47.1