https://gcc.gnu.org/bugzilla/show_bug.cgi?id=121174
--- Comment #4 from Jakub Jelinek <jakub at gcc dot gnu.org> --- Given that #include <mdspan> int main() { int v[] = {1, 2, 3, 4, 5, 6}; std::mdspan<int, std::dextents<std::size_t, 2>> view{v, 2, 3}; return 0; } compiles, I think more likely libstdc++-v3/src/c++23/std.cc.in needs the // <mdspan> #if __glibcxx_mdspan export namespace std { using std::extents; using std::layout_left; using std::layout_right; using std::layout_stride; using std::default_accessor; using std::mdspan; // FIXME layout_left_padded, layout_right_padded, aligned_accessor, mdsubspan } #endif part to include using std::dextents; and when implemented also using std::dims; using std::layout_left_padded; using std::layout_right_padded; using std::aligned_accessor; using std::strided_slice; using std::submdspan_mapping_result; using std::full_extent_t; using std::full_extent; using std::submdspan_extents; using std::submdspan; (not sure if other stuff as well). r16-442-ga33b5db97c4e87b added just using std::extents; even when it also added std::dextents.