https://gcc.gnu.org/bugzilla/show_bug.cgi?id=111539
Bug ID: 111539 Summary: __is_range_adaptor_closure_fn is too loosely defined Product: gcc Version: 14.0 Status: UNCONFIRMED Severity: normal Priority: P3 Component: c++ Assignee: unassigned at gcc dot gnu.org Reporter: hewillk at gmail dot com Target Milestone: --- template<typename _Tp, typename _Up> requires (!same_as<_Tp, _RangeAdaptorClosure<_Up>>) void __is_range_adaptor_closure_fn (const _Tp&, const _RangeAdaptorClosure<_Up>&); // not defined template<typename _Tp> concept __is_range_adaptor_closure = requires (_Tp __t) { __adaptor::__is_range_adaptor_closure_fn(__t, __t); }; The standard requires range adapter closure object type T to model derived_from<range_adaptor_closure<T>>. However, the above definition does not consider whether the template parameter of range_adaptor_closure is T, which makes libstdc++ accept the following #include <ranges> struct _; struct closure : std::ranges::range_adaptor_closure<_> { int operator()(auto&&); }; int main() { auto r = std::views::iota(0) | closure{}; }